Tuesday, October 29, 2013

Fun with the Microchip PICKit 3 and the Sure Electronics DB-UD11111 ZIF adapter

For many years now I've been using my old, trusty Picstart Plus programmer for my PIC-based projects.  Having used PICs since about 1990 - and having a reasonable suite of development tools, including the CCS C Compiler.

Since (before?) the introduction of MPLAB-X a while ago, the Picstart Plus was not been actively supported - and it never did/will support some of the newer, fancier devices anyway, so I started to look around for a replacement.

I quickly came to the realization that the most reliable path to the newer chips was the PICKit 3, a USB-based device which is much more convenient - and faster - than the serial-based Picstart Plus, but I soon realized that in order to use it with a wide variety of devices I'd need to have some sort of external board with several different sockets on it:  Unlike the Picstart Plus - which would actually "rewire" itself to program about any PIC you threw at it, the PICkit 3 simply had several pins on it which connected to the user's board or to an external socket using the ICSP (In-Circuit Serial Programming) capabilities of modern PIC processors.

In looking around the web I spotted the Sure Electronics DB-UD11111 for just $9.95 - and it looked as though it would fit my needs:  It was fairly cheap and it looked as though it would be able to handle most of what I needed it to do having 40, 20 and 18 pin ZIF (Zero-Insertion-Force) sockets to accommodate the different PICs.  While it appears to have been originally designed for the PICkit 2, the same ICSP programming is used in the newer PICkit 3 so it would work equally well for both.  At $9.95, I knew that I would probably have trouble even buying three ZIF sockets for that price!

It would appear that the Sure Electronics DB-UD11111 has vanished from the above web site (at least I couldn't find it there!)  It does seem to be available from other vendors on the web for a higher price than the original $9.95 and, at the time of this update (August 2015) it seems to be available on EvilBay, being sold by Sure Electronics in their very own store for around $15 - search for "DB-UD11111".
I'm sure that there are other ZIF socket arrangements that will permit the PicKit 3 to work with a wide variety of devices, but I am not familiar with them.

Several weeks after placing the order with Sure, both it and the PICkit 3 arrived from China and I sat down to study the board.

Unfortunately, the documentation supplied is very sparse to say the least as it is just a .PDF of the schematic diagram of the board!  In looking at the board - which seems to be fairly well built but a bit awkward to use (more about that shortly) - I could see that it was labeled for 18, 20 and 40 pin devices.

What about programming  8, 14 and 28 pin devices?

In seeing the 18, 20 and 40 pin sockets, I wondered about the other devices.

To answer this question looked at the data sheets for typical 8, 14 and 28 pin devices (e.g. PIC12F675/683, PIC16F688 and PIC18F2620, respectively) on the Microchip web site and compared them to the schematic of this ZIF board to satisfy a hunch - which I verified to be correct:  Microchip had thoughtfully placed the Vdd, Vss, Vpp, PGC and PGD pins - everything that you'd need to program modern PICs - in locations that physically translated to the the appropriate pins on different package sizes of these PICs.

In other words:
  • 18 pin PICs:  You program those in the 18 pin socket - that's obvious!
  • 28 pin PICs:  The programming pins on the 40 pin socket also align physically with those on the 28 pin PICs, so pin 1 of the 28 pin devices go in pin 1 of the 40 pin socket.
  • Figure 1: 
    The PICkit 3.  As you can see I
    added a label to remind me of the
    "power" configuration to make it
    work with devices plugged into
    the passive ZIF socket adapter.
  • 20, 8 and 14 pin PICs:  The connections on the 20 pin socket physically align with those required on both 8 and 14 pin PICs, so use that socket, aligning pin 1 of the 8 and 14 pin devices with the socket's pin 1.

Making it work:

After getting the PICkit 3 and firing up MPLAB, I initially had trouble getting it to see any PICs at all - that is, until I remembered that the PICkit 3 had the option of powering the PIC chip being programmed - or not. As it turned out, it defaults to "not" so I had to go into the "power" sub-menu and check the box that told the PICkit to power the target device.

After that, I could read and write to a PIC16F88.  "Great!", I thought,  so I tried a PIC12F683 - an 8 pin device.

No dice!


Using an ohmmeter I verified that all of the pins went where the schematic said that they should go (they did) so why didn't the PICKit 3 "see" the 12F683?

On a hunch I put the voltmeter on the power supply pins of the PIC and saw about 4.75 volts - lower than the 5.00 volts that I'd selected in the "power" menu.  Setting it to 5.5 volts, I saw no change, so I dropped it to 4.5 volts and not only did the voltage on the PIC now read 4.55 volts (close enough!), but I could now read and write to the PIC12F683!

What's the deal, then?

As it turns out, the PICkit 3 gets its voltage for powering the PIC being programmed from the computer's USB port which, by definition, is somewhere around 5 volts - and in the case of this particular computer was right at 4.85 volts.  What this means is that the PICKit3 could never supply more than about 4.75 volts to the PIC as there is about 0.1 volts drop within its circuitry.
Figure 2:
The Sure DB-UD11111 socket adapter.
It's pretty well built, but the cover gets
in the way of the levers!  After
taking this picture I took the cover
off, saving it by sandwiching it
on the bottom cover to avoid
losing it somewhere.

What seemed to be happening was that at a voltage that was "too high" to be supplied by USB connection, the Vdd being supplied to the PIC could not be regulated by the PICkit 3 and was likely "dirty."

Setting it to a lower voltage safely below that which was provided on the USB port - say, 4.5 volts - "fixed" this problem.

Unfortunately, there doesn't appear to be a way in MPLAB to have it supply power to the target device by default so I have to remember to re-select these anytime I make a change to the configuration or start the program.  After a bit of fiddling I "discovered" that if you exit MPLAB (Version 8.9x) with the unit configured for 4.5 volts that it may remember that voltage setting next time it is powered up - but it still seems to require you to tick the box for the PICkit supplying power to the target device and verify that the programming voltage is still set to 4.5 volts.

In further testing with a few different PICs I found that I could read/write them at least as low as 3.5 volts, but I couldn't immediately find a specification as to the low end of Vdd range.  Since I was not using LVP (Low Voltage Programming) and the PICkit 3 was supplying the Vpp, this specification is likely somewhat relaxed for most devices.

One suggestion that has been made online is to connect the PICkit 3 to a powered USB hub that is supplied with 5.25 volts.  Interestingly, the voltage specification for USB 2.0 is 5.0 volts +/- 0.25 volts so their inclusion of 5.5 volts as a  valid programming voltage would never be satisfied by an in-spec USB connection!

A few more comments about the Sure DB-UD11111 socket adapter:

This socket adapter is built fairly well, but I removed the translucent top cover after taking the picture in Figure 2 as it also somewhat obscured the silkscreen notation on the board, including the indication of the location of pin 1 of the connecting cable. (it's the white wire on the right, if you are curious).

Another problem with this cover is that it gets in the way of the levers on the ZIF sockets:  You may need to use either your fingernails or a small tool to operate some of these levers unless you have fairly small fingers.

Was removing this cover much of a loss?  No, since I'll just make some labels, anyway to remind me where/how to orient 8, 14 and 28 pin devices.

On the bottom plastic cover on the board I attached some small, stick-on rubber feet to keep the unit from sliding around on the desk, as well.

Some final comments on the PICkit 3:

In the forums, blogs and comment boards, the PICkit 3 has been oft-maligned - and I can see why:  It is decidedly less user-friendly and idiot proof than the Picstart Plus in many ways as there are many things that can go wrong - particularly if you have integrated a PIC with ICSP in your project where there is more than jut the PIC being programmed to deal with.

