Repairing a ("smoked") MFJ-998 1.5kW automatic antenna tuner

Figure 1:
The (repaired) MFJ-998 front, now working, sitting
atop my Heathkit SA-2060 manual tuner and underneath
my homebrew neon bar-graph VSWR/Power meter.
Click on the image for a larger version.

To inspect or not inspect 

When I buy some types of ham gear second-hand, I'll treat it like as I would if I were to buy used Heathkit gear that had been put together by someone with "average" (or unknown) kit building experience:  I take the cover off, tighten the screws, reflow suspicious solder joints and do a visual inspection.  Regardless of brand, it's probably a good idea to pop the covers and take a look at gear that you buy second hand before using it.

For some reason - when I recently bought a second-hand MFJ-998 1.5kW automatic antenna tuner on EvilBay - I didn't do that.

When this '998 arrived I did a cursory look at the case, connected it, and tried it out at 100 watts - and it seemed to work OK - but over the course of a few weeks being used at just 100 watts I noticed something odd:  It would occasionally start re-tuning during a transmission for no obvious reason and, perhaps, there was a faint whiff of "something" in the air - but I never connected the two and didn't investigate.  I'd previously operated using my old Heathkit SA-2060 (non-"A" version) antenna tuner for several years on this same antenna and hadn't noticed a randomly changing VSWR that might explain this tendency for the '998 to "hunt".

Figure 2:
A of relay K2 from the top:  Evidence of
damaged glass-epoxy PC board material
is very clearly evident!
Click on the image for a larger version.

I checked for the usual suspects external to the tuner:  Loose coaxial or wire connections, a branch touching the antenna (an 80 meter horizontal loop) somewhere - but there was nothing obvious.  This "problem" wasn't consistent, either - not happening frequently enough to cause me to expend more effort to track it down.

Magic smoke escapes!

Several weeks ago I had occasion to run a bit more power and threw about 800 watts through it and things seemed OK for a while, but then it started retuning itself again - this time, accompanied by a very distinct burning smell.

I immediately pulled the tuner out of the circuit, going back to the Heathkit SA-2060 manual tuner, and things were fine once again, further indicating that the "instability" was related to the auto-tuner and not the antenna itself.  Later, when I had time to do so, I pulled the cover off the MFJ-998 and immediately saw the problem:  As can be seen in Figures 2 and 3, the PC board was carbonized in the vicinity of relay K2 which is used to select antenna #1 or antenna #2.

Figure 3:
The damage to the PC board as seen from the
bottom side.  Sections of the board have become
carbonized, offering current paths to RF, causing
the degradation to accelerate.
Click on the image for a larger version.

Upon seeing this, I ordered some replacement relays - ten of them, as they weren't particularly expensive - and tuner sat around for several weeks until they arrived.  I did briefly consider just omitting this relay, "hardwiring" it for just one of the antenna outputs, but decided to proceed with the repair:  If this happens again I'll reconsider doing this - or perhaps changing the way the relay(s) are configured.

What happened?

Clearly, the PC board material had "carbon tracked" at some point:  A bit of leakage between the traces had obviously occurred and with the higher RF voltage resulting from my running higher power, this "slight" leakage had gotten very much worse, heating the board material, decomposing the epoxy base of the PC board material and causing it to become conductive - and the it gets worse and worse from there.

I have the suspicion that a carbon track was present before I owned this tuner and it likely occurred due to attempting to tune an antenna that wasn't connected (possibly resulting in very high voltage), an intermittent antenna fault, or perhaps even lightning.  At the time it failed,  I'd been using antenna #1 and had nothing connected to the connector or post of antenna #2 so I'm not sure why it so readily burned across the traces between the antenna connections - but it did.

Figure 4:
Damage to the original relay.  I don't think that
the relay itself initially failed, but rather that it
was damaged by the intense heat of the glowing,
carbonized board material.
Click on the image for a larger version.

Analysis of the damage

With the "carbon tracking" between the tuner and the antenna "#1" and "#2" connections, the circuit board was nearly burned-through in a few places and the relay was destroyed - but it looked as though the damage of the relay was caused by the heat from the (burning!) PC board.  Despite looking really bad, the damage was very localized - and it provided an opportunity for improvement.

The fact that the damage was worse on the bottom of the board than the top also indicated that it was likely on the bottom side that the issue first started.

What to do?

In many cases - when high voltages are present across a section of PC board - manufacturers will place an "anti-tracking slot" between the two points:  Rather than rely on the surface of the PC board to withstand high voltages - the ability decreasing if moisture, dust or other contaminants are present - a physical slot is cut in the PC board material between those connections, greatly increasing the path length and high voltage stand-off ability.

Figure 5:
Using a rotary tool to remove ALL carbonized PC board.
All potentially-conductive board material must be removed
or else the same thing will happen again!
Click on the image for a larger version.

This "slotting" technique is frequently found on mains-powered devices that have human contact - such as phone chargers and other power supplies.  This is done where excess leakage between the high voltage from the wall plug and the low voltage output could result in injury or death if someone touched an accessible metal contact.  

While this application isn't a "life-safety" issue like a power supply, it would have made sense, given the high voltages that are possible, to implement such measures here.  For "reasons", MFJ did not choose to manufacture the PC board with such "anti-tracking" slots in this particular location - on that is very likely to have the highest voltages present across adjacent contacts to be found in the tuner.

Using a small routing bit in a rotary tool I ground away ALL of the carbonized (conductive!) material:  NOT doing so would have risked additional "tracking" in the future where it could have found a new conduction path.  The result of this work can be seen in the photos - a bit of "Swiss-cheese" of the circuit board - but now, there was only air between the contacts across which there was likely to be high voltage.

Installing the new relay

With all of the carbonized material removed, I vacuumed up the debris and cleaned the area around the relay on both the top and bottom side with alcohol, removing deposits from the soot of the burning PC board material.  I repeated this process after the new relay was soldered in.

Figure 6:
After cleaning the board with alcohol, the new.
relay was installed and the connections made
using 16AWG wire.  The "air gap" between
pins should make it more resistant in the future.
Click on the image for a larger version.

On this relay, the "common" (armature) pins are in the center with the normally-open and normally-closed pins on either side - but since I removed most of the PC board material around these pins, under the relay the "common" connection was completely missing.  This problem was easily solved using a piece of tinned wire as can be seen in Figure 6:  The air gap between the other relay contacts was maintained.

I was fortunate that the PC board connections for the coil (the two solder pads near the top of Figure 6) and both the "Normally Open" and "Normally Closed" terminals were still intact (e.g. the board wasn't burned in those areas) and this provided a solid mounting for the relay.   It was only the "common" relay contacts - those that connected back to the tuner itself - that were no longer extant so I folded a piece of 16AWG copper wire and made the connection back to the remaining PC board trace as seen in Figure 6.  (Note:  This relay, K2, is a DPDT relay of the same type as all of the other relays and both of its sections are connected in parallel.)

