Wednesday, October 1, 2014

What to do if your IFR 1200 Service Monitor display just shows "@" signs on the display - and if your deviation meter goes non-linear...

Figure 1.  A properly-working IFR-1200 in receive mode.
A few months ago the old IFR FM/AM-1200S service monitor on the bench at work started getting flaky.  If you turned it on, cold, it would just display a string of "@@@@@@@@@@@@@" ("at" signs) 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 this trick stopped working and 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 so, eventually, I decided to tear into them both and fix the problems:  More on the deviation meter problem, later...

Fixing the "At sign" problem:

Figure 2:  The display showing "at" signs, indicating
that the CPU cannot properly start.
Click on the image for a larger version.
Pulling the Processor board I noticed immediately that there were exactly FOUR electrolytic capacitors - at least two of them were the typical power supply bypass capacitors, but one of them was related to a line that had something to do with the CPU's startup/reset - in this case, C6, the one near the upper-right corner.  This particular capacitor, if it becomes leaky, tends to hold the CPU's RST line high and the internal pull-down resistor cannot take it out of reset and you get it out of the "@@@@@@" mode.

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:

The four capacitors are easy to spot:  Three of them are clustered together near the bottom-left corner and the other (C6) is just to the left of the CPU, the large chip in the upper-right corner of Figure 3.

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!

Figure 3
The Processor board from the IFR FM/AM-1200S (the older "A-3" version).  The culprit is C6, the capacitor just to the left of the CPU near the upper-right corner.  The newer version of the board looks completely different and may/may not have a similar problem.  If it does, it would be due to C3, a 4.7uF capacitor along the top edge of its board.
Click on the picture for a larger version.
I replaced these capacitors with 47uF, high-temperature 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, using spaghetti tubing to insulate the extra leads.
The reason for this is that there is very limited clearance between this board and its neighbor:  If the capacitor is too tall it will hit the next board, either making the installation of this (or the adjacent) board impossible, or the aluminum end of the capacitor will short out components on the other board!

By the way, the capacitor that causes the problem with the version of the board shown in Figure 3 is C6, the one in the upper-right corner!

Another reason why your IFR-1200 may be throwing "At" signs:

There is another reason why your IFR-1200 may be throwing "At" signs:  Its SRAM backup cell may be dead, or nearly so!

If you look at Figure 3 you will see a lithium coin cell in the lower-left corner:  Measure the voltage between the top of the cell and the ground plane of the board:  If it is lower than about 2.8 volts you should really replace it while you are at it and if it is lower than 2.6 volts, that is too low for reliable operation.

What happens is this:  When you power off the IFR the backup battery is supposed to hold the contents of the RAM, but when the battery gets too low (below approx. 2.6 volts) there is a reasonable chance of corruption at some point.  The way the software is written it seems possible that the SRAM could be corrupted in a way that the CPU  cannot start up and detect this condition and the unit "hangs".

The only way to "fix" this condition once it occurs is to clear the SRAM's contents, which may be done one of two ways:
  • Under anti-static conditions, remove the SRAM chip.  This is the chip to the left of the windowed chip in Figure 3 that, in the picture, is labeled "TC5517APL-2".  The SRAM chip in your unit may have different nomenclature.  When this chip is removed, stick it in a "bug rug" (conductive foam) or set its pins on a wet paper towel for a few seconds to completely discharge it as its intrinsic capacitance can actually hold its (corrupted!) contents for a while - even out of the socket!
  • Momentarily (for no more than 1 second) short pins 14 and 28 of the SRAM chip to clear the memory.
Unless you replace the cell, this problem is likely to happen again, so you have two options:
  • Remove the cell completely.  You will get a "Checksum Failed" type of error whenever you power it up - and occasionally you may not be able to power it up without turning it off again and waiting a minute or two - but you shouldn't have to take it apart.
  • Replace the cell.
Clearly, the latter option is preferred.  In this unit the cell was a "2325" type lithium coin cell with tabs on it and it had lasted well over 20 years.  This sort of tabbed cell may be ordered directly from Digi-Key (you will have to specially-order it with solder tabs!) but you could substitute it with the more readily-available 2025 or 2032 type of cell, particularly if you were to use a plastic coin cell holder - keeping in mind the available clearance between the two boards.

