|Figure 1. A properly-working IFR-1200 in receive mode.|
on the display and would not do anything else.
If it was allowed to sit in this state for a while and warm up, another power cycle could often bring it to life, but eventually, it was completely dead.
Because we had another IFR 1200 kicking around, we used that for a while - but it had a nagging problem with its deviation meter in that it didn't read quite correctly (more on this later) so, eventually, I decided to tear into it to fix the problem.
Pulling the CPU board I noticed immediately that there were exactly FOUR electrolytic capacitors - at least two of them were the typical power supply bypass capacitors, but at least one of them was related to a line that had something to do with the CPU's startup/reset.
Since, on old equipment, you can almost always blame the capacitor - and be right much of the time - I decided just to replace them all. The picture below shows the board and the four capacitors:
|Figure 2: The CPU board from the IFR FM/AM-1200S.|
All four of these capacitors should be replaced while you are at it. Note that your circuit board may also be of the "blue" variety and if so, it may be a bit fragile: When removing the capacitors use a temperature-controlled soldering iron and proper techniques as these types of boards (e.g. the "blue" ones) are reportedly easy to damage. I didn't have any trouble when working on this board, but this is good advice, nonetheless!
I replaced these capacitors with 47uF, 105C devices. It is important to note that the capacitor that you use to replace it must be either:
- Shorter than the height of the tallest chip+socket on the board, or
- If the capacitor is taller, you must use extra lead length and lay it on its side.
I never really did look up exactly which capacitor of the four that is involved with the CPU initialization (I think that it's the upper-right one) but now the IFR FM/AM-1200S "boots" up perfectly, every time!
* * *
About that problem on the "other" IFR 1200S: Non-linearity on the deviation meter.
It was noticed that the deviation reading wasn't "linear". In other words, if I did a first Bessel Null with a 905.8 Hz tone and set the deviation meter for a reading of 2.18 kHz, it would read higher than 5.00 kHz at the second Bessel null of the same 905.8 Hz tone.
Interestingly, the deviation as displayed on the oscilloscope was correct, but the deviation meter was "off" by an ever-increasing, non-linear amount as the deviation went up. I then noticed that this was NOT true if I switched to the "Medium" mode - which is actually the "Wide" filter with a low-pass filter in the audio path.
What it turned out to be was one or more of the 10.7 MHz crystal filters in the IF.
Fortunately, I was able to use an off-the-shelf 2-pole ECS 10.7 MHz 15 kHz wide monolithic crystal filter to replace it with, retune the filter for lowest distortion (as indicated on the IFR's own distortion meter) and this fixed the problem!
For reference, this is Mouser part: 520-107-15B - This comes as a matched pair of two 2-pole filters, which is exactly what you need.
The Digi-Key part number for the same, exact item is: X703-ND - except that these come individually, NOT as a matched pair!
Because these new filters operate at 1.8k ohms instead of the higher impedance of the original crystals I had to parallel the input/output (R8 and R10) resistors with 3.0k resistors to achieve the 1.8k source/termination impedance to properly match these filters.