To be sure, the "problem" with the Vdd supply being derived from the USB power source is a problem for those who use the PICkit 3 to supply power to the target device - and it would be really nice if there was the option in MPLAB to allow all of the power options to "stick":  A warning comes up anyway about powering 3.3 volt devices from 5 volts, so why not have that warning include any settings that you have overridden, too?

In my (thusfar) limited use, it is much faster than the Picstart Plus and it has been very reliable - once you know the tricks - with it being powered entirely by the USB interface, which is much more convenient than the Picstart with which required not only a USB-to-serial adapter to work with my laptop, but also a "wall wart" just to power the programmer.

Additional comment:

I occasionally have trouble programming PIC12F675's and PIC12F683's with the PICkit 3 because of this same problem.  In those cases, attaching a separate Vdd supply of 5 volts to the programming socket - to power the chip being programmed - always solves the problem.

I have no idea why it works most of the time and then fails at other times...

Why PICs?

These days, the PICs seem a bit passe to many as the Arduinos and their variants have caught the fancy of many experimenters.  True, there are many "shield" modules with lots of cool peripherals to be found and, perhaps most attractive of all, huge libraries of code available to get done what you might want to do.

While I have used the pre-built Atmel/Arduino devices peripherally (pun intended!) and they look cool, they have usually been a bit of an overkill - and certainly much more expensive (hardware-wise) than just a PIC and a few parts.  In most cases, when I have a project, it would take a bit of effort to shoehorn an already-made Arduino board into place to fit my needs than it would have been to simply design the device from the ground up to do just what I need:  After all, to get the processor, alone, working it may take as little as a capacitor, IC socket (in some cases, less than $2 in parts) and the processor itself to provide the basis of the hardware to which the rest of the project to which you'd need to connect any processor that you might use (PIC or not) to the rest of the project!

Since I have long had the development tools for the PIC (e.g. CCS C compiler, since the "Version 2" days) and was familiar with it, I already have a fairly large library of my own code from which to borrow - and occasionally, I may even "borrow" some ideas from other platforms as well!

In doing my PIC projects I have found the 8-pin devices to be the most all-around useful as it is often the case that just 6 I/O pins is enough to do what I need to do - anything from flashing some lights to controlling a fire siren to even doing some audio DSP/filtering - and I've probably dropped hundreds of these things into projects that I did on my own and for others.  Where 6 pins of I/O isn't enough, the 18 pin devices are about the right size for most other things - but I have used the 14, 28 and 40 pin devices where it made sense to do so!

Having said that, the Arduinos do look fun and one of these days I may just try one of the "canned" boards and then think how I might incorporate that into another project.  More likely, though, I'd probably just build something around the raw chip itself - just as I've been doing for years now with the PICs...

Since this was post was originally written I got another PicKit 3 and DB-UD11111 adapter - just so that I would have two:  One for the bench, and one for "on the go" when I needed to haul the programmer somewhere else, or for those times that I can't find the other one because there's too much junk on the workbench!


This page stolen from ka7oei.blogspot.com

Monday, October 28, 2013

Repair of the speedometer on a Polaris Sportsman 500 4-wheeler

If you are at this page you probably have a Polaris with a bad speedometer which means that you can't put it in 4 wheel drive and your reverse override doesn't work.
If this has happened to you, you probably ran it without a good battery - or with no battery at all:  Read on to find out why you should never do that if you can help it!
There are at least two versions of speedometers used in the late 1990's and early 2000s - the one that, inside, looks like Figure 4 with "through-hole" components and a slightly newer version that largely uses surface-mount components.  While both versions seem to have the same fate when used with a bad (or no) battery - and the "fix" is likely similar, I have not personally seen the newer surface-mount version, but you can find information on its repair near the bottom of this article as provided by one of the readers.
If you have successfully repaired one of these newer versions and are willing to share the specifics, I'd be happy to post it here.  If you have a "dead" speedometer of the newer version and would likely me to take a crack at fixing it (absolutely no guarantees about success!) then feel free to contact me via comments, below.

Last year, a friend of mine had the battery go bad on his 1999 Polaris Sportsman 500 4-wheeler.  Aside from the inconvenience of having to pull-start it, it seemed to work OK.

Sort of.

Soon, it was noticed that the speedometer had died and interestingly, a few other things quit working at the same time such as the ability to put it into four-wheel drive. When the battery was finally replaced the speedometer still did not work so a he dug around in the internet and found that this is quite a common problem with that vintage of Polaris vehicles - and it seems to work out this way:

The charging system's voltage regulator on these vehicles are fairly simple, but they depend strongly on the presence of the battery to moderate the wildly pulsating DC coming out of the alternator/regulator system to maintain the average voltage in the range of 13.5-14.2 volts or so.  If the battery goes completely bad or is removed, the charging system goes haywire and the voltage can (apparently) exceed 20 volts (and is probably higher) and one can risk burning out the various indicator, marker and headlights.

Another fatality under this conditions seems to be the speedometer module itself!

What (probably) happens:

It's probably not the high voltage that actually kills the speedometer:  The voltage regulator circuit in the speedometer seems to be fairly robust, using high voltage (>=300 volt) transistors to withstand the voltage spikes that are endemic to any vehicle electrical system.  What seems to kill these things is heat.

Let me explain.

The job of the voltage regulator circuit inside the speedometer is to assure that the voltage feeding the circuit inside doesn't exceed about 15 volts or so and from there, it is regulated down even lower by other circuitry for the computer that provides the odometer readings and (probably) the speedometer as well as having something to do with the reverse limiter designed to prevent you from accidentally driving backwards at a high speed and the lockout/controls for the all-wheel drive switch.  There are also several small light bulbs inside the speedometer that provide backlighting for the display at night and these, too, are protected from high voltage by the 15 volt regulator.

Under normal conditions the voltage on the vehicle's electrical system is around 14 volts or so and the regulator's job is to suppress spikes and brief excursions above that and in this mode, the regulator itself isn't doing much.  If the voltage rises, however, it has to drop the excessive voltage and and a natural by-product of this is that it develops heat.

Apparently, quite a bit of it!  In testing the speedometer after the repair I applied 20 volts to it and the main regulator transistor soon got too hot to touch:  If this had been a hot, summer day with the transistor crammed inside the waterproof speedometer casing with no free air ventilation, it would have been much hotter.

So, with the bad battery and a subsequently malfunctioning charging system it is easily likely that the speedometer's regulator saw an average of 20-30 volts on its input.  At some point the transistor overheated and eventually failed internally, shorting itself out.  Fortunately, the majority of the circuits in the speedometer seemed to survive this since once the regulator itself had quit, all power feeding the rest of the circuit was lost completely, preventing further damage.

While a new speedometer is available as a replacement part, it will cost you several hundred dollars, new!

Fortunately, it may be that you can fix it!

The obligatory warnings, etc.
  • Assume from the beginning that the speedometer is a total write-off and that you would have to replace it, anyway.  This way, if you can fix it, you will be money ahead - but if you can't, you haven't lost anything more! 
  • Before you start, read this entire posting so that you'll know what you are in for!
  • Repair of the speedometer requires some knowledge of electronics and board-level electrical components.
  • Repair also requires good unsoldering and soldering skills and equipment - and a soldering "gun" doesn't count.  If you don't have the proper tools and experience in the replacement and installation of individual, through-hole components, do not even attempt this!
  • The speedometer is part of the electrical system of the vehicle and as such, it is possible that its malfunction - possibly due to a failed repair - could cause additional damage to other components.
  • No, I won't repair your speedometer as with shipping, time, "hassle factor" and labor, I'd have to charge a sizable percentage of the cost of a new one.  I suggest that you find someone versed in electronics to help you out if you need to do so.
  • I know ONLY about the speedometers on Polaris Sportsman 500's for the years 1999 and 2000:  If you ask me about speedometers for any other make, year or model, I can't help you!  (They may be the same - they may not - I don't know.) 
  • You do this repair at your own risk!  Do not get mad at me if you blow something up, set fire to your four-wheeler or cause all of your dog's hair to fall out! 
  • Again, there seems to be a (newer?) version of the speedometer with more surface-mount parts that can suffer the same fate.  While I have not seen one of these, personally, one of the readers has provided information about his successful repair:  With his permission, I have posted this update near the end of this article.
  • You have been warned!

