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!
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.
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.
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.
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!