Testing and comments

It worked!

As there was no damage to any other circuitry, the relay properly selected between antennas 1 and 2 as designed and the memory pre-sets (on the bands other than where I was operating when the failure occurred) were just as they were indicating that the matching conditions were identical to before.

In doing research on the relays used in this tuner (all of the relays are identical - for my tuner, they are Hui Ke HK14FH-DC12V-SH - which is the same as the American Zettler AZ576-1C-12DE, the Songle SMIH-12VDC-SL-C and many others ) I noted a few things about their specifications.  As expected, they have 12 volt, non-latching coils (e.g. power must be applied for it to hold the tuning configuration) and their contacts are rated for 16 amps (resistive) for DC and mains-frequency AC, and they also have good isolation between the contacts and the coil (rated for 5kVAC at mains frequency).

What did concern me a bit was the fact that they have only a 1kVAC rating between "open" contacts - and a quick check with a "Hi-Pot" tester verified that it did, in fact, break down (arc internally between the armature and the normally-open contacts) at about 1.7KVDC.  It would seem that the designers of this tuner considered this aspect of the relays' limitations to an extent as all of the capacitors in the tuner are switched in/out using two relays in series to accommodate higher voltages - but this technique was not applied to K2, the antenna switching relay.

A quick check of the ratings of this tuner indicate that it is rated for up to 1600 Ohms at full power (1500 watts).  Knowing the power and impedance - and presuming a resistive (e.g. resonant - neither capacitor of inductive) load - we can use the following formula:

V = √(P x R) 

Where:

V = Voltage

P = Power in Watts

R = Resistance in Ohms

and based on this, at 1500 watts and 1600 Ohms, the voltage in a purely resistive load would be about 1550 Volts RMS (or about 2200 peak volts) - well above the known breakdown voltage of the contacts of the relay.  It's worth noting that it's often the case that at radio frequencies, insulation and breakdown ratings are lower than they might be at mains frequencies and DC - something else to consider!

What this implies is that under such conditions there would be enough voltage for the armature of the relay to arc to those of the unused antenna and, perhaps, the designers of this tuner should have put two relays in series for antenna switching, too:  If the "new" relay breaks down with voltage, it wouldn't be too difficult to wire a second one in series to increase the stand-off voltage, driven by the same coil driver.

What about the other extreme, where the impedance is low?  The tuner is rated for as low as 12 Ohms where the voltage would be lower and current higher and here we use a different formula:

I = √(P / R) 

Where:

V = Voltage

P = Power in Watts

R = Resistance in Ohms

 

In this case 1500 watts and 12 Ohms (resistive) yields a current of about 11.2 amps RMS - somewhat lower than the relay's contact rating of 16 amps, plus there's the fact that both sets of contacts of this relay are in parallel which further increases their durability.  At such high currents, the concerns aren't necessarily with the relays, then, but rather potential I*R heating of the inductors and capacitors whenever an extreme match condition is encountered.

 

In short:  If you are running anywhere near 1500 watts and have a high impedance being presented to the tuner by your antenna (e.g. using something like directly feeding a half-wave end-fed antenna with no other matching device) and using both antenna ports,  you should probably reconsider your arrangement!

 

* * * * *

 

This page stolen from ka7oei.blogspot.com

 

[END]

 

Ali Express "SDR TX/RX Switch" - A design NOT well thought out...

Figure 1: 
Front panel of the SDR TX/RX antenna switch showing the
the 3.5mm audio connectors and the red/green RX/TX LED -
which have been swapped to be in their proper location.
Click on the image for a larger version.

        IMPORTANT:        

If you have one of these devices, DO NOT connect it to your transceiver and second receiver (SDR) UNTIL you have read and understood the issues described here. 

Failure to understand how this device works may result in you blowing up your SDR when you transmit! 

If you wish to use it ONLY as an audio muting switch when you transmit (e.g. when using a remote receiver) see the section labeled "Audio Muting" near the bottom of the page.

I've you've been following this blog you'll note that I've used SDRs (Software Defined Radios) quite a bit - particularly for reception.  Transmitting in the vicinity of any receiver - or trying to use an outboard receiver in conjunction with a transmitter on the same antenna - is a bit problematic for several reasons:

• If the transmitter and receiver are in close proximity and on very nearby frequencies (e.g. on the same band) then it is (nearly) inevitable that the receiver WILL be overloaded when the transmitter is active.
  
• Unless the frequencies (transmit and receive) are very well separated AND both the receiver and transmitter have adequate filtering, the receiver will be overloaded by the transmitter.
• It is possible that even if you have a separate receive antenna, it may intercept enough energy from the transmitter to damage/destroy the receiver.  If the two are on the same band, this is more likely - but even if the receiver is being operated on a very different frequency range than the nearby transmitter and there is insufficient filtering at the receiver the receiver could sustain damage.
• It is often the case that one might have a single antenna on which there are two receivers (e.g. the receiver built into the transceiver and an outboard SDR receiver).  In this case one clearly must protect (e.g. disconnect) the outboard receiver when transmitting.

It's worth noting that most SDR receivers do NOT have particularly strong filtering in them:  Unlike an amateur transceiver - which may have separate filtering for each amateur band (or groups of bands) - this is rarely the case for wide frequency-range software-defined radios:  RTL-SDRs, SDRPlay, Funcube and others have either no band-specific filtering or rather broad (e.g. covering about an octave or even wider) filtering in them.

What this unit does

Some modern radios actually have external receive ports on them to allow you to "share" the RF while protecting the external receiver.  If your radio doesn't have that, there are/were several devices to allow this that may be found on the market (e.g. the MFJ-1708) but my attention was brought to an inexpensive unit (pictured above) that has appeared on the various seller web sites (Amazon, EvilBay, Ali Express, etc.) so I obtained one via a U.S. seller.

The description of this device is typical of those found on at the stores of Chinese sellers, curiously being both under and over-descriptive at the same time:  "160MHz 100W Portable SDR Transceivers Aluminum Alloy Box Device Radio Switch Antenna Sharer Practical Signal Equipment Accessory"

By the description, with this device it should be possible to connect your transceiver and SDR (receiver) to the same antenna, perhaps receiving using both (e.g. the addition of a waterfall to an older radio) without fear of damaging the SDR or the transceiver.

This device also has another feature:  To re-route audio when transmitting - which is probably the most usable feature of this device as it comes out of the box as we'll see.

As we'll see, this device doesn't quite work as you might think that it should.

"Documentation?  What documentation?!" 

This unit arrived in a package with (surprise!) no documentation at all - which was somewhat disappointing:  Sometimes one gets a (badly!) translated half-sheet of paper that hurts one's brain to parse - or even a URL to a page with... something... but not the case here.