How to do it

Remove the speedometer:
The first step is to remove the speedometer from the vehicle.  It's a bit of a pain, but it's not terribly difficult to do as it is almost the same procedure as would be followed for replacing the headlight.

Inside the housing that covers the headlight you'll find two connectors that snap into the speedometer, held in place with release tabs, as well as two nuts that hold the bracket in place:  Note how these go together before taking them off - make a drawing and/or take a picture before you take everything off if you aren't sure.

Open the speedometer:

Place the speedometer face down on a clean, un-cluttered work area on a surface that you don't mind scratching:  It's recommended that you put a rag or old towel between the face of the speedometer and the work surface.

Now, notice the soft, aluminum ring around its perimeter:  This holds the clear, plastic cover to the body of the speedometer by virtue of crimping between those two pieces a rubber gasket.

Wearing leather gloves to prevent being stabbed during this step, use a medium-sized blade screwdriver - preferably one that is somewhat worn out with rounded edges - and slide it between the aluminum ring and the plastic body of the speedometer on the back side, prying the ring open as you go along, straightening it out.  With a bit of practice you can firmly slide the screwdriver along the perimeter and straighten out that soft, aluminum ring and you will probably have to go around several times to do the job.

Figure 1:
The soft, black, aluminum ring around the perimeter of the speedometer that holds the faceplate to the body.  Carefully pry this straight.
(This picture was taken after I'd already opened and repaired it - and partially closed it again.)
Click on the image for a larger version.

Once you get 75-90% of the backside of the ring straightened out, you'll be able to pop the ring off the front:  Set it aside.  With the ring removed you should be able to use your fingernails and pry the front, clear cover from the body of the speedometer.

Be careful with the black plastic rod under the push button, noting carefully how it is installed and taking care that it doesn't fly off somewhere!

Once you have the cover off, set it aside with the black, plastic rod laying inside the cover.

Remove the needle:

This is sort of tricky and it is possible to ruin the speedometer with this step:  Since you have already declared the speedometer to be a total loss, you shouldn't feel too bad if you do.

First, note how far the needle is pushed on to the spindle:  You'll want to remember this when putting it back on.

If you have very strong fingernails, try pulling the needle straight off the speedometer, but whatever you do, apply tensions EVENLY - that is, pull straight out on the needle as you do not want to bend the spindle!  When you pull on the needle make sure that the speedometer is on the workbench with padding on it because if it comes off suddenly, you don't want to slam your hand or the speedometer into the workbench and break something!

If you can't remove the needle with our fingers, you'll need to apply a bit more force.  Cut some pieces of paper or thin cardboard (such as from a cereal box) so that you cover the entire face of the speedometer, but allow access to the needle and its spindle - this being done to prevent you from accidentally marking up the speedometer face.

Now, using two medium-size blade screwdrivers, pry the needle evenly off the spindle using the paper/cardboard to prevent damaging the speedometer face:  You may want to wrap a rag around the body of the speedometer and clamp it gently - but firmly - in a vise.  Hopefully, the needle will come up without breaking anything else!  If you do break something else, save up for a new speedo!

Remove the speedometer module from the body:

Using a small screwdriver blade or, preferably, a similarly-sized and shaped piece of plastic, carefully pry up on the face of the speedometer.  The face is actually printed on a piece of fairly heavy, self-adhesive plastic that is about as thick as a postcard - and it is this, not the actual body of the speedometer itself - that you want to pry up.  It can be a bit tricky to get purchase on the speedometer face and you might bend into a hook a small piece of metal such as a paper clip to act as a tool.

Figure 2:
By carefully prying up the speedometer face's plastic, you can access the three screws that hold the module in the body.  The other two screws are located 1/3rd of the way around, at approximately "10" and "40" MPH.  If you can't get the needle off before this step, make sure that you move it out of the way when you pry up on the face plate so that you don't accidentally break it.
Click on the image for a larger version.

Start by prying up on the plastic face below and in the middle of the LCD odometer display, at the bottom of the speedometer and once you lift it a little bit, you will see underneath it a Philips type screw:  Holding the speedometer face up with a small screwdriver, use another screwdriver to remove that screw.

There are two other screws, each located 1/3rd of the way around on either side:  Remove those, too.

Now, the only thing holding the speedometer module inside the case is friction and the silicone used to seal around the wiring connector pins on the back.  Using a small blade screwdriver, work your way around the perimeter of the inside of the speedometer, wedging gently between the outer body of the speedometer and the module itself, reaching down slightly past the face of the speedometer to do it.  After going around several times, applying a bit of twisting and/or prying force, the module will hopefully break loose and gradually come out.

When it does this, the pins from the electrical connectors on the back will be pulled through the case and soon, you'll have the module separated from the case.

  • There are one or two cylinders with granules packed inside them in the case that contain moisture-absorbing compound.  As you  remove the body of the module, they may come out, or they may be (at least temporarily) stuck in place in their own crevice inside the module - but in any case, note where they originally sit.   Take them out and place them in a "Zip-Lock" (tm) bag and suck out the air to protect them from additional moisture while you are working on the speedometer.

Removing the LCD and accessing the back of the circuit board for soldering:

In order to get access to the "solder" side of the circuit board you'll need to remove the portion with the face plate, after you have removed the needle.  In so-doing, you'll also be removing the LCD odometer display.  It is recommended that you do this over a workbench covered by a rag or towel in case the fragile LCD falls out.

On the back side of the speedometer module (the "component" side of the circuit board) you'll find four black screws that correspond approximately with the four corners of the LCD display.  Laying the speedometer face-down on a piece of cloth, remove these four screws:  The front faceplate portion will separate from the circuit board.

This front portion also retains the LCD in place and it may fall out.  If it doesn't come out on its own, carefully remove it - noting the markings on the LCD and which way they were oriented with respect to the board.

Figure 3:
The repaired board showing the LCD.  Note the orientation of the writing on the LCD with respect to the board.  If you look carefully, you will notice markings on the upper-left edge of the LCD which may be used to indicate which way is up.
 Toward the 2-o'clock position of the speedometer board you can see the repair to the damaged trace.  Because the board is coated with a sticky conformal coating compound used for moisture protection it's difficult to avoid discoloring when soldering due to the heat and flux.   
Click on the image for a larger version.

The LCD's electrical connections to the circuit board are made via two small strips of pink-ish conductive rubber sandwiching darker rubber (often called "Zebra Strips") and these usually stick to the LCD.  If they are stuck to the board, very carefully remove them, but if they are stuck to the LCD, don't worry and leave them in place.  Set the LCD and the rubber strips aside in a clean, dust-free place such as a clean, dry food container.  You may notice that the LCD itself has a part number printed on it:  Note its orientation so that you can reinstall it correctly.

You now have access to both sides of the circuit board.

