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!