From a practical standpoint, it's somewhat "self documenting" in the sense that if you ordered this device in the first place, you already had an idea as to what it was supposed to do, so it's possible to figure things out.  Referring to Figures 1 and 2 (the front and back panels) we have:

Front panel (Figure 1):

• LED on the left-hand side.  This LED is illuminated when the unit is in "Receive" mode - that is, the "SDR" rear-panel RF connector is connected to the "ANT" rear-panel connector.  (The PC board shows this as a green LED, but on mine the red and green were interchanged during assembly:  I swapped them back.)
• 3.5mm jack labeled "SDR".  This is a stereo (2-channel) audio jack and, during receive, both channels are connected to the "Audio Out" connector.  It is disconnected during transmit.
  
• 3.5mm jack labeled "AUDIO OUT".  This is a stereo (2-channel) audio jack that is intended to be connected to speakers.
• 3.5mm jack labeled "TRX".  This is a stereo (2-channel) audio jack that is intended to be connected to the transceiver during transmit.  Is is disconnected during receive.
• LED on the right-hand side.  This LED is illuminated when the unit is in "Transmit" mode - that is, the "TRX" RF rear-panel connector is connected to the "ANT" rear-panel connector.  (As noted, this should have been a red LED according to the marking on the PC board but mine was populated with a green LED, which I swapped.)
  

Figure 2:
Back panel of the SDR antenna switch.  SO-239 connectors
are for the radio (transceiver) and antenna with the SMA for
the SDR.  The 3.5mm PTT and power connectors are visible.
Click on the image for a larger version.

Rear panel (Figure 2):

• RADIO connector.  This is an SO-239 (female) UHF connector to which the transmitter/transceiver is to be connected.
• ANTENNA connector.  This is an SO-239 (female) UHF connector to which the antenna is to be connected.
•  PTT ("Push To Talk") connector.  This is a 3.5mm connector in which the center pin (tip), when grounded, will switch the unit from"Receive" to "Transmit" mode.  It's likely that a 3 or 4 wire 3.5mm cable was provided in which case only the tip (PTT) and sleeve (outer-most ring - ground) are needed.
  
• SDR connector.  This is an SMA connector to which the SDR (or other auxiliary receiver) is to be connected.
• 13.8 VDC connector.  This is a 2.1x5.5mm coaxial power connector (center positive) through which DC power is supplied.  This voltage is not critical and could be anywhere from 11.5 through 15 volts.

Also in the box my unit came with an SMA-SMA jumper, SMA-BNC adapter to adapt the SDR connector to BNC, three "audio" cables with 3-conductor 3.5mm connectors on each end, and a 12 volt switching supply (with a European "pin" plug) and a universal plug adapter:  The 12 volt switching supply seems to be the cheapest, meanest possible unit with no brand name and should NOT be trusted or used - but at least its DC cord is useful!  (In other words, do not use this power supply - particularly as it is unfiltered from an RF standpoint and it would be a really bad idea to use it on an RF receive device of any type.) 

How it actually works

As you would expect, the antenna is to be connected to the "ANTENNA" port.  When in receive mode, the "SDR" connector is also connected to the "ANTENNA" port - but the "RADIO" port is not!

What this means is that as shipped from the factory, if you connect your transceiver, antenna and SDR to the unit, when it's in receive mode, you will get no receive signals on your transceiver.  This is by design, apparently.

It is expected that the PTT connection on the back should be grounded when the transceiver is in transmit mode - and when this happens, the RADIO and ANTENNA ports will be connected to each other.  There is also an RF sensing circuit that is supposed to detect when the transmitter is producing RF, but this has its own issues as will be discussed later.

There is a jumper...

If you take the unit apart (via the four screws on the back panel) you'll see a jumper (J5 - see the schematic of Figure 3 and the photo of the board in Figure 4) and some awkwardly-worded text indicating that if you remove the jumper that you'll have "dual receive" - which means that the Radio's receiver and the SDR will be connected to the antenna at the same time.

This is technically true - but there are a number of "gotchas" here

First, let's take a look at a reverse-engineered schematic of the unit, below:

Figure 3:
Reverse-engineered schematic diagram of the unit.
The parts designators are those shown on the silkscreen of the circuit board.
Click on the image for a larger version.

Circuit description:

DC power

The DC input power via J4 is protected with F1, a 500mA self-resetting fuse and D10, a diode for reverse polarity protection while L2, a 220uH inductor, isolates the connection at RF.  Capacitors C6 bypasses RF while C7, a 220uF electrolytic, provides smoothing/filtering - likely enough to even allow an AC power source (from a 12-ish volt transformer) to be used. 

It's worth noting that there are two "grounds" on this device:  The "antenna ground" of the rear panel and RF connectors and the "shack ground" of the DC power and audio which are isolated by L1, a 220uH inductor.  On paper, this isn't a bad idea - but on this unit there's a flaw:

The back panel being used to mount the connectors and it - and the entire case - is at "RF" ground - which would be fine as that would be the same "ground" as your radio.  The problem is that the circuit board's ground planes are not set back from the edges of the board meaning that it's possible that the green insulating coating could scrape off the board and contact the case it the mounting slot, connecting the two "grounds" together - perhaps intermittently.  (Practically speaking, most people would not be likely to ever have a problem.)

Oops.

Keying

J6 is the PTT input, activated by grounding the tip of the 3.5mm connection with the outermost sleeve being the "shack" (not antenna) ground.  Diode D6 blocks positive voltage and when the PTT is keyed, the gate of Q3, an N-channel MOSFET, goes low, de-energizing all of the relays.  When the PTT line is "un-grounded" capacitor C3 and R9 charge, preventing Q3 from re-activating the relays instantly, providing about 100msec or so of delay through the charging by R3.

Comments about the keying via the PTT port

This relay keying scheme assumes that there is either NO voltage, or that there is a POSITIVE voltage on the keying line from the radio when in receive that is greater than a few volts.  There are several caveats to this:

When powered up, the relays are energized whenever it's in "Receive" mode (PTT ungrounded or no transmit RF)  What this means is that if power is removed, it's as if it's in "transmit" mode.

• This connects the ANT to the RADIO port and the SDR port is grounded when in TX mode or powered down.
  
• Additionally, the front-panel AUDIO jack gets connected to the TRX jack. when in TX mode or powered down.
  

The keying line from the radio must go to GROUND when keyed.  If the keying line is shared with an amplifier, that amplifier CANNOT put a negative voltage on the keying line as that will hold the SDR switch in "Transmit" mode at best, damage the SDR switch in worst case - and in either case it would hold the amplifier in a "keyed" state.