Identifying the bad components:

Important note (again) about through-hole versus surface-mount versions:

The pictures and descriptions below assume a through-hole version of the speedometer.  It would seem that a later model of this same speedometer uses surface-mount components for some of those that fail.
While I have not seen a surface mount version of this speedometer in person, one of the readers of this blog entry with a failed surface-mount version of the speedometer was sufficiently inspired to attempt a repair - and was successful!
With his kind permission, some details on the repair of a surface-mount version may be found at the bottom of this blog entry.

It seems that the one part that is sure to go is a large-ish power transistor, but there can be two components next to it that are also destroyed - and this damage may be evidenced by some burn marks on the circuit board:  See Figure 4, below, for identification of these components.

Figure 4:
Location of the likely bad part(s).  When I took this picture I had already replaced the TIP48 and the MPSA42 - but not the Zener diode.  There have been reported instances where the parts have gotten so hot that the solder has melted and that they have simply fallen out:  If this is the case with your speedometer, be sure to test the parts before reinstalling them or, if you don't have the facility to do that, simply replace them.
Click on the image for a larger version.

Even though only one or two of these components may be bad, I would recommend replacing all three of them.  These are:
  • A TIP48 high-voltage NPN power transistor.  This is the most likely component to be damaged and is a transistor with a metal tab.  In Figure 4, above, the leads are, left to right, B-C-E.  (The NTE equivalent is the NTE-198.  Note that previously I'd inadvertently listed the '197, which was incorrect.)
  • An MPSA42 high-voltage NPN low-power transistor.  This is a small, black transistor located next to the TIP48.  In Figure 4, the leads are C-B-E from top-to-bottom.  (The NTE equivalent is the NTE-287.)
  • A 1N5245 15 volt, 1/2 watt Zener diode.  This is located next to the MPSA42 transistor and is a small, (usually) red/orange glass device on axial leads.  In Figure 4, the "banded" end is the lower end.  (The NTE equivalent could be the NTE-5024A or its 1-watt version, the NTE-145A.)
The only critical things about the two transistors are the fact that they are NPN devices rated for at least 300 volts to withstand the normal transients found in a vehicle electrical system.  If you are unfamiliar with electronics I would suggest that you replace these with exact types which are readily available from Mouser Electronics or Digi-Key Electronics - or you can get cross-reference equivalents in the NTE family (noted above) from MCM Electronics or even find a local distributor that carries the NTE parts.

If you are familiar with electronic components:
  • The TIP48 may be replaced with about any NPN bipolar transistor found on the "mains" side of an AC-powered switching power supply - such as a dead computer supply.   Typically, these transistors  are rated for more than 400 volts and are in a TO-220 style case - often with a plastic or insulated tab - which really doesn't matter in this application.  Just make sure that it's rated for at least 300 volts and has the same pinout (B-C-E as viewed left-to-right with the leads facing down and the label facing you) as the original TIP48!  Chances are this transistor will have a number that begins with "2SC" (or just "C") followed by 4 digits:  Look up the device's data sheet online and verify that it is, in fact, a high-voltage NPN device.  The one that I happened to used came from a junked VCR, was rated for 500 volts and happened to have a plastic tab rather than the metal tab of the original device.
  • The MPSA42 is a high-voltage, low-power NPN transistor of the type typically used in video drivers for cathode-ray tubes on older TVs and suitable equivalents may be found on the small circuit board attached to the end of a CRT on a discarded television.  I happened to have some ZTX458 transistors - devices that had equal or better voltage/current specs than the original - laying around from another project and used one of them.  Just make sure that you take into account any differences in the pinout of the replacement transistor!
  • I didn't have a 1N5245 1/2 watt, 15 volt Zener around, but I did have a 1N4744 Zener which is a  somewhat beefier 1 watt version with the same voltage rating, so I used it.
Replacing the components: 

  • Unless you are experienced in component replacement and circuit board repair, I would not suggest you do this procedure at all!
  • Use a temperature-controlled soldering iron.  Too little heat, you'll damage the board trying to get the components off.  Too much, you'll damage it that way, as well.
  • Do NOT use a soldering gun for this repair work:  If that's all that you have, you really should not try it as you'll likely ruin the board! 
  • Using your phone or camera, take some close-up pictures of both sides of the board before you start as a possible aid in reassembly.
  • You must have proper desoldering equipment.  A vacuum-operated desoldering tool is the ideal, but "Solder Wick" (tm) or even a spring-loaded "solder sucker" or "desolder bulb" will work.  If you don't have any of these it will be challenging to get the through-holes cleared to install the replacement components without tearing traces off the board. 
  • Both sides of the circuit board and most of the components are covered with a moisture resistant coating.  This coating is slightly rubbery and when soldered, it gets discolored.  Fortunately, you can "solder through" it although extra care should be taken to assure that the solder joints are clean and good.  When you are done soldering, you can clean the flux with alcohol and a cotton swab, but the discoloration will probably remain.  (If your solder uses water-based flux, make absolutely certain that you have removed it using clean water as many of these types of fluxes can slowly corrode connections.)
  • When removing the old components (the two transistors and the Zener diode) it may be easiest to just clip them from the board first using small, sharp diagonal-cut pliers.  This will remove the body of the device and allow each lead to be removed independently. 
Using "solder-wick" or a vacuum desoldering device, remove the two transistors and the Zener Diode, noting the original orientation of each device.  Make sure that you also clean out the holes through the board!

In my case, the heat of the original TIP48's destruction and its subsequent removal from the board actually damaged a trace so I had to repair it with a short piece of wire (I used #30 wire-wrap wire) on the bottom-side (that's the short green wire visible in Figure 3) but it's likely that your damage won't be that severe.

After inspecting for damaged traces, install the new components.  The notes above indicate which lead is which when replacing the two transistors and the diode.

Figure 5:
The newly-repaired speedometer.  It shows a speed because I was injecting a signal on the back to simulate input from a wheel sensor.
If you look carefully you might note that the 10th's of a mile digit on the LCD is missing part of its top half:  To fix it I had to take it apart again, clean the rubber pad ("zebra strip") the circuit board contacts and the LCD with some denatured alcohol and reassemble it to remove the mote of dust that prevented part of that particular digit from working properly.
Click on the image for a larger version.

Putting it back together:

As they say, "Assembly is the reverse of disassembly"!

