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Old 26th Aug 2019, 12:04 pm   #1
Blobcat
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Location: Retford, Nottinghamshire, UK.
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Default Marconi 2955 AC1 board issues.

Hi - I'm new to this forum so apologies if I display spectacular ignorance. But "hello".

I bought a Marconi 8955 at a radio rally recently about which I was told "power supply issue" and that's it. I can see a flicker on the screen when it starts but it's clear that the AC1 board CRT driver is pooped - big style.

Looking at the board, there's clearly been some serious overheating of R18, R19 and R34 and TR3 (regulator?) is melted. Having started replacing components, it's clear that the fault has taken TR1 and TR2 (video amp) with it too.

On the manual that I downloaded from the WWW, there's a hand written note on the scan "TR3 most likely to fail" - OK, well thanks to whoever wrote that

For those who've played with these instruments, is it usual for such a fault to take the video amp transistors with it? Should I be looking at something else too? There's some quite impressive overheating on this part of the PCB and I'll have to rebuild a few tracks.

Any help much appreciated. Thanks. Dave
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Old 28th Aug 2019, 7:53 pm   #2
Biggles
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Default Re: Marconi 2955 AC1 board issues.

Hi Dave and welcome to the forum. I have repaired a few 2955's in the past and one of the main faults was the line output transformer failing, but the fact that you have a flash of screen when you switch on may indicate the transformer has survived. If the whole thing fails to start, I would check the main system oscillator 10MHz? is running. The oscillator seems to drive quite a few things. Once over there was a LOPT mod kit but that was quite a few years back. Other common problems are the attenuators becoming intermittent, and of course the OM345 hybrid circuits failing, although there is a fix around for this I believe. Otherwise they seem quite reliable. Mine are both around thirty years old. I have a full genuine Marconi service manual if you need more info.
Alan.
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Old 28th Aug 2019, 8:04 pm   #3
Blobcat
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Default Re: Marconi 2955 AC1 board issues.

Thanks, Alan. Much appreciated. I'll replace all the components that look fried, plus the electrolytics on that board and - assuming no smoke - will start with the oscillator.
cheers
Dave
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Old 28th Aug 2019, 9:17 pm   #4
mole42uk
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Default Re: Marconi 2955 AC1 board issues.

Can you check that the 10MHz OCXO is running and on-frequency? That is the most critical part. There are a couple of failure-prone capacitors in the CRT driver board, but unless the 10MHz oscillator is running all bets are off.

Another point to note is that the CRT scans up and down, not side to side. It's peculiar until you get used to it.

Keep asking questions, there are a few of us on here who have experience of repairing the 2955 and we may be able to help....
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