If there IS a positive voltage on the keying line when "unkeyed" it must be at least 5 volts just to assure that Q3 will turn on reliably when the radio is un-keyed - and this lower voltage may affect the duration of the "un-key" delay.  If the voltage is less than 10 volts, the full delay caused by C3 and R3 may not occur.  The 100msec or so of "un-key"delay afforded by R3 and C3 is insufficient to prevent the relays from "chattering" during SSB and CW transmissions if RF sensing is used!

• If you are a CW operator, the unit will not switch back to receive mode as quickly as your radio might.  If you are a CW operator that prefers QSK (full break-in) you probably don't want to have this unit inline.
  

There is also an RF-sensing keying circuit:  Transmit RF is tapped from the RADIO RF line (from the transmitter) by C1, a 47pF capacitor and rectified by D1 and D2 and used to turn on Q1 - grounding it in the same way that grounding the PTT line does - which in turn keys the transmitter.  There is a fatal flaw in the design of this device exacerbated by RF sensing which I will discuss shortly.

Audio switching

Figure 4:
The circuit board, showing J5 in the center.
The power supply filtering is in the upper-right with the audio
relay (K3) on the left.
Click on the image for a larger version.

Relay K3 may be used to switch audio based on keying.  Let's assume that you are using a separate SDR with a computer to receive audio:  By connecting the computer speakers to the "Audio Out" connector and the computer audio output to the front-panel 3.5mm "SDR" jack the SDR audio will be muted when transmitting at which any audio from the radio connected to the front-panel 3.5mm "TRX" jack will be passed through to the speakers.

Practically speaking, you would probably never use the "TRX" jack to mute your radio's audio, but a more likely scenario is that if you are using an online WebSDR (see the WebSDR.org web site for a list) to listen on the air, you could use this to mute your speakers when you transmit to prevent your own transmitted audio from coming back with a delay and causing an echo.

RF Switching

This is where it gets a bit scary.  First, consider the configuration - from the factory - with jumper "J5" in place.

Remembering that when powered up, the relays are energized, you can see that when in "Receive" mode, the ANT port is connected directly to the SDR port - but you'll also note that the RADIO port is not connected to anything (e.g. floating).  When you transmit, the relays de-energize, connecting the RADIO port to the antenna and grounding the SDR port.  This means two things:

• There is no receive RF at the transceiver.  Most people that I know don't use their transceiver's receiver instead of the SDR, but use them at the same time - perhaps turning down the volume on the one not being used or, more likely, using their transceiver for audio and the SDR to display a waterfall.  Not having antenna RF to be able to receive anything on the transceiver is likely not what you really want to do.
  

• When using the RF sensing, the transmitter is connected to an open circuit before the relay switches.  This has several implications:
• If you don't have the PTT line wired to your radio, there will be a split second when RF first appears that the radio will see an infinite VSWR before the relays de-energize and the contacts close, connecting the radio to the antenna.  This repeated burst of infinite SWR can progressively damage a transmitter's finals, despite the SWR protection circuitry within the radio.
• When relay K1 does de-energize and connect to the antenna, it will be "hot switching" the relay contacts, which is to say that they will be carrying RF power at the instant that the antenna is connected.  This tends to burn contacts and shorten the life of the relay.
• The RF sensing circuit doesn't have adequate "hang time" to ride through word pauses and CW elements meaning that it will likely "chatter", repeatedly causing the hazards noted above.

As can be seen from the picture of the circuit board in Figure 4 there's a jumper, J5, on the board with the following somewhat confusing text:

J5 Usage
Open = Dual receive wheh [sic] RX ("wheh" was probably intended to be "when")
Short = Normal Operation

By removing J5, relay K1 is never energized meaning that it is always connected to the antenna:  This helps to mitigate the problem that - when RF sensing is used - that the transmitter is connected to "nothing" as it would be the case with J5 installed - but this also means that in receive mode, the transceiver and the SDR are connected in parallel. 

Simply paralleling two (nominally) 50 ohm devices (the transceiver operating in receive mode and the SDR) isn't a great idea - but it will generally work "OK", particularly if the SDR and the transceiver are tuned to the same frequency range.  When the transceiver is OFF - or on a band  other than that to which the SDR is tuned - it may cause its filters to "suck out" RF and a loss of signal/sensitivity on the SDR.   

(Note:  A "properly-designed" device that shared the antenna for receive would likely include a built-in 2-way splitter which can reduce such problems, and it will also contain circuitry to protect the second receiver should excessive RF manage to find its way into it.)

The bad part here is that if you transmit - and, for some reason relay K2 doesn't de-energize instantly, as would be the case with RF sensing only - you will transmit directly into your SDR, likely destroying its front end.

Oops, again.

What this means is:

• If you remove J5, DO NOT operate the unit UNLESS you are using the PTT cable - which is to say DO NOT rely on RF sensing alone as transmit power will briefly enter the SDR's front end before the relay can switch.  The SDR is likely to be damaged due to the lack of RF power protection on that port.
  

• If your PTT cable accidentally becomes disconnected - or external keying is turned off in your radio's menu - you will transmit into the SDR and destroy its front end due to the inability of the RF sensing to act instantly and due to the lack of protection to the SDR.

The reason for this as as mentioned above:  Not only are the transceiver and SDR connected together without any protection circuitry, but also the RF sense needs to detect transmit power before it will activate - and by the time that it does, a brief burst of full transmit power may have found its way into your SDR.

A hardware bug

There is also a more subtle bug that I uncovered.  While testing the unit on the bench, I disconnected J5 - but was confused when the ANT and RADIO ports were not connected.  What was happening was that when I removed J5 - while the unit was powered up and in receive mode - enough current was flowing through LED D8 and resistor R4 to hold relay K1 closed.

Simply removing the power temporarily caused K1 to release - and there wasn't enough current to close it again, but if you are messing with the configuration, this "bug" could bite you, too!  I suppose that it's also possible that jarring the unit could cause the armature of K1 to hold in place - but I didn't try this.

The "fix" for this - if you want to bother with it - is to change resistor R4 to a 10k resistor:  This also tones down the TX LED's brightness a bit, too.

Overall comments

I get the sense that whoever designed this thing may have been copying the general idea from other, similar devices - but not really understanding what was being done, and why.  For example, the separate "grounds" implies an understanding that having them separated would be a good idea - but whoever laid out the circuit board made the "rookie mistake" of making it possible for the two "grounds" to be connected, anyway if the green coating on the board and the black anodization of the case wear through.

The description of this unit implies that it's useful up to 160 MHz.  I suspect that this is probably "true-ish", but the VSWR starts to climb when one gets to and above 6 meters (50 MHz) meaning that it's likely most useful at low power at these higher frequencies.

The cardinal - and unforgivable - sin has to do with the fact that if you want to use both your transceiver's receiver and your SDR simultaneously, you WILL want to remove J5 - but if you do - and you don't absolutely have the PTT connection working properly, you WILL blow up your SDR!

Fixing this problem is possible with the addition of some simple protection circuitry to allow the SDR to survive brief bursts of transmit power - perhaps the topic of a later post. 

