Thursday, October 18, 2012

Smoke and flames from my IFR-1000S...

I'd not really intended to have two posts in a row about repairing service monitors, but fate/opportunity intervened...

Figure 1:
The gray, charred stump of the failed tantalum capacitor in the center
of the image, just above the potentiometer shaft.)  The original,
nylon extension shaft broke several years ago and here, it is
shown having been mended with two overlapping
layers of heat-shrink tubing.
Click on the image for a larger version. 
The other night I had a few minutes to spare and I decided to take a quick look at my old IFR-1000S service monitor:  I'd remembered that the last time I dragged it to a repeater site there was something about it that was flaky - but I couldn't remember what - the 'scope, I thought...

So, I plugged it in, turned it on and everything was fine until I flipped the switch to apply power to the 'scope.  At this point the front panel lights dimmed momentarily, following by a loud bang (even though it was muffled by the unit's metal case) and smoke billowed out from every gap in the front panel along with a bright, flickering yellow light from a fiercely burning flame within.

Of course, I turned it off and fortunately, the flame quickly died out!

Hmmm...

Undoing about a dozen screws I soon had the cover off and discovered the culprit:  A dipped tantalum capacitor (150uF, 15 volts) on the the high voltage power supply board for the oscilloscope had incinerated itself.  Fortunately, aside from leaving a sticky, smoky residue on the nearby components, adjacent chassis panels and the inside of the wrap-around case, there didn't appear to be any real damage.

I should say that once I saw what had "flamed out" I wasn't too surprised:  Dipped tantalum capacitors don't fail too often, but when they do, they usually fail spectacularly, often burning holes in the circuit board and destroying nearby components!

Figure 2:
Inside the wrap-around cover - evidence of smoke and flames!
Click on the image for a larger version.
Grabbing the service manual I quickly located the faulty capacitor on the schematic diagram and noted that it wasn't anything too critical - a bypass capacitor on the power supply to filter the ripple from the 'scopes high voltage power supply (essentially an oscillator) from the main 12 volt power bus - this, to keep "noise" from getting into other circuits.

Carefully unsoldering the remnants of the capacitor (now a small chunk of charred tantalum) I shook out the other pieces of the capacitor that had fallen inside the unit and the powered it up.

Everything looked good!

Now, to replace the capacitor.  The original was a dipped tantalum unit, this type chosen because of its low ESR (Equivalent Series Resistance) and its ability to effectively filter the high-frequency noise produced by the high voltage inverter.  For this task I wasn't going use an "ordinary", cheap capacitor since its filtering ability may be somewhat diminished at the frequencies involved - around 20 kHz.

Back when the unit was made the best capacitors for high-frequency filtering were tantalum units or specially-made low-ESR electrolytics, but the latter weren't extremely common.  These days, with the proliferation of switching power supplies it's quite common to find high-performance, low-ESR electrolytics designed for just this task so I rummaged around and found a 330uF, low-ESR 105C (high temperature) capacitor that appeared to be well-suited for the task.

Figure 3:
The new (blue) CDE 330uF low-ESR electrolytic.
Click on the image for a larger version.
While it would have been ideal to have completely pulled the circuit board to install the replacement capacitor, I knew this to be a chore - having done it several times before - so I was able to do a careful "top soldering" job, heating the component's through-hole vias from the component side of the board.  Not having pulled the board out of the unit also meant that some of the sticky, smoky residue remained in some of the inner recesses and on some of the adjacent components, but I was content to clean off what I could reach using denatured alcohol.

The upshot?

The unit is now working again and the new capacitor seems to be doing its job.  If I have a reason to do so in the future, I'll pull the scope module go through it to remove the last traces of the smoky residue and, perhaps, preemptively replacing the other tantalum, but for now...

I still don't remember for certain the problem for which I was checking out the service monitor!

Additional random comments:

A few months later (8/13) I noticed that sometimes the IFR-1000S would hum when it was powered up - but not always.  Clearly 120 Hz ripple, it was pervasive enough that it would register as 200-300 Hz of deviation on an otherwise unmodulated carrier, appearing as a "dirty" waveform on the output signal as well as being visible on the scope and audible via the speaker.

Taking the cover off, the hum stopped, but I checked the filter capacitors in the power supply (some of which I'd replaced a few years ago) and found them to be good.  After having used it a few more times hum-free, the problem appeared again and this time I happened to notice, as I was picking it up while it was powered on, that the hum changed.  Pushing on the case and wiggling things I discovered that the hum changed radically when I wiggled them main AC power connector - "Jones" plug.

Upon disassembling the unit I saw that the solder joints on the connector were just fine, but that the Battery - lead from this connector (which can be used to operate the unit from DC power) shared a heavy black lead that came from the main power supply, bonding it to the chassis.

Hmmm...

