Lovingly restored COM-120A

[VK3GJM]

Hi All,

Long-time reader, first post.

A friend donated a badly maintained COM-120A some years back that is in relative good cosmetic shape but the inner vitals needed a massive effort to return it from the abyss.

Well, after more than 300 hours of effort spanning 4 years, I have life and the unit is up and running with 99% functionality close to spec. Electrolytics on other boards and BBT, power supply and other issues all sorted now…..

The unit also has a number of options, they are:-
- (option 02) the high stability 10MHz Wenzel Oscillator.
- (option 04) Variable Audio Gen
- (option 05) Generate Amp
- (option 07) only states BER
- (option 12) Tracking Gen

The above options are great features and I am lucky, but would love to integrate SSB if the 120A allows it, I think the 120B and up allows this?

I do quite a bit of SSB work for the older fellow Amateurs as a community service.

The above options is one thing, but a number of problems elude me and I am struggling with the following:-

1. The 10MHz Wenzel stability is superb, however this is not reflected at the TX/RX Gen ports, the stability variation is somewhat greater but all tests pass and synthesizers are locked. I am struggling to pin down this instability issue.
2. Although I have the above listed options, when I go to the options screen, only the Data and Track Gen show? RTC clock and memory appears OK!
3. At power up, the unit does a self-test, all pass with the exception of Deviation meter. However after power-up and if you perform another test, all passes. Again, something I am struggling with.
4. The unit is vintage with version data showing compiled 1993 and the main program V1.05, Bios V1.03, GSP V1.01 and Keyboard V1.01. This may be very early firmware. Is later firmware available? I will admit, I am not sure what CPU is in it, most likely 188.
5. The unit also has all the hardware for the GPIB, but this option does not appear to be enabled, I assume IFT did this and you simply pay a fee to enable GPIB?

The above is an extensive list of please can someone assist, even small snippets of information and guidance will go a long way. It’s been a real struggle, especially with virtually no doc/schematics available. I am keen to see it returned to its original glory.

By the way, I will be building an extension card to assist me with the remaining issues, that will be fun,..... not....

Thank you in advance.

Regards

Gerald
VK3GJM

[Wowbagger]

In order to have SSB, you would need to get the 3kHz IF filter and SSB board - it's not just a software option. As for some options not showing up on the options screen: only options that have a software impact will show up, so things like the hi stability OXCO don't because software doesn't care about that.
As for later firmware: The COM-120A CPU was a 80188 with very limited RAM and ROM (in fact, we blew through the address space of the processor and had to go to bank switching before we even released the first version). The COM-120B had a different CPU (80486SX) and a much larger address space, and so all the later firmware was only for that processor and won't run on the A version CPU.
The whole COM-120 family is no longer supported at by Cobham, so there is no official channel for enabling options. I can do so (after all, I wrote the code), but I am walking a fine line there. If you can give me a very good reason you want GPIB enabled, I can do that, but as I have said before, I am not just going to enable all options for everybody - unless you all want to sign up to take over paying my salary if I get sacked (and I make a comfortable six figures).

There really isn't anything you can do over GPIB that you cannot do over RS-232 - it's the same interpreter.

[d119]

If you can give me a very good reason you want GPIB enabled, I can do that, but as I have said before, I am not just going to enable all options for everybody - unless you all want to sign up to take over paying my salary if I get sacked (and I make a comfortable six figures).

That was a wholly unnecessary Adam Henry type comment, Wowbagger... Check yourself? If you're that worried about it, don't even offer dude. I can't stand Adam Henries.

[VK3GJM]

Wowbagger, thanks for the response.

I did a quick Serial dump and I got the following:

*IDN?
IFR SYSTEMS INC,COM120A, 1385 ,0105-0103-0101-0101
*OPT?
144

The option sticker states O01020412. Sorry I had assumed 05, error on my part, I read -13 not +13.

What is 144 under "*opt"?

All understood on 01 which is internal battery, 02 on High Stab Osc.