AS IT IS, I WOULD NOT USE IT FOR ITS INTENDED PURPOSE, AS AN SDR ANTENNA SWITCH - at least not without significant modification.

"Audio Muting"

What it IS useful for, out of the box

What it IS useful for is an audio switch to mute your computer audio when you transmit - as you might do when using a WebSDR or other remote receiver.  For this, I would:

• Remove internal jumper J5
  
• Connect the PTT to your radio's PTT
• Connect your computer's speakers to the "AUDIO OUT" jack
• Connect your computer audio output to the "SDR" jack

If you are hell-bent on not using the PTT cable, the RF sense may be useful, but you would also need to:

• Be sure that you have removed internal jumper J5
• Connect the transceiver to the "RADIO" port
• Connect the antenna to the "ANTENNA" port 
• DO NOT connect anything to the SDR port

As noted before, the "hang time" imposed by the time constant of C3 and R3 may not be enough and the relay may "chatter" - in which case R3 could be replaced with a higher-value resistor of, say, 330k - or with a 1 Meg potentiometer in series with a 47k resistor to allow "hang time" adjustment.  (Adjustment of R3 is preferable to increasing the value of C3 much as the latter could also slow the activation time when RF is detected.)

Final comments

Other than to switch audio as described above, I wouldn't use this device as it is shipped for any other purpose without appropriate modification.  This makes this device a possible  "starting point" for another project (e.g. there are already some relays and a metal box!) - one to provide proper RF protection for the SDR and make the RF sensing more useful when using modes that have variable power levels (e.g. CW, SSB) which can cause the current design to "chatter".

* * * * * * *

This page stolen from ka7oei.blogspot.com

[END]

Repair of a (lightning struck?) Yaesu FT-1000 MP Mark V

The background - and initial assessment:

A month or so ago an acquaintance of mine in the local amateur community asked me to take a look at his radio - a Yaesu FT-1000 MP Mark V.  While I don't routinely repair other people's radios, I decided to make an exception, as I've known this gentleman for decades.

The complaint was "The receiver is dead" - which is a very broad assessment, but it was an obvious starting point and upon putting this rather large, ponderous piece of equipment on my workbench - which itself proved to be a challenge - I noted that it took about a 0dBm signal to attain an "S-9" reading:  Yes, the radio was deaf, to the tune of around 70dB!  Knowing that the antenna A/B relay and/or antenna tuner could be a problem with this particular radio, I tried different configurations - even the (differently-routed) rear "RX Antenna" jack - but no difference.

Figure 1:  A blast mark!

Getting it apart:

This radio's technology and construction is what I would refer to as "transitional" -  mostly reminiscent of 80s Japanese radios in that there is a mix of through-hole and surface mount, and the boards are mostly interconnected with a myriad of white wires with unmarked plugs on them that go everywhere in the case - with the occasional gray coaxial cable that use the odd board-mounted plug-in cables.  Fortunately - unlike some of those older radios - the boards are mostly double-sided epoxy rather than single-sided paper phenolic.

Of course, the main receive RF board was buried under the very large heat-sinked power amplifier assembly, so I first did a test on the "sub" receiver board, which was accessible:  That receiver seemed to be reasonably sensitive when I injected a signal directly into it, but there was no sign of RF getting to that receiver via the radio's rear coaxial cable connector.

The next, obvious step was to remove the PA board - which, like many things about working on this radio,  was a real pain:  Only four screws, but two of them were buried under a mass of white wires -  I pulled the amplifier module out from the radio with trepidation, knowing that I'd eventually have to get it back together.

A visual and an olfactory inspection of the receive board - before removing it - was done, and nothing was obviously amiss, so I removed a bunch more screws and cables - marking them as appropriate (you will hate life if you don't mark where they went as you remove them!) I was able to pull the board away and immediately saw the first indications of the problem:  A very obvious black mark on the aluminum chassis under the board and a corresponding area of char on the board itself (see Figure 1, above and Figure 2, below).

What might have happened:

Figure 2:  Carbonized crater in the RF board.

While it is difficult to determine what, exactly, happened to this radio, I suspect that it was likely a nearby lightning strike that did the initial damage, with peripheral damage occurring later.  In inspecting the radio and referring to the service manual, which I found online, I saw that capacitor C1278 on the "RF unit" - and its related circuit board trace, and a portion of the board itself - had been wholly transformed into the black mark on the aluminum chassis.  The fix for this was pretty simple:  Scrape away all of the now-conductive PC board carbonized residue and replace the vaporized surface mount 0.1uF capacitor with an axial-leaded unit to both replace both the component itself and the trace as seen in Figure 3.

Knowing that this was not likely to be the only problem, I half-installed the board again and checked the receiver:  Very slightly less terrible - but still deaf as a post, but with access to the board, I cranked up the signal generator to 0 dBm and started poking around with the oscilloscope to see where RF disappeared.

Figure 3:  New cap, carbon excised!

Fortunately, I was seeing RF at the input to the RF board - and even though many of the key probing points were on the (inaccessible!) bottom side, I found a few places where I could test on the top and discovered that the RF signal stopped at D1056, a PIN diode found at a low-pass filter stage in the RF path.  Pulling the board again I started probing with the multi-meter and discovered that not only was this PIN diode completely open, but R1157 and L1086 - both part of the DC path of this and (the still-working D1055) were open:  I had a suitable SMD replacement for R1157, but used a small, molded 1mH inductor for L1086 to accommodate the damage due to a blown-off pad and trace that hadn't been immediately obvious and I used a much more rugged PIN diode than the original to replace D1056.  While I was at it I did more probing around and found that R1150 - part of what appears to be a DC drain on the antenna circuit - was also burned open:  Because this resistor can only be "reached" via the 1 mH inductor in series with it, this meant that it was not likely RF that blew it up, but rather a transient with a strong DC component - or at least a low-frequency AC component - maybe lightning?

Initial re-testing:

Putting the board back in, temporarily, I re-checked the receiver sensitivity and found that it was "OK", in that I could touch a screwdriver and put it in the rear-panel RF connector and hear signals, but a quick check with the signal generator indicated that something was still amiss as it seemed to be off by about 15dB based on the specs in the alignment procedure.

At this point I decided to check the transmitter and to my gratification, I was able to get about 90 watts out of it.  My initial satisfaction was short-lived, as I soon realized two three things:

• I should have been able to get at least 150 watts out of the transmitter.
• The SWR indicator on the radio was showing a mismatch, with the tuner bypassed, into a known-good load.
• I smelled epoxy smoke from the "RF Unit".

Figure 4:  Original TX isolation relay - welded!

