VAX 4000 PSU (H7874) Schematics

[Holm Tiffe]

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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