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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 13th Sep 2020, 12:31 pm   #1
coopzone
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Location: Cannock, Staffordshire, UK.
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Default Amstrad PCW 8256 Power supply fault

So, I've spent several days getting an old PCW up and running. Just got it to boot from floppy and into cp/m ok (either 3" or 3.5"). And now kapoof! PSU dead.

I left the machine on while i walked away to format a disk to try in it. When i got back power was off. Did a few tests and found the fuse had blown - not good. So i replaced the fuse (well you never know it mite have worked) and kapoof it blew as soon as the power was turned on.

Next did a few ore tests etc, nothing obvious, no short on the input etc. Still no good.

Next i wired in an old AT PSU +12 +5v only and powered it up from that, it works fine (screen flickers a bit on disk access, but ok). So at least i know it's the PSU.

So my question is - is there a guide or hints page to get the PSU working, the only thing i have found so far are people saying - give up, just use another PSU and strip the old one out (or at least disconnect it)

Derek
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Old 13th Sep 2020, 1:53 pm   #2
GeoffB17
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Default Re: Amstrad PCW 8256 Power supply fault

My understanding is that the power supply is eminently repairable, BUT, there are some specialised components in there that may be a problem getting hold of.

I take it there is no immed sign of something burnt or overheated. I've never had to do this myself, so this is based on things I've read on other forums (esp the CPC Wiki forum where there are a number of PCW users, and repairers) but I think the initial step is the check the voltage reading around the main PSU chip/IC as it's often just part of the power that has blown.

I think that CPC used to be the best source of spare parts, there were 8 million PCWs about back in their day, and power supplies going bang was a regular problem.

Oh, just in case of confusion, the two references to 'CPC' are relating to two quite different things

Geoff

Last edited by GeoffB17; 13th Sep 2020 at 1:57 pm. Reason: CPC initials
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Old 14th Sep 2020, 9:47 am   #3
Slothie
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Default Re: Amstrad PCW 8256 Power supply fault

Disclaimer: I know little about the PCW 8256 specifically, but I've looked at the schematic!
The PSU looks like a fairly standard switched mode supply. I note from the schematic I found there is a filter capacitor across the primary, and these are notorious for failing. Usually they self distruct a while after being switched on for the first time in ages releasing a lot of smoke, but its not inconcevable that it may have just failed with an internal short that blew the fuse.
The simple test is to just remove the capacitor, replace the fuse and try again. (After looking for suspicious skid marks or burnt looking components and leaky capacitors, which I assume you would have done already).
Beyond that debugging a SMPS is a far more complex task - not one I would relish but them I'm more of a digital guy than analogue!
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