At this point a felt a bit silly:  I should have checked to see that the unit had a functional TX/RX isolation relay - and a quick check with a signal generator and probing with the oscilloscope told me that it did not, the apparent culprit being  RL6414, a reed relay on the "Tuner-Main" unit that was clearly welded - a fact soon verified by multimeter.  The radio's power amplifier had dutifully cut back its power because it was seeing a mismatch caused by the receiver being connected at the same time as the dummy load.

At this point I again removed the RF unit and replaced D1056 (again!) - the apparent source of the smoke that I'd smelled and - without transmitting - restored the operation of the receiver.  In reviewing the service manual and online forums, I discovered that the failure of RL6414 was semi-common, and also that this particular relay - seen in Figure 4 - was difficult to source.  Nevertheless, I sent an email off to Yaesu Parts to find out.

A few days later I'd heard back from Yaesu:  This relay was available - but it would take 6-8 weeks.  In the meantime I'd tried to find an exact replacement elsewhere, but to no avail:  The original relay had a non-standard pin-out and was a brand that was simply not carried by U.S. parts suppliers - and I couldn't be sure if this particularly relay was still made!  In speaking with the owner of the radio I gave him two options:  Order the part and wait 6-8 weeks, or get a more common part and adapt it to fit:  He opted for the latter, so I placed an order with DigiKey, set the radio aside and waited.

It's worth noting that RL6414 is an SPDT relay with the RX signal path connected in the "Normally Open" position with the "Normally Closed" position grounded - that is, the relay must be energized for the receiver to be connected to the antenna.  If the radio is turned off, there is no direct path, so whatever "killed" the receive, must have either happened with the radio turned on, while it was receiving, or it was sufficiently energetic enough to weld the relay and, apparently, blow away the ground "N.C." contact:  Yet more evidence of a "high energy discharge" from lightning.

Resuming work:

Figure 5:  New relay on homebrew carrier board.

About a week later I again had some time to work on the radio and the relay from Digi-Key had arrived.  This relay - a Comus BFH-1C-12C (Digi-Key P/N:  1835-BFH-1C-12C-ND) was an actual RF reed relay with decent voltage and current ratings - but the pin-out was different from the original, requiring me to construct a simple "carrier board" to reroute some of the connections of the relay footprint to match the tuner board:  With plenty of headroom, the extra height of the combination was not an issue.

At this point, I will note again how difficult it is to work on this radio - highlighting, in this case, the antenna tuner.  This module consists of two boards, face-to-face, meaning that no components are accessible unless they are separated.  Unfortunately, there are several wires that appear to have been pulled through from the "lower" board (with the tuning capacitors) and then cut short - and none of the other wires were any longer than they absolutely needed to be.  Disassembly was pretty easy (especially with proper desoldering equipment) but re-soldering the short wires was an exercise of patience and the careful manipulation of small tweezers and screwdrivers to try to align all of the wires simultaneously without causing one or more of them to pop out of place!

Figure 6:  New TX isolation relay - looks right at home!

With the TX isolation relay (RL6414) once again functioning I found that the transmitter's output now exceeded 150 watts with no reflected power on the radio's meter - and no smoke!

Alignment - and more problems:

Not having part of the transmitter signal path in parallel with the receiver input helped the sensitivity a bit, but it still wasn't right so I did a bit more checking with the signal generator and scope, finding no-where that things were obviously amiss on the RF Unit - but I did notice that the secondary receiver was more sensitive than the main:  Since they share the same signal path that more or less ruled out an obvious problem with the RF unit when operated in the normal fashion so I decided to perform a realignment.

To my surprise, the I.F. stages in both receivers were quite far out of alignment - particularly on the main receiver - and after finishing this, the receivers "woke up" and met specifications:  I was surprised to see this on a modern receiver and am at a loss to explain it.  It seems unlikely that it detuned itself with age, so it was either not properly aligned at the factory, or someone else tried to "improve" it - but I'm suspecting the former.

In performing the alignment steps, I then "discovered" that the "VRF" circuit - an electronically-tuned preselector - was inoperative, about 25 dB of excess attenuation:  This problem was traced back to being caused by RL1005 having welded contacts, not allowing this preselector circuit to be fully switched into the signal path.  This not unexpected as this is in the same signal path as the vaporized trace and capacitor.  Fortunately, these exact relays were readily available from Digi-Key as well.

All's well that ends well:

The replacement of RL1005 turned out to be the last problem that was found and the radio easily met its specs when the job was done.  The reassembly of the radio was, as expected a bit of a challenge:  The inboard mounting flange of the PA unit shares the same channel as dozens of thin, white wires and there is practically no visibility as one tries to maneuver the screws into place while trying to avoid them getting knocked off the magnetic driver while, at the same time, trying to make sure that none of the dozens of small, white wires get pinched in the process - all the while trying to align to invisible holes!

In the end, the radio was handed off to the owner, with the recommendation of added lightning protection or, at least, disconnecting things when not in use - good advice for anyone!

This page stolen from ka7oei.blogspot.com

[End]

An automatic transfer relay for UPS/Critical loads, for the ham shack, generator backup, and home

It is quite common to use a UPS (Uninterruptible Power Supply) to keep critical loads - typically computers or NAS (Network Attached Storage) devices - online when there is a power failure as even a brief power failure can be inconvenient.  Like any device, a UPS occasionally needs to be maintained - especially the occasional replacement of batteries - and doing so often necessitates that everything be shut down.

A simple transfer relay can make such work easier, allowing one to switch from the UPS to another load - typically unprotected mains, or even another UPS - without "dumping" the load or needing to shut down.

This type of device is also useful when one is using a generator to provide power:  Rather than dumping the load when refueling the generator, another generator could be connected to the "other" port, the load transferred to it, and the original generator be shut down and safely refueled - such as during amateur radio Field Day operations.

Figure 1:
Exterior view of the  "simple" transfer relay depicted in Figure 2, below.
The "Main" power source is shown as "A" on the diagram.
Click on the image for a larger version.

But first, a few weasel words:

• The project(s) described below involve dangerous mains voltages which can be hazardous/fatal if handled improperly:  Please treat them with respect and caution.

• Do NOT attempt a project like this unless you have the knowledge and experience to do so.

• While this information is provided in good faith, please do your own research to make sure that it suited to your needs in terms of applicability and safety.

• Do not presume that this circuit or its implementation is compliant with your local electrical codes/regulations - that is something that  you should do. 

• There are no warranties expressed or implied regarding these designs:  It is up to YOU to determine the safety and suitability of the information below for your applications:  I cannot/will not take any responsibility for your actions or their results. 
  

• With a standard DPDT relay, it's possible that one set of contacts can "weld" (get stuck) causing half of it to switch to one source.  The hazard of this is that this could "back-feed" power to the grid if the power is off not to mention possibly damaging equipment - the load equipment due to improper voltages and/or the UPS itself.  There are relays with mechanical interlocking contacts which make it virtually impossible for this to happen that might be considered.
  