A few comments:
  • When putting the LCD back in place, it may be a good idea to wipe down the top and bottom edges of the rubber strips with an alcohol-wetted cotton swab, and this should also be done for all surfaces of the LCD itself and the metal contacts on the circuit board.  Doing this will make sure that there aren't any dust particles, fibers or hair that may cause one or more of the LCD's segments to not work.
  • The two rubber strips for the LCD just sit in place, on edge, in the LCD mount and the LCD goes on top of it.  When mating the face of the speedometer back to the circuit board, make sure that you have re-installed the LCD right-side up and oriented properly (remember when you made note of the marks on the LCD?) so that you don't accidentally crush and break it when putting the speedometer face back into place.  Make sure that the four black screws are tightened firmly - but not so tight that you crack the plastic.
  • Before sealing everything up, carefully remove obvious fingerprints from the LCD (using a cotton swab) and the inside of the face plate as well as the speedometer. 
  • If you have a bench-type power supply capable of 12 volts at about 600 milliamps, you can do some preliminary testing of the speedometer by +12 volts to pin "A" and the ground to pin "B".  If all goes well, the lights will turn on and you'll see the odometer displaying numbers.  See below for a description if the pinout.
  • If you have applied power and everything looks OK, gently push the needle partway onto the spindle, aligning it with "0".  With the power applied, move the needle up scale (say, to 20-40 miles-per-hour) and watch it go back down to zero:  If it stops slightly off zero (it may take 10-15 seconds to settle when the unit is powered up) then pull the pointer off and re-align it, re-doing the above steps again until you get it to land on zero.
  • If you can't power up the speedometer, line the pointer up with zero and then move it up-scale to 20 or so.  After 5-10 minutes (yes, it may take that long to slowly move back!) look again to see where it is pointing:  If it isn't at zero, remove and re-align the pointer and try it again.
  • Once you are satisfied that the pointer is correct, firmly push it in on the spindle as far as you noted that it had been pushed before you'd removed it.  Move the needle up-scale by hand again and verify that it will (slowly) move down, indicating that it isn't binding anywhere.  If the pointer isn't pushed in far enough it can jam on the inside of the faceplate.
  • In the case you may have noted one or two small cylinders with granules inside them that are either still in place, or had previously fallen out and it was recommended that they be put in a sealed storage bag.  These are moisture absorbers and they sit in a slight recess one one side of the case.  It is recommended that these be reinstalled
  • When you put the speedometer module back into the case make sure that you move the needle out of the way when you pry up on edges of the speedometer face to reinstall the three screws.
  • Strongly recommended to maintain waterproofing:  Apply a thin layer of silicone grease to the gasket that goes between the body of the speedometer and the clear front plate, taking care not to get it on the face plate where you can see it.  (Just wipe the grease off if you do get it on the display or the back side of the clear cover.)  You may use silicone-based "Plumber's Grease" available at practically any hardware/home improvement store.  Do not use a petroleum-based grease for this!
  • When putting the face plate back on, remember to re-install the black push-rod that operates the button!
  • Test-fit the face plate to make sure that you pushed the needle on far enough and is not rubbing on the inside face plate. 
  • Slide the aluminum ring back into place.  With the speedometer face-down in a cloth (preferably a gap to accommodate the button on the front panel) use a piece of wood to re-crimp the aluminum ring to attach the face plate to the body.  You'll probably have to go around the perimeter several times to get it tight.
  • Squirt a dab of silicone grease (but not silicone seal!) this in each of the electrical connectors on the back side:  This will help prevent ingress of moisture as well as prevent possible corrosion of the electrical connectors.  Again, "Plumber's Grease" or the sort used to lubricate "O" rings is this same sort of grease and will work fine and this is available anywhere you can buy plumbing supplies and parts - including big-box home-improvement stores like "Lowes-Depot."  Remember:  It is not recommended that one use "normal" petroleum grease (e.g. axle grease, Vaseline (tm)) as this will degrade the plastic and connectors!

Comments about pin-out and testing:

Note:  If you do an internet search, you should be able to locate some online drawings showing the pinouts of the speedometer's connectors.  If you do this testing, you would do so before reinstalling it in the case.

With the speedometer facing down, orient it so that the 6 pins of the larger connector are toward the top running horizontally and that the 3 pins of the smaller connector (the one for the wheel sensor) is to the right of it with its pins running vertically.

For the large connector, pin "A" is on the left and they are designated on available drawings as A-F, left-to-right.  For the 3-pin connector they are A-C starting from the top and working down.

Applying voltage to these pins using a regulated, current-limited power supply set to 12-15 volts at a maximum current of 600 milliamps to 1 amp, you should be able to power up the speedometer.
  • Do NOT power the speedometer being tested directly from a battery as that could supply virtually unlimited current in the event of an accidental short or fault.
  • DO NOT connect the polarity backwards - even for an instant.
  • If your current-limited power supply "sees" a dead short, remove power immediately and check for solder bridges around the components that were replaced.

If it works, the lights should come on and if you move the speedometer needle with your finger, up-scale, it should reset itself to zero much more quickly than it had with the power off.  An additional test is that if you increase the voltage above 14-15 volts (but not above 20 volts!) the lights will not get any brighter - a sure sign that the regulator is now working properly.  If you do this, now is the time to double-check that the needle points at zero.

If you are curious, you can apply a square wave signal from an audio generator (3-5 volts RMS) between pins B and C of the 3-pin connector and vary it from about 5 Hz to 200 Hz and you should see the speedometer go up with increasing frequency as you simulate, with your audio generator or "function generator", the input from the wheel sensor.  Pin "C" is ground while pin "B" is the signal input.  The unit must be powered up in order for the speedometer - and odometer, for that matter - to indicate.  If you do not have a piece of test equipment to generate such a signal, a "555" timer chip may be wired up (with the appropriate components) to generate a variable frequency square wave train.

Final words:

If all goes well, your speedometer, 4-wheel drive switch, "reverse override" button and odometer will now work properly again!

Note that there's no guarantee that it will be as waterproof as it was before since you probably lack the special machine required to properly crimp that aluminum ring down so it's probably best to keep it out of rain as much as possible - a good practice, anyway!

Comment:  I occasionally get asked a question via this blog's comment tool.  Unless you include a return email address, you'll have to check back here to see if I've answered it as I won't be able to reply any other way.

As of October, 2015 August 2016 October 2017 November 2018 the repaired speedometer is still working fine.

Repairing the Surface-Mount version of the Speedometer

One of the readers (Mathias) was inspired by this blog entry and decided to attempt a repair of the newer surface-mount version of this same speedometer.  It is not known, exactly, over which model numbers and years this speedometer was in use (or if it still is!)

What follows below details the repair of a unit from a 2000 Polaris Sportsman 335.  Again, unless you are familiar with work on small electronic components and have the appropriate tools to do so, please seek out someone who does have the the familiarity/tools before attempting a repair!  You have been warned!

Figure 6
 Used by permission of Mathias, this is a photograph of the surface-mount version found in a 2000 Polaris Sportsman 335.  It is believed to be (more or less) the same circuitry as that shown in figure 4, above - but using more surface-mount components.
The thread of the related conversation - including a reference to the picture above - may be found in the comments below:  Look for those related to user "Mathias999us".
Click on the image for a larger version.
Figure 6, above, shows the surface-mount version that seems to suffer the same fate when the unit is run without a battery to moderate the voltage.  The above drawing is annotated with some clarifications added below:
  • TIP48 - This is the same power transistor (through-hole) as used in the older version.
  • "SMD marked 1Dt" - This is the same function as the MPSA42 used in the older version, except surface mount.  The case marking of "1Dt" (assuming that the writing was deciphered correctly) is a bit ambiguous, but it refers to an NPN transistor.  It is believed to be a BC846 transistor - a general-purposes NPN device that is rated for 65 volts.
    • It should be noted that the MPSA42 is available in the same SMD footprint as the above and is known as the "PMBTA42".  This may be a better choice than the BC846 in terms of being able to handle voltage spikes, should the Polaris be operated without a battery.
  • "Unidentified diode" - There are two of these:
    • The large one appears to be just a standard diode.  If open/shorted, the surface-mount version of the 1N4003 (through 1N4007) should be fine.
    • The small one is likely a surface-mount version of the Zener diode called out above - namely a 15 volt unit.
  • Capacitors:  These aren't marked in Figure 6, but they are the large, metal cans next to the TIP42.
    • These are often damaged by the heat and voltage when the speedometer is run without a battery.
    • If one/both of these capacitors look at all bulged or swollen, if it looks like their plastic labels have shrunk and split and/or if there is some sort of liquid coming out of them - either from the seam on the top or at the circuit board - they MUST be replaced!  
    • These are reportedly 470uF, 25 volt capacitors.
    • Before removing them note which end of the capacitors are marked with the minus sign so that the new ones may be installed correctly.


This page stolen from ka7oei.blogspot.com

Wednesday, October 16, 2013

Sleuthing the source of a SARSAT signal?

