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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th Oct 2018, 12:36 pm   #1
LARRY1962
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Join Date: Oct 2018
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Thumbs up Marconi 2955 fixed

I bought a Marconi 2955 in an auction, sold on an untested basis. On switching it on, it would only display a screen full of nonsense characters although some relays did click reassuringly.

I had read about the common 10MHz OCXO issue but in this instance the problem was simply resolved by cleaning the edge connectors on all the plug-in cards (AB2 to AB6) with some IPA and a small brush, as described by others. The display was instantly restored to normal and the issue did not recur.

At this point I could run the self-tests for the first time and was rewarded with FAIL results for virtually all of them!

I tackled the frequency counter first, feeding in a 19MHz signal from a sig gen with the 2955 set to TX test mode. The fault was quickly confirmed to be the common issue with the AA2 board - a failed OM345 - confirmed by checking for an amplified signal at the output, pin 5, and finding that the signal was heavily attenuated compared with the input at pin 1. I decided to buy some of the ready-made OM345 replacements (MMIC-based type) and replaced both IC1 and IC2 while I was at it. This board was fairly easy to work on except that I was very surprised how much heat was needed from the desoldering gun to desolder the pins of the hybrid - the ground plane acts a rather effective heat sink. Eventually I got the new modules in and the PCB neatly cleaned up. The linearity of the modules did not concern me much here because the signal is being amplified and clipped hard to feed the frequency counter. This repair fixed the frequency counter and the 2955 passed its first four self-tests. Progress!

Looking at the design, it's very important to have the frequency counter in good working order if you want the automatic detection of the signal frequency to be working, since this is what sets the local oscillator to the right value to generate the IF.

I then turned to the Power Meter. Checking the values displayed in TX test mode with an external signal from a sig gen immediately confirmed that the power meter values were way out. With an input of 0.7Vrms at 19 MHz, I was seeing a pathetic 133uW reported for TX Power. It took me rather a long time to find the cause. This level did change when the input level was changed but was clearly far too small. After some hours spent looking in the wrong places (interesting though this was), it turned out to be that little plastic and gold link between the AA1 and AA2 boards. I know this issue has been reported by others. I had already cleaned and re-seated the link but when I eventually checked it with an oscilloscope probe, it revealed itself to be a massive attenuator. So the power meter was telling the truth! Moving the link around did improve the contact but it was noisy and unreliable so I decided to go for the brute-force solution of soldering a short wire link across. I was able to squeeze the plastic link back on, over the top, so it still looked neat. This instantly fixed the issues with the power meter readings.

By the way, the 2955 automatic level control and power ranging circuits are very good at adjusting the signal level and getting a normal IF level out even when there is this kind of fault - this had misled me for a while as the IF looked perfectly healthy.

At this point, I ran the self-tests again hoping for some more passes but, no, the same fails were still there. This pointed to the signal generator also being faulty.

Checking the 2955 output level in RX test mode with an oscilloscope confirmed that the output levels were definitely too low. I opened up the AA4 tray and, now being more familiar with the OM345 issue, I went straight to testing the outputs of the two OM345 modules. Sure enough, one was faulty. This time, I decided to try adding a 4.7kohm resistor temporarily between pin 5 and pin 1 to restore the dc bias, as described in other posts, as this was easy enough to test without removing the board. This instantly restored the functionality of the faulty OM345 and the 2955 was able to pass all of its self-tests! I decided to fit the 4K7 permanently to the OM345 rather than disturbing the board by removing the apparently quite usable hybrid. I trimmed the resistor leads carefully and was able to make a neat job of soldering it directly to the OM345's pins. I suspect this may actually be more linear and closer to the original calibration than replacing the OM345 hybrid with a MMIC-based module.

With a happy 2955 that now passed ALL its self-tests reliably, I carefully replaced all the covers. It is great to have the working instrument on the bench the right way up again!

Thank you to everyone who posted their 2955 repair stories on this forum - they helped me enormously.
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Old 15th Oct 2018, 4:20 pm   #2
Phil G4SPZ
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Default Re: Marconi 2955 fixed

That's a really good result and a nice write-up. Well done!
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Old 19th Oct 2018, 10:41 pm   #3
Biggles
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Default Re: Marconi 2955 fixed

Thanks for the write up. The more threads we have on these elderly but useful bits of kit the easier it will be to fix them. I have had various faults with mine over the years but didn't know about re-biasing the dud OM345's with a resistor. I have got a couple of replacement modules in reserve for mine if and when they fail, but I will try that dodge next time. I would miss my 2955 as it has become a firm friend over the years right from when I used one in the trade.
Alan.
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