• It is recommended that this type of device not be plugged in permanently if connected to the mains grid:  If used on the grid, the "B" side would be best connected only if you are transferring from "A" to "B", minimizing risk.
  

• You have been warned!

The simplest transfer relay:

The simplest version of this is a DPDT relay, the relay's coil being powered from the primary power source - which we will call "A" - as depicted in the drawing below:

Figure 2:
The simplest version(s) of load transfer relays - the load transferred to "A" ("Main") upon its presence, switching to "B" (Aux) in its absence.
The version on the left uses a relay with a mains-voltage coil while that on the right uses a low-voltage transformer and relay coil - otherwise they are functionally identical.
Click on the image for a larger version.

How it works:

Operation is very simple:  When the primary power source "A" is energized, the relay will pull in, connecting the load to source "A".  Conversely, when power source "A is lost, the relay will de-energize and the load will be transferred to the back-up power source, "B".  In every case that was tried, the relay armature moved fast enough to keep the load "happy" despite the very brief "blink" as the load was transferred from one source to another.

Two versions of this circuit are depicted:  The one on the left uses a relay with a mains-voltage coil while the one on the right uses a low-voltage coil - typically 24 VAC.  These circuits are functionally identical, but because low-voltage coil relays are common - as are 24 volt signal transformers - it may be easier to source the components for the latter.

Figure 3:
The interior of the "simple" transfer relay.  Tucked behind the outlet is the
DPDT relay with the 120 volt coil, the connections made to the relay.
using spade female spade lugs. The frame of a discarded light switch
is used as a mounting point for a standard "outlet + switch" cover plat
with neon panel lights being mounted in the slot for a light switch.
The entire unit is housed in a plastic dual-gang "old work" box.
Click on the image for a larger version.

The actual transfer takes only a few 10s of milliseconds:  I have not found a power supply that wasn't able to "ride through" such a brief outage but if a UPS is the load, it will probably see the transfer as a "bump" and briefly operate from battery.

Why a DPDT relay?

One may ask why use a DPDT (Double-Pole, Double-Throw)  relay if there is a common neutral:  Could you not simply switch the "hot" side from one voltage source to another?

The reasons for completely isolating the two sources with a double-pole relay is multi-fold:

• This unit is typically constructed with two power cords - one for each power source.  While it is unlikely, it is possible that one or more outlets may be wired incorrectly, putting the "hot" side on the neutral prong.  Having a common "neutral" by skimping on the relay would connect a hot directly to a neutral or, worse, two "hot" sides of different phases together.

• It may be that you are using different electrical circuits for the "A" and "B" power in which case bonding the neutrals together may result in circulating currents - particularly if these circuits are from disparate locations (e.g. long cord.

• For readers outside North America:  While typical outlets are 120 volts, almost every location with power has 240 volts available which is used to run larger appliances.  This is made available via a split phase arrangement from a center tap on the distribution transformer which yields 120 volts with respect to the neutral.  It is because of this that different circuits will be on different phases meaning that the voltage between two "hot" terminals on outlets in different locations may be 240 (or possibly 208) volts.

• There is no guarantee that a UPS will "play nice" if its neutral output is connected somewhere else.  In some UPSs or inverters the "neutral" side may not actually be near ground potential - as a neutral is supposed to be - so it's best to let it "do its thing."

How it might be used:

With such a device in place, one simply needs to make sure that source "B" is connected, and when  load "A" - typically the UPS, but it could be a generator -  is disconnected, everything will get switched over, allowing you to performs the needed maintenance.

UPS maintenance:

When used with a UPS, I have typically plugged "A" (Main) into the UPS and "B" (Aux) into a non-UPS outlet.  If you need to service the UPS, simply unplug "A" and the load will be transferred instantly to "B".  Having "B" as a non-UPS source is usually acceptable as it is unlikely that a power failure will occur while on that input - but if you choose not to take that risk, another UPS (or a generator) could be connected to the "B" port.

I have typically kept input "B" (Aux) plugged into non-protected (non-UPS) power as a failure of a UPS would not likely interrupt the power to the backed-up device(s) - but if you do this you must keep an eye on everything as unless it is monitored, the failure of a UPS may go unnoticed until there is a power failure! 

This same device has also been used in a remote site with two UPSs for redundancy, not to mention ease of maintenance.  One must, of course, weigh the risk of adding yet another device (another possible point of failure, perhaps?) if one does this.

Generator change-over:

During in-the-field events like Amateur Radio Field Day such a switch is handy when a generator is used.  It is generally not advisable to refuel a generator while it is running even though I have seen others do it.  If, while gear is running on a generator, it is necessary to refuel it - another generator can be connected to input "B" and once it is up to speed (and switched out of "Eco" mode if using an inverter generator) input "A" is un-plugged  for refueling, checking the oil, etc.

If you are of the "OCD" type, two generators can be used:  The generator on "A" would be running the gear most of the time, but if it drops out, a generator on "B" - which will have been under no load up to that point - will take over.

Disadvantages of this "simple" version of the transfer relay:

For typical applications, the above arrangement works pretty well - particularly if power outages and maintenance needs are pretty infrequent - and it works very well in the "generator scenario" where one might wish to seamlessly transfer loads from one generator to another.

It does have a major weak point in its design - and that's related to how the relay pulls in or releases.

For example, many UPSs or generators - especially the "inverter" types - do not turn instantly "on", but rather they may ramp up the voltage comparatively slowly, but by its nature the relay coil may pull in at a much lower voltage than nominal - say, 80 volts.  When a load is transferred at this lower voltage, it may momentarily cause the power source to buckle, causing the load to be dropped and/or the relay to chatter briefly or, possibly simply cause the load to drop owing to too-low battery voltage.  The typical "work around" for this is to allow the "A" source to come up fully before plugging back into it - which is fine in many applications.

A "slow" pull-in on a relay can also be hard on relay contacts - particularly a "slow" rise the voltage from power source "A" - in which the contacts may not close quickly enough to prevent extensive wear.  In severe conditions, this can even result in one or more of the contacts welding (sticking together) which is not at all a desirable condition.  For this reason it is a good idea to use a relay with a significantly higher current rating than you are planning to pull.

A slightly more complicated version:

What can help this situation would be the addition of a short delay, after power source "A" is applied  but before the load is transferred to it - and better yet, we would like this load to be transferred only if its voltage is above a minimum value:  The circuit in the diagram below does this.

Figure 4:
This version of the transfer relay offers a short delay in transferring to load "A" as well as providing a low-voltage lock-out/detect.
The relay is a Dayton 5X847N - a 40 amp (resistive load) DPDT contactor with a 120 VAC coil.  This relay is likely overkill, but it should handle about anything one can throw at it - including capacitor-input power power supplies that tend to be very hard on relay contacts due to inrush current.  Not shown on the diagram above:  It's recommended that a "snubber" circuit consisting of a 100 ohm resistor and 0.1 uF capacitor - of appropriate voltage rating - be connected across the contacts of RLY2 to suppress back-EMF that might damage its contacts when this relay opens.
Click on the image for a larger version.