It was August 11, 2006 - a late Friday afternoon - and I received a telephone call from Gordon, K7HFV and with him on the line was John, WA7UUJ with an unusual request:  The FCC's Denver field office had called him to ask if he - or someone he knew - would be willing look for a signal that NOAA had reported as having had appeared on several recent SARSAT passes.  If they could recruit some Salt Lake area locals to find this signal, it would save them a trip.

For some reason, I got called.

About the SARSAT system:

The SARSAT system was originally intended to provide a means for locating, via satellite triangulation, a signal emanating from a vehicle in distress - specifically, a ship at sea or a downed aircraft using an ELT.  Originally operating on 121.5 MHz, other frequencies such as 243.0 and those in the 406 MHz area had also come into use.  Today, the 406 MHz area frequencies are used almost exclusively, with the spacebourne capability of the other two frequencies having been phased out in 2009.
Note:  The 121.5 MHz frequency continues to be used, but only for short-range earth-based searches as many emergency beacon transmitters emit a much weaker, continuous-duty signal on this frequency to aid search parties as they close in.
Asked if I was interested in looking for the source of this signal, I agreed and John passed on a frequency - 406.2 MHz, some coordinates, and the number of a contact at the FCC.  Calling that number and talking to the contact I quickly determined that the coordinates given were not, in fact in decimal degrees, but rather in degrees, minutes, and seconds - and these coordinates, 40 degrees, 48 minutes North by 111 degrees, 49 minutes West - reportedly had a "circle of probability" of 1.1 km in terms of accuracy, placing the source of the signal 1.9 miles (about 3km) northeast of the "U" on the mountain north of Salt Lake, about halfway between Twin Peaks and Black Mountain.

It was reported that there had been several "hits" on this signal going back several days and since this territory was somewhat remote and not likely to have any power nearby so it was presumed that whatever it was that was causing the problem was likely to be battery-powered and would not persist for much longer.

Being that this frequency (406.2 MHz) is well-below the 70cm amateur band, Yagi antennas cut for that band were not likely to be useful in their directional characteristics.  Fortunately, Glen, WA7X, had previously acquired a circularly-polarized GOES downlink antenna surplus for only a few dollars with the intent of modifying it for 70cm use - but had not yet done so.  Since this antenna was designed to work at 402 MHz, it was still usable at 406 MHz and an additional feature was that the elements of this antenna could easily be removed, making it practical to carry while hiking.

That evening, after work I went over to Gordon's house.  He reported being able to hear a signal on his scanner near the frequency of interest and had already driven around a bit to see if he could make sense out of what was being heard as well as to scout out the trail head for the most likely access.  A point of confusion was that even though this signal went away when he removed the antenna, he couldn't hear it on any other receiver - and neither could I - so it seemed likely that it may be emanating from the radio itself or could possibly be an image response of some kind.

Sunset over Ensign Peak, as viewed from the ridge just below Twin Peaks.
Click on image for a larger version.

We piled into Gordon's Jeep and drove up through some fairly fancy neighborhoods until we arrived at the trailhead to Ensign Peak.  At this point Gordon was wondering where his scanner had gone and a fairly thorough search of his Jeep (assuming that such was possible) did not yield it.  Fearing the worst, Gordon drove back home while there was still some sunlight to look for it while I proceeded up the trail, talking occasionally with Glen who also relayed my transmissions when Gordon as I didn't have a simplex path to him from my location due to geography.  Once he arrived home he searched some more for the scanner and carefully retraced some of his steps to see if he could find it - as well as keeping a careful eye along the road:  No luck.  As best as he could determine he'd set the scanner on the roof of the Jeep and taken off, probably leaving the scanner on the road somewhere...

By the time Gordon got back to the trailhead I'd arrived at a ridge to the west of Twin Peaks where I could see into City Creek Canyon.  Up there, for the first time, I noticed a fairly weak signal on 406.28 MHz that sounded either like a repeater blowing squelch noise or, more likely, a data transmission of some sort.  Getting out the Yagi, the signal bearing seemed to indicate that it was coming from the direction of Ensign Peak, so I decided to break out another receiver, a Kenwood TM-733A to get a second opinion and to help eliminate the possibility that it was an image or spur of some kind.  Confirming that my FT-530 HT was, in fact, really hearing a signal, I decided to do a lateral traverse of this ridge to double-check the bearing.  The destination was a minor peak about a half mile to the west and it was hoped that by taking several bearings along the way, I could determine if the bearing that I'd taken was, in fact, likely to be a "true" bearing and not merely a reflection.

While making the traverse I was in occasional contact with Gordon, who was well underway (sans scanner, unfortunately) and was keeping him apprised as to what I had found.  Upon arrival at the minor peak I checked the signal strength and bearing once again and it was, in fact, still coming from the direction of Ensign Peak and of about the same strength - a reading entirely contrary to what the satellite coordinates had seemed to indicate.  Armed with this information, I headed back to the saddle.

By the time I got to the saddle Gordon had been there for a few minutes and it was starting to get quite dark with the city lights having turned on a short time before.  By now I was wishing that I'd done a bit more research as to the passband of the SARSAT system and had a few specific other specific technical details as to how it worked:  For example, I was wondering if a signal at 406.28 MHz would be bothersome to a system that received at 406.20 MHz?  I also wished that I'd queried the contact at the FCC a bit more about any relevant details of this signal that he might have such as whether or not that it was modulated - and if so, what was the nature of this modulation?

The block diagram of a typical SARSAT transponder.  This is some of the information that I'd wished that I had with me on the first trip to look for the signal.  This diagram includes the older 121.5 and 243.0 MHz subsystems which are no longer used onboard the spacecraft.
Click on image for a larger version.

Being that this saddle was still a few kilometers west of the coordinates we decided to follow the roads and trails to the east towards Black Mountain while keeping an eye on the clouds, virga, and lightning that we were seeing to the south and west.  Following the trail we wound around to the back side of Twin Peaks and before too long, we split from the old jeep road and began to follow a foot/bicycle trail to the east, lighting our way via flashlight.  At some distance past Twin Peaks we started to break out from behind the peaks at about the same time that the wind, rain, and lighting started to pick up.  After a few lighting strikes that appeared to be much less than a mile away, we beat a hasty retreat to the only thing resembling shelter that we could find - a shallow ravine along the trail near some scrub oak with a few 10's of feet of ridge above us.  There, we hurriedly donned our ponchos and waited out the now-heavy rain, fierce wind and lightning.  Fortunately, this storm wasn't too long-lived and it moved off toward the east and after it had passed we stuffed our rain-soaked ponchos into our packs and, defying common sense, continued on our way in the direction of Black Mountain.

By this time, I was really starting to wonder about some of the technical details of the SARSAT system so I called Glen on the cell phone and requested that he see what he could find on the web.  Before long we found ourselves in a clear area on a ridge top a little more than a kilometer from the coordinates that I'd been given so we decided to see what we could hear.  At about this time Glen called back on the radio with the results of his quick research on the web.  He reported that the SARSAT satellite operated on a center frequency of 406.025 MHz and NOT 406.2 MHz.  He also reported that in this frequency range there operated "Personal Locator Beacons" (PLB's) that could be activated by someone in distress.  These PLBs operate with several watts of power and transmit a short data burst that contains not only a unique serial number with which a person, boat, or vehicle could be associated, but a GPS-derived location as well.

