VAX 4000 PSU (H7874) Schematics

[Aaron Jackson]

Hopefully you have changed the caps before the leak all over your board
:)


Bill Degnan writes:

> I have one or two mostly new 4000 supplies here, plus two working 4000's.
> Did not know until recently this was an issue, I guess I have been lucky.
> b
>
> On Fri, Jun 8, 2018 at 9:04 AM, Holm Tiffe via cctalk <cctalk at classiccmp.org
>> wrote:
>
>> Aaron Jackson via cctalk wrote:
>>
>> > > On Thu, 7 Jun 2018, Aaron Jackson wrote:
>> > >> The VAX turns on and the status LED stays on F. The DC lamp does not
>> > >> illuminate on the PSU. 5V rail appears fine but there is nothing from
>> > >> the 12V rail. There were some *very* dodgy looking caps which I have
>> > >> replaced, and some *very* exploded MOSFETs, which I have also
>> > >> replaced.
>> > >
>> > > If that's all that you replaced, no wonder it still doesn't work ;-)
>> > > Usually shorted power transistors in SMPSUs also break one or more
>> fusable
>> > > resistors, maybe some diodes, maybe the driver IC or transistors.
>> > > So if you have anything that "exploded" in the power supply, be sure
>> there
>> > > is more work to be invested that replacing the obious parts.
>> > >
>> > > Christian
>> >
>> > Haha, okay, seems reasonable. I'll have to probe about a lot more
>> > then. Not giving up quite yet, even though everything I have read about
>> > this power supply has said that it is difficult to repair. So far, I
>> > would agree with such a statement.
>> >
>> > Thanks,
>> > Aaron.
>>
>> Difficult to repair? Probably since it is the worst switching PSU Desing
>> I ever meet, at least from seriveability and the mechanical point of view.
>>
>> I own an 4000/300 with an working again PSU (after some capacitor
>> changes) and I habe an PSU from a friend here..its works sometimes,
>> mostly not.
>>
>> Regards,
>>
>> Holm
>>
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>>
>>


-- 
Aaron Jackson
PhD Student, Computer Vision Laboratory, Uni of Nottingham
http://aaronsplace.co.uk