How it works:

This circuit is based on the venerable TL431 - a "programmable" (via resistors) Zener diode/voltage reference - U1 in the above diagram.  A sample of the mains voltage is input via T1 which, in this case, provides 9-12 volts AC which is then half-wave rectified by D1 and then smoothed with capacitor C1.  LED D2 was included on the board mostly for testing and initial adjustment - but it also establishes a 8-12 milliamp static load to help discharge C1 when the mains voltage goes low - although the current consumption of the relay does this quite well.

Figure 5:
An exterior view of the version of the transfer relay depicted in Figure 4,
above.  The unit is mounted in a 6x6x4" electrical "J" box.
The 10 amp rating is a bit arbitrary and conservative considering that
the contactor itself is rated for 40 amps and the fact that capacitor-input
supplies are likely to be connected to it.
Click on the image for a larger version.

The DC voltage is divided down via R2 and R3 and this is further filtered with capacitor C2, with R3 being adjustable to provide a variable threshold voltage to U1.  The combination of R2 and C2 causes the voltage at their junction to rise comparatively slowly, taking a couple seconds to stabilize.

When power is first applied, C2 is at zero volts, and will take a couple seconds to charge.  When the wiper of R3 exceeds 2.5 volts, U1 will suddenly turn on (conduct), pulling the "low" side of the coil of relay RLY2 to ground, turning it on which, in turn, will apply current to the coil of RLY1.  When it does, the base of transistor Q1 is pulled toward ground via R6, turning it on and when current passes through R4 into the junction of R2 and R3, the voltage will rise slightly, resulting in some hysteresis.  For example, if R3 is adjusted so that RLY2 will be activated at 105 volts, once activated the voltage threshold for U1 will be effectively lowered to about 90 volts.

If power source "A" disappears abruptly, RLY1 will, of course, lose power to its coil and open immediately - and a similar thing will happen if the voltage goes below approximately 90 volts when RLY2 will open, disconnecting power to RLY1 - and at this point Q1 will be turned off and it will require at least 105 volts (as in our example) for RLY1 to be activated again.  Diode D4 may be considered optional as it will more-quickly discharge C2 in the even the power on "A" goes away and suddenly comes back, but it is unlikely that its presence will usefully speed response.

As noted in the caption of Figure 4, the relay used is a Dayton 5X847N which has a 120 volt coil and 40 amp (resistive load), self-wiping contacts.  While 40 amps may seem overkill for a device with an ostensible 10 amp rating as depicted in Figure 5, it is good to over-size the relay a bit, particularly since many loads these days (computer equipment, in particular) can have very high inrush currents due to capacitor-input rectifier, so a large relay is justified.

Note:  The 5X848 is the same device, but with a 240 volt AC coil while the 5X846 has a 24 volt AC coil:  All of three of these devices are suitable for both 50 and 60 Hz operation.

Circuit comments:

Figure 5:
Inside the transfer relay unit.  The large, open-frame DPDT relay is in the
foreground while the 12 volt AC transformer is tucked behind it.  Mounted
to the wall behind it (upper-left in the box) is the piece of prototype
board with the smaller relay and delay/voltage sense circuitry.
Click on the image for a larger version.

U1, the TL431, is rated to switch up to 200 milliamps, but it's probably a good idea to select a relay that will draw 125 milliamps or less.  Because the contacts of the relay are simply switching power to the main relay (RLY1), RLY2 need only be a light-duty relay.

When I built this circuit I used a 5 amp relay with a 9 volt coil because I had a bunch of them in my junk box and in checking it out, I found the coil resistance to be 150 ohms meaning that at its rated voltage, it would draw 60 milliamps.  The voltage across C1 when RLY1 was not active was measured at about 16 volts so it was presumed that with the load of the relay that this would drop by a volt or two meaning that a series resistor that would pass 60 milliamps across 6 volts (the difference between the 15 volt supply and 9 volt coil voltage) should be used - and Ohms law tells us that a 100 ohm, 0.5-1 watt resistor would do the job.

Adjustment:

A variable AC supply (e.g. a "Variac") is essential for proper adjustment.  To start, the wiper of R3 is adjusted all of the way to the "ground" and then the applied AC voltage is set to 105 volts - a nice, minimum value for U.S. power mains.  Then, R3 is adjusted, bringing the voltage on its wiper upwards until RLY2 and RLY1 just close.  At this point one can lower the input voltage down to 80-90 volts and after capacitor C2 discharges, the relays will again open and one can then move the voltage back up, slowly, and verify the pull-in voltage.

Figure 6:
The back side of the front panel of the J box:  A large, square hole was cut
in the front and an plastic dual gang "old work" box with its back
cut away was used to facilitate mounting of the two outlets  to the front panel.
Adhesive was used around the periphery to prevent the box from sliding
around on the front panel.
Click on the image for a larger version.

If less hysteresis is desired, the value of R4 can be increased to, say, 22k.  Note that despite the operation of Q1, some of the hysteresis is cancelled out by the voltage across C1 decreasing under load when the circuit is triggered, by the current through RLY1, so a bit of hysteresis is absolutely necessary or else the relays will chatter!

Construction:

As can be seen in figures 5 and 6, a 6x6x4 inch gray plastic electrical "J" box was used to house the entire unit - a common item found in U.S. home improvement stores.  A pair of "duplex" outlets were mounted in the front cover by cutting a large square hole in it and using a modified "old work" box with its back removed, giving a proper means of mounting the outlets.

A pair of front panel neon indicators indicate the current state:  The "B" indicator simply indicates the presence of mains voltage on that input while the "A" indicator is wired across the relay's mains-voltage coil and is thus indicative of the delay in the relay's closure.

The circuitry with the TL431 and RLY2 is constructed on a small piece of prototype board, mounted to the side of the box using stand-offs.  The 9-12 volt AC transformer - the smallest that I could find in my junk box (it's probably rated for 200 milliamps) is also bolted to the side of the box.  Liberal use of "zip" ties are used to tame the internal wiring with special care being taken to absolutely avoid any wire from touching the armature of the relay itself to prevent any interference with its mechanical operation!

Final comments:

Both versions work well and the "simple" version depicted in figures 1 and 2 is suitable for most applications.  For more demanding applications - particularly those where a transfer may occur frequently and/or the mains voltage may rise "slowly", the more complicated version is recommended.

Again, if you choose to construct any of these devices, please take care in doing so, being aware of the hazards of mains voltages.  As mentioned in the "Weasel Words" section, please make sure that this sort of device is appropriate to your situation.

This page stolen from ka7oei.blogspot.com

[End]