VAX 4000 PSU (H7874) Schematics

[Bill Degnan]

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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