I asked about GPIB, as the I/F PCB and cable is loaded, it would make sense to enable that by default, that's what's so nice about HP gear. Nice to have but not mandatory, I use GPIB throughout the shack.

No harm asking, you only live once. Wouldn't mind having "all" software option available like option 03, 09, 11, 13, 14, 15, 16. Happy to receive a PM from you, you have the unit S/N!

I am still puzzled at 144 though?

I did ask a question that you did not answer at the time, any pointers, I am struggling to find the root cause, see below:-

1. The 10MHz Wenzel stability is superb, however this is not reflected at the TX/RX Gen ports, the stability variation is somewhat greater but all tests pass and synthesizers are locked. I am struggling to pin down this instability issue.

Thanks for reading and commenting. Happy with the outcome thus far, the build date was 9338, so it's done well. Nice to have it up and running, very satisfying.

Again thanks. d119, comment noted, who is Adam Henry??

Regards

Gerald
VK3GJM

[Wowbagger]

That was a wholly unnecessary Adam Henry type comment, Wowbagger... Check yourself? If you're that worried about it, don't even offer dude. I can't stand Adam Henries.

Sorry, but no, it was necessary. I have in fact had several people IM me asking for me to do exactly that - enable all options. I had offered to help people who had their clock batteries die by restoring the options they had, and some people went over the line.

[Wowbagger]

IFR SYSTEMS INC,COM120A, 1385 ,0105-0103-0101-0101
*OPT?
144
The option sticker states O01020412. Sorry I had assumed 05, error on my part, I read -13 not +13.

The response to the *IDN? is giving the version numbers of the software: main code, BIOS, GPU, Keyboard. The response from *opt? is the options as far as the software goes:
Digital signaling (POCSAG), Tracking Gen if I remember correctly.
The options are the 01, 02, 04, 12 (and I don't recall what those map to - those where the numbers the production line used).

I asked about GPIB, as the I/F PCB and cable is loaded, it would make sense to enable that by default, that's what's so nice about HP gear. Nice to have but not mandatory, I use GPIB throughout the shack.

GPIB was always physically present, since that was what Production used to run the calibrations. If you are already using GPIB I can understand why you might want to have it enabled. Plug the following numbers into the options screen:
2717929728
136873559

1. The 10MHz Wenzel stability is superb, however this is not reflected at the TX/RX Gen ports, the stability variation is somewhat greater but all tests pass and synthesizers are locked. I am struggling to pin down this instability issue.

Much of that is how the actual modulator sections were designed - a bit of warble will always be present. The unit was designed for mostly simple FM systems, and the hardware took a couple of shortcuts.

[VK3GJM]

Hi Wowbagger,

First of all, thank you for the options numbers, much appreciated.

FYI, POCSAG never appeared before but now is there in options with GPIB?

OK on some short cut design and native warble, I actually looked a the synth sync out (RTS on DB9 ) and they closely match and track the slow warble/wander.

Regards

Gerald

VK3GJM

[Wowbagger]

Yes, I decided I'd be nice and turn it on for you as well.

[VK3GJM]

Hi All,

I did not realise my last post was 5 years ago, time ticks away.

Anyway, fast track 5 years on, Lock down again V5.0 here in VK3, and time to spare. I finally have the COM120A fully restored. With that bit of spare time I re-visited the BB PCB and found 2 caps placed wrong way around, what was I thinking.

With better test gear now, I re-calibrated, and at last I finally have an error free unit. Next task is to tackle the timekeeper. My vintage 9338 build unit has a working DS1286 , date stamp is 9323, so it is a matter of when. I have a few DS1286's so more experimenting to ensure long Gevity.

Once right, it is stable and it isn't a bad machine, slow, it works. Calibration values are within documented levels, actually, deviations on TX and RX are very stable and less then 0.1% error for now.

Great support with endless suggestions, well done guys, especially David N0YKG.

Regards

Gerald
Vk3GJM