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Old 14th Nov 2016, 6:25 pm   #1
Biggles
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Default Another Marconi 2955 repair

I had one of these radiocomms test sets in to repair for a friend recently. It failed every self test apart from the last one in the list. After replacing two of the usual faulty OM345 chips the unit passed a few of the self tests but was still showing wildly inaccurate readings on AF voltmeter, demod and power. This turned out to be caused by failure of the -12V supply on the AB5 board, which does some of the clever processing. A 220 microfarad decoupling capacitor had gone short circuit and taken out the filter choke which was in series with it. Replacement of both failed components restored the test set to perfect order. Calibration was checked against one of my own units and was found to be acceptable. Apart from the line output transformer and OM345 faults which seem common, this is the first time I have had a capacitor go short on one of these, although it's not surprising as they are probably 30 years old by now.
Alan.
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Old 14th Nov 2016, 11:09 pm   #2
Nymrod121
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Default Re: Another Marconi 2955 repair

I used one of these a fair bit during 1989-90; the BBC's Local Radio stations had been the subject of a re-allocation of VHF frequencies for 'studio to/from reporters's radiocar' talkback links. Selective calling (ZVEI in this case) functionality was extremely useful. What began as a turnkey project got a bit complicated due to conditional stability issues with some of the Band III mobile equipment; once those were resolved, myself and an engineer from Glasgow OB Comms went round the country converting 21 base stations and 28 mobiles in just under four weeks. Busy, busy ... !!
Good to hear you've got this going again ...
Best wishes
Guy
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Old 15th Nov 2016, 6:53 am   #3
mole42uk
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Default Re: Another Marconi 2955 repair

I've got three of those.
One works.
Parts of the other two have been in Saltash, Cornwall for a couple of years.
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Old 15th Nov 2016, 6:13 pm   #4
Biggles
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Default Re: Another Marconi 2955 repair

They were the industry standard for a lot of years in the radio comms field. I used one on a daily basis for about twenty years. Easy to drive and manageable weight of about twenty five Kgs. One lived in my vehicle for much of that time while I was looking after radio sites. It got quite a bit of stick and occasionally fell over (I used to use it sitting up on end). Good bits of kit although they are showing their age now. Marconi used to do a repair and cal for a fixed price of about £250 if I remember correctly, in the early days. They are very fixable, no SMDs and quite a good service manual available. The OM345s are virtually unobtainable though, and unfortunately are a common problem, although I believe there is a "fix" available using a small module instead of the chip.
Alan.
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