While the 2325 cell is a 160-210maH cell (typically), the 2032, even though it is smaller, has approximately the same specifications (190maH) and the 2025 has around 160maH if you choose a cell with a known, good brand.  If you used one of these smaller cells, instead of 20 or so years, it may last only 15 or so - still a reasonable lifetime!

Note that you cannot solder wires to one of these coin cells without damaging it, reducing its lifetime significantly:  The only real way to attach one of these cells to the circuit would be to spot weld tabs to it or to use a holder.

* * *

About that problem on the "other" IFR-1200S:  Non-linearity on the deviation meter.

It was noticed that the deviation reading on the other IFR-1200S 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" bandwidth 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 on the IF on the "10.7 MHz Gen/Rec" board - and the reason that I determined this was that I noticed that the audio coming from the FM demodulator was variously distorted in the "FM Narrow" mode, but not distorted at all in the "FM Mid" or "FM Wide" modes.

Because, as we know, the "Mid" and "Wide" share the same filter (a fairly wide ceramic filter) we could rule out the demodulator itself as the culprit for the source of the distortion.  I also noted that the actual amount of distortion varied with the amount of deviation:  Because the location and amplitude of the sidebands of an FM signal vary with the amount of deviation (and modulation frequency) this also told me that there was something in the signal path that was asymmetrically disturbing this signal as it passed through - and it could only be one thing:  The "FM Narrow" IF filter!

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!  Fortunately, these components are fairly cheap and as of the time of writing, still readily available.

For reference, these are Mouser part:  520-107-15B  - This comes as a matched pair of two 2-pole filters, which is exactly what you need - in other words, you buy "one" of these items and get two devices that have been matched at the factory. The Digi-Key part number for the exact, same item is:  X704-ND.

Because these new filters operate at 1.8k ohms instead of the higher impedance of the original crystal filters I had to parallel the input/output (R4 and R10) resistors with resistors to achieve the 1.8k source/termination impedance to properly match these filters:  For this I used 3.0k resistors in parallel with R4, R80, R6 and R81.

The specified ratings for the above pair of filters are 15 kHz at the -3dB points and at least -40dB at +/- 25 kHz.

The input/output termination transformers for these filters should be readjusted (using a non-metallic tool to prevent breakage of the core) according to the manual, although adjusting for minimum distortion with a 1 kHz tone set for 5-6 kHz deviation (using the unit's own distortion or SINAD meter, which also measures distortion/noise, but presents it differently) and then do similar for the "discrimination" coils on the demodulator, as well as slightly adjusting the various bandpass transformers along the signal path as well.  Please note that you will need to check/readjust the deviation for all three ranges of the deviation meter when you are done.

After I did this the deviation meter, once recalibrated according to the manual, behaved normally!

The two filters related to the FM signal path are YFL1 and YFL2.  These are the two metal-can crystal filters located closest to the top of the board (nearest to the connectors) in the center, un-sheilded area.

Note about the AM signal path and filter: 

The "AM" signal path uses a still-narrower filter that is not used in FM demodulation.  If this goes bad a suitable substitute may possibly be one of the narrowband FM filters of the same product line as the above.

It won't have the exact same bandwidth as the original filter, but the input/output impedance of these filters appears to be the same as the original.

The Mouser part number for the single device is 520-107-7.5A while the matched pair is 520-107.7.5B.  The Digi-Key part number for the X701-ND for the single device and X702-ND for the matched pair.

The devices noted above, when used as a pair, are rated at 7.5 kHz bandwidth at the -3dB points and -40dB at +/- 14 kHz with an input/output impedance of 1.8k.

The AM filters are YFL3 and YFL4 and these are the "crystal cans" located just below YFL1 and YFL2 noted above.  There may be small chip ceramic capacitors connected directly to the leads of these crystal filters on the bottom side of the board:  When you remove them, note exactly where they went.  Unless you have the test equipment to "sweep" the AM IF filter precisely, to not worry about putting these capacitors back into place as they were matched with the original crystal filters and the properties of those specific units when they were new.

Since the aforementioned replacements are slightly wider in bandwidth, and since AM is generally more "forgiving" than FM in terms of and phase imbalance across the passband when measuring things like distortion (but not necessarily frequency response) - and the fact that many amateur radio operators would not really be using it for AM very much anyway - just plopping in new filters to replace obviously-defective units will probably result in reasonable performance, anyway!


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