Armed with this new information we started to look around in the 406.025 MHz range as well and sure enough, we heard a few bursts of what sounded like data transmissions.  Another interesting signal source seemed to appear at about 406.18 MHz - a bit off the SARSAT's center frequency - but since we weren't sure of the bandwidth of the SARSAT system, we couldn't readily discount it, either.  This latter signal proved to be problematic, however:  Although it could be heard on both the TM-733A and the FT-530, it seemed to resist attempts at determining a bearing.  The real clincher came about when I wielded the step attenuator and noted that adding just 3dB of attenuation seemed to cause the signal to go completely away:  It seemed to us that this indicated that this signal was likely an intermod product, but we were still somewhat puzzled by the fact that it was a clean, unmodulated carrier and we were wondering what signal sources would be likely to cause that?

At this location we had quite a good view of the Salt Lake Valley and over toward the Oquirrhs.  Not too long after arriving we were looking across the valley and our eyes caught a tremendously bright flash of light on the ground near the north-center of the valley.  What was striking about this flash was that not only did it seem to last the better part of a second, but it clearly had some obvious color to it and after disappearing for a few seconds, it appeared again, lasting for about the same amount of time.  Not having any clue what it was, we made several guesses, including the possibility that it was a malfunctioning power transformer.

This was just some of the electrical activity witnessed occurring over the Salt Lake valley that evening.
Click on image for a larger version.

Another thing that we could see from this spot was another thunderstorm moving in from the West but since this one was quite a ways to the south and since the skies above us were relatively clear, we weren't too worried.  As we sat there, I took advantage of the viewpoint and snapped several dozen short time exposures, hoping that I would be able to catch a half-decent lightning strike on at least one of them:  The results speak for themselves - see the picture above.

After a few minutes of listening, looking and taking pictures we decided that because it was approaching midnight and that the storm seemed to be inclined to approach us after all, it was time to go.  At this point, when putting gear back into my pack, that I'd realized that I couldn't find my transit compass:  I'd tried to find it a little bit earlier when we were trying to take bearings on the signals that we had heard, but it now seemed not to be buried in my pack after all.  All that I could figure was that in the haste to don the poncho in the rain I must have either dropped it or inadvertently taken it off, so on the way back we stopped at the place where we'd taken cover from the rain (after several back-and forth attempts to try to positively identify the exact location) and tried to find the compass by flashlight - but to no avail:  It seemed that the radio gods weren't satisfied with just Gordon's scanner, but they wanted my compass as a tribute as well!

The trip back down to the vehicle was mostly uneventful:  The only pause was a brief stop to attempt to take more lightning pictures - this time of a storm that was starting to appear over mid-southern Oquirrhs.  At about the time we got the the vehicle, it was sprinkling very lightly and the wind started to pick up, but by the time we got back to Gordon's house the wind was gusting fairly strongly and the lightning was much more intense and and accompanied by a hard rain.  We decided that we'd timed our retreat about right!

The next day I toyed with the idea of going back up to listen for the signal and to look for the compass but my plans were changed by the appearance of a strong thunderstorm that dropped marble-sized hail at my house.  Apparently Gordon toyed briefly with the idea as well, but he, too, was dissuaded by mother nature.

The next day (Sunday) was very nice, but I didn't feel inclined to tromp around in the 90 degree weather and I figured that the batteries running whatever it was that was generated the nuisance signal probably weren't likely to last 5 days, anyway...

A topographical map showing the hiking route (in blue) along with the precise satellite fix and the "circle of probability".Click on image for a larger version.

More reports of signals:

On Monday (8/14) I received another email from the FCC that not only reported that the signal was still there, but it had satellite "hits" going all the way back to the 3rd of August - 11 days before.   An interesting piece of information was that this signal usually seemed to appear in the late afternoon or early evenings - a possible clue of its origin.  Also included in the email was a new position fix:  40 deg, 48' 00" North and 111 deg, 54' 36" West.  This new fix was about 5 miles west of the original fix, indicating that it had either moved, or that the quality of the earlier fix wasn't quite as good as was believed.  By this time we'd also done enough research to know that the proper frequency to listen on or near was, in fact, 406.050 MHz, plus or minus 40-50 kHz.

As it turns out, the modern SARSATs have two separate receive systems onboard for 406 MHz:  One of these simply receives and does a store-and-forward of the data packets from the PLB's.  The other part of the system is a passband-limited,  frequency conversion receiver that translates the spectrum centered on 406.050 MHz with a passband of about +- 40 kHz, converts it down to 170 kHz and then phase-modulates it (along with the converted baseband signals of the other frequencies) on a 1544.5 MHz carrier.  In this way the effects of the Doppler shift are almost exclusively confined to the receive system onboard the satellite itself owing to the fact that the satellite downlink was not a direct frequency translation.

Note:  Another mode of operation for the SARSAT is with the transponder centered on 406.025 MHz with a passband of +-10 kHz:  We do not know for certain what mode the SARSAT transponder(s) were in when the signal was received.

With this new information Gordon and I decided that we might, in fact, be able to find this thing.  To be sure, the new coordinates were much "friendlier" than the others as they put the signal source just off the east side of Victory Road - just above its junction with Beck Street - all in places where we were likely to able to drive.

When trying to locate a signal such as this it is helpful to have an all-mode receiver capable of operating in the frequency range.  While one would normally listen for a signal in FM, being able to listen for the same signal on a receiver with a BFO (e.g. an SSB-capable receiver) is extremely helpful for several reasons:  It is possible to detect far weaker signals using SSB than with FM alone, it is easier to determine the precise carrier frequency of the signal being sought when using an SSB receiver, and it is fairly easy to analyze the "signature" of the signal to determine something about its origin.  This last point can be particularly important as not only can one often identify one specific transmitter in the presence of others, but its observed frequency stability may provide a clue as to whether the signal being heard is one that is stable and on-frequency or more likely to be a free-running spurious emission of a malfunctioning device.

The blue cross at the center of this map shows the location of the "new" SARSAT fix obtained on 8/14.
The ACTUAL location of the transmitter turned out to be near the lower-left corner of this
map, at approximately the location of the first letter of the word "LINE".
Click on image for a larger version.

While the most likely candidate for all-mode use was my Yaesu FT-817, it does not cover below 420 MHz.

What to do?

An answer occurred to me:  Throw together a quick-and-dirty frequency converter.

Fortunately, John, K7JL, had onhand several UHF bandpass cavities and he was able to tune one of these well out of its original frequency range all the way down to 406 MHz so that it could function as a bandpass filter.  The next step was to provide a local oscillator reference and this was provided by my Schlumberger 4031 service monitor - a device that can produce an SSB-quality carrier and run from 12 volts.  As it turns out, I had some diode ring mixers handy, but they were rated only to 250 MHz.  A quick test revealed that they did, in fact, function reasonably well at 406 MHz, albeit with a few extra dB of insertion loss.  The final component needed was a good preamplifier, but I was able to quickly retune a hombrew 70cm GaAsFET that I had onhand to operate at 406 MHz.  Cabling everything together I set the service monitor to generate a 280 MHz signal for the local oscillator, tuned the FT-817 to 126 MHz and, using another service monitor, I checked the sensitivity of this lashup and found that, on FM its 12dB SINAD sensitivity was about -115 dBm - more than good enough for our purposes!

After heaving all of this equipment into the car (along with a pair of 100 amp-hour batteries to run the service monitor, which draws 8-10 amps) I set off and picked up Gordon from his house.  From there we drove through downtown Salt Lake and past the state Capitol building and as we climbed the hill we could hear some activity in the 406.025 MHz area that sounded like data transmissions.  We had heard what we thought had been similar-sounding signals on the previous Friday when we were overlooking the valley and had presumed that they might, in fact, be some Personal Locator Beacons transmitting.  The fact that there was an outdoor exhibitors' show in town seemed to make it even more likely to us that one or more of these transmitters was active.

