Cambridge Workstation woes - VMS 2691 PSU smoke

Jules Richardson jules.richardson99 at gmail.com
Sun Jul 30 14:17:21 CDT 2017


On 07/30/2017 11:28 AM, Tony Duell wrote:
> On Sun, Jul 30, 2017 at 5:18 PM, Jules Richardson via cctalk
> <cctalk at classiccmp.org> wrote:
>> On 07/30/2017 10:59 AM, Ian Frost via cctalk wrote:
>>>
>>> Any body any experience in fixing these old Acorn PSUs - or managed to
>>> source alternative modern supplies?
>
> In general smoke from a power supply that keeps on working is from the
> mains filter capacitors. Some makes are well-known for this. I would
> carefully examine them, they crack when they fail

Aha, I missed the "kept working" bit... agreed there, then, it's probably 
one of the input caps which failed (and that seems consistent with brown 
goo on the PCB, too).

> The former are the ones that normally fail, but I would change the lot.

I'd be inclined to agree in the case of any class X/Y caps as they're a 
known failure point in any piece of vintage equipment... but not every 
single cap in the PSU. I know that's not what you're suggesting, but there 
seems to be a trend amongst collectors to blindly replace every cap in 
sight as part of some "refurbishment process" these days, whether they're 
good or bad. Personally I like to keep my systems as original as possible, 
which generally precludes replacing perfectly-good vintage components with 
perfectly-good modern ones :-)

> I certainly traced out the schematics of mine, but I forget which PSU was
> fitted to my machine (something tells me it was made by Farnell).

That could well be - I do think that I've seen another type of PSU used in 
addition to the VMS one, and Farnell would be an obvious choice.

cheers

Jules



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