Grabbing a screwdriver, I immediately noticed that this screw was a bit loose.  As it turned out it was this connection that was getting flaky, developing a slight amount of resistance.  Since it came from the power supply this caused the regulation (and consequently, ripple rejection) to suffer.  I put a drop of anti oxidant grease on the connections and properly tightened the screw, thus fixing the problem!


About the IFR-1000S:

This service monitor covers from a few hundred kHz to 999.9999 MHz in AM, FM and the reception of SSB and is able to both generate and analyze signals over this entire range.  With its calibrated signal generator one can determine the performance of a receiver (sensitivity, distortion, alignment) and it can also test the power and modulation of a transmitter.  Being the "S" version, it also has a built-in spectrum analyzer:  Not really well-calibrated, but still very useful.

Being made in about 1980, it's pretty basic, having no computers in it and lots of analog circuitry, but it is fairly bulletproof and uses relatively few impossible-to-find components other than the obvious such as meter movements or the cathode-ray tube for the 'scope.  Since most of the components are readily-available, off-the-shelf parts such as ICs, transistors, diodes - many of which are still made - it should be repairable for some time to come!

Although labeled as a "1000S", it was apparently factory-upgraded by a previous owner and is, in fact, really an 1100S - a slightly fancier model that saw widespread use in the military and aeronautics industry.  The main differences between the 1000S and 1100S are the way the audio synthesizers work (the variable and fixed 1 kHz), the change of a few front-panel controls and input/output connections and also the fact that the 1100S has an additional amplifier allowing it to produce up to at least 0dBm (1 milliwatt) of RF output - about 40dB more signal than the standard IFR-1000 and the power supply.  Soon after I got the unit I replaced the remaining incandescent indicators with red, green and yellow LEDs as appropriate, just like the factory 1100S.

The only thing that the factory seemed to have missed was the connecting of the output of the fixed 1 kHz tone generator (prior to the output level control) to the front-panel accessory connector that is used to interface the (optional) MM-100E meter unit:  Without this connection having been made I pulled my hair out trying to figure out why the SINAD and distortion meters didn't work before finally realizing that this reference signal was missing!

5 comments:

  1. I also have a IFR1000S which has been rock solid. However, lately I have noticed a "warble" in the signal generator. It is very much like the note you hear from a receiver if you tune the receiver under test to say 455khz, THEN change the output of the IFR from some other frequency to 455khz. It occurs randomly and powering off/on seems to fix it for a while. I have never had the case off so I thought I would run this by you first before I start trouble shooting.
    Thanks es 73,
    Pat AA4PG

    ReplyDelete
  2. Pat,

    I was brought an IFR-1000 that had a severe "warble" problem a few years ago. After checking out the power supply capacitors (just to make sure that they were OK - the warble did not have much 120 Hz ripple on it) we traced it down to one of the VCOs: This was the one in a rectangular metal can and not the one in the machined aluminum turret. (I don't have a service manual for it handy so I'm going by memory.) I believe that the status of this "other" VCO is indicated directly by one of the two "lock" lights on the front panel - which didn't really go out as this unit warbled.

    What it turned out to be was a flaky potentiometer. Inside the VCO box is a "sub" regulator that takes the voltage from the main supply down to a slightly lower voltage (say, 12 volts down to 9-10 volts - I don't remember the exact voltage.) What had happened was that this voltage had crept up due to the pot going somewhat noisy so the originally-lower voltage was now (approximately) the same as that of the '1000's main supply, which itself is not really all that well-regulated. Fortunately this module was relatively easy to access and it could be operated while least one of the covers was removed.

    What was done was to work some "potentiometer and switch cleaner" into the pot (a drop was put onto the adjustment slot) and the pot was then exercised by rotating it several dozen times, both cleaning it by abrasion and working a small amount of the cleaner into it as evidenced by the fact that it became easier to rotate. After doing that the voltage was then set to the 'spec in the manual and the unit worked normally. We did not replace the pot since we didn't have one of the exact same size on hand: If it happens again, we'll know where to go first when tracking the culprit.

    Having worked on several IFR-1000s I know that the "CW Note" on these is very slightly warbly even when they are working fine, but good enough that SSB may be demodulated: Before we found this problem there was no way that SSB could be demodulated as the frequency was wandering all over the place (e.g. +/- 1-2 kHz).

    I hope that this helps.

    73,
    Clint
    KA7OEI

    ReplyDelete
    Replies
    1. LOCK Lamp IFR FM/AM-1200S always flash, do you have sugestion for me?

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    2. Not quite enough detail to diagnose properly, but I would first check that the power supplies are at the proper voltage and "clean".

      Delete
    3. I had a 1000s with what could be called a "warble". It turned out to be a "Microphonic" problem due to a loose sma connector. Check to make sure all are snug.

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