Driving past the Capitol building the signal that we'd been hearing wavered a bit, but became fairly consistent as we went down the Victory road, and it sounded very much like what we'd heard on Friday.  Near the bottom of the hill - just above the junction with Beck Street - I pulled off the road into a gravel parking lot.  A quick check revealed that we were just a few 10's of meters south of the precise coordinates relayed to us based on the most recent satellite data.  Getting out the Yagi, we tuned around and listened.

After a few minutes of thinking about what we were hearing, it occurred to us that we were hearing only one transmission at this location.  While it sounded just like data bursts, we also noted that these transmissions were not of consistent length and it sounded as though they were being truncated randomly.  Switching to SSB, we also noted that the signal that we were hearing was fairly unstable, having several kHz of observable "chirp" when it keyed up.  From this evidence, we concluded that this was most likely our suspect:  We surmised that it could still be a PLB, but one that was malfunctioning but this seemed to be unlikely as such a device would likely be operating from a battery that should have long been exhausted.  The other obvious possibility was that it could be an existing, malfunctioning transmitter throwing out spurious transmissions.

Wielding the Yagi we determined that its bearing was west-southwest from our current position so we set out to find another vantage point from which we could hear this signal.  Wanting to guard against the likely possibility of being misled by a reflection Gordon suggested somewhere above the capitol so we drove in that direction to find a good overlook.  Driving behind the capitol we happened to drop behind the ridge, effectively shielding us from anything to the west and we noticed that the signal that we were listening for disappeared, indicating that it was not likely to be either to the south or east of us.  After a few more minutes of driving we found a nice observation point at the end of the road, just below Ensign Peak, and parked.

Judging by the sizable crowd up there we concluded that this was a popular spot as it commanded a spectacular view to the south and west and once we started waving the Yagi around we not only got some stares, but also an indication that the signal was quite strong, yielding a bearing that was almost due west.  As we were getting ready to depart several people asked what we were doing and (nearly correctly) had surmised that we were tracking animals.  "Close!" we told them as we explained what it was that we were really doing.  Interestingly, we again noticed a very bright flash of light on the ground in a midvalley location - just like the one that we'd seen on Friday:  We still don't know what it was!

Armed with the new bearing we wound our way down from the foothills and headed west on 6th North.  As we dropped, the signal became quite weak for a while and it wasn't until we got to about 20th West that it started to pick up again.  Soon, we turned south, finding ourselves passing the Executive Airport terminal and the local FAA offices and we parked for a moment to find another bearing:  From here, the signal appeared to be coming from the northwest - but it wasn't particularly strong.

Needing another clear location, we decided to head toward the International Center on the west side of the International Airport in the hopes that we'd be able to get a counterbearing.  En route, we lost the signal, but we assumed that the path to the source had been obstructed but once we got to the International Center we still could not hear it.  What we could hear, however, seemed to a very occasional data transmission on the frequency (perhaps once per minute) that sounded different from the one that we were seeking.  A quick listen on SSB revealed that, unlike our suspect, its carrier frequency was quite stable.

At this point we decided to go back to where he'd heard the signal before - in the general area of the Executive terminal.  Returning and parking in an empty lot we listened for a few minutes and didn't hear anything other than the "occasional" transmission that we'd noted previously.  I then surmised that being a spur, it may have drifted off-frequency a bit and eventually found it about 30 kHz higher than it had been when we'd first heard it earlier in the evening.  I then decided that since it was likely a spur, it would seem reasonable that the frequency on which the malfunctioning transmitter operated was likely to be nearby, so I decided to start tuning around on the TM-733 in my Jeep while monitoring and tracking the unstable suspected signal on the FT-817.

After a few minutes of tuning I found a signal on 410.075 that sounded very much like our suspect and after comparing its appearance and modulation with that of the spur, they appeared to match exactly.  For this signal the beam heading was due south from our location, but there was a problem:  Because that the antenna was designed for operation on 402 MHz, we weren't sure if it would function properly and yield a usable bearing that far off frequency.  Fortunately, we had a nearby-frequency signal with which we could calibrate our antenna:  A backhaul feed of the main National Weather Service transmitter on 410.575 MHz.  The problem was that we didn't know where this transmitter was located, so I called John, K7JL.  He wasn't sure of its exactly location, either, so he looked it up in the telephone book and we determined that it was just a few blocks south of us - a location that correlated with our bearing.  At this point it also occurred to us that the bearing of our suspect signal was about the same as the NWS signal.

Getting back in the car we drove a few blocks south and stopped in a parking lot adjacent to the NWS office and observed that the 410.075 MHz signal was extremely strong with a bearing pointing into a side yard that, in the dark, appeared to have a number of antenna-like structures.  To be certain, we drove around to the south side of the building and again, we resolved a bearing in their side yard.  At this latter location, I looked at this signal - and its spur - on the service monitor's spectrum analyzer and determined that the 406 MHz component was 38 dB below that of the 410.075 MHz one:  If the transmitter was running at just a couple of watts, this would imply that the power in the spur was a few milliwatts - more than enough to be heard around town and even by an orbiting spacecraft!

John, who was still on the phone, happened to know  the technician at the NWS responsible for their transmitters and left a message indicating that there may be a problem requiring his attention.  At this point we were satisfied that we'd uncovered the source of the signal and went home.  After all of this we were somewhat surprised that our snooping around government buildings, after dark, wielding odd-looking gear hadn't seemed to have attracted any attention!

The next morning the technician from the NWS returned the call and John explained what had been happening and what had been found:  The tech immediately arranged to have the transmitters looked at and, as a future preventative measure, installed bandpass cavities.  As it turned out, this transmitter was part of  the ASOS - the Automated Surface Observing System - which is used for gathering close-in meteorological data at airports.  Interestingly, the FCC's NOAA contact pointed out that, several years ago, they had a spate of similar interference issues at airports caused by these same-model transmitters in which a weak 400 MHz-area spur had caused some interference with some operations.  He also then mentioned that this was the first time that a "non-airport" transmitter had caused some problems, but this NOAA facility, which was adjacent to the airport, had escaped prior scrutiny since its transmitter wasn't actually on airport property like the other errant transmitters.

The irony did not escape us that this signal was generated from an NOAA facility and that it was, in fact, NOAA that had reported this signal to the FCC in the first place!

Interestingly, the latter satellite fix (the one indicating a site near Beck Street) was about 5km northeast of the actual location.  This error is understandable when one considers the nature of the signal:  It was likely fairly weak on the satellite.  It also was quite unstable in frequency, a factor that would tend to increase the uncertainty when trying to make a Doppler fix on the location.  Furthermore, being that the satellites used to determine these sorts of location fixes are in very high inclination (being nearly in polar orbit) they would have their best accuracy when determining latitude:  The accurate determination of longitude is a bit more problematic as its uncertainty relies on observations over multiple passes and/or through different satellites, hence the fairly close agreement on latitude, but the much larger longitude error.  (Note:  This system is "reverse" of the way the old TRANSIT (link) satellite system operated.)

The observation that this signal seemed to be only present in evening hours also made sense:  Only when the outside temperature was in the mid 90's Fahrenheit did it seem to drift into the SARSAT passband.  This temperature peak would, in fact, occur in the late afternoon and as it cooled off, it would drift back up in frequency, out of the transponder's passband - possibly disappearing altogether from the transmitter's spectra.

A few days after the transmitter's servicing another email arrived reporting that since the work had been done, no further SARSAT hits had been noted.  An email from the FCC indicated that they were pleased that they did not have to make a special trip to Salt Lake just to find this signal and offered to take us to the Red Iguana next time they were in town.


P.S.  The FCC folks still haven't taken us to the Red Iguana!


This page stolen from ka7oei.blogspot.com