old PDP/DEC PSU hardware question

Matt Burke matt at 9track.net
Sun Nov 17 18:55:32 CST 2019

On 17/11/2019 20:28, Dr Iain Maoileoin via cctalk wrote:
> I have a query out on VCFed, but I am not gettin gany interest.  Neither did my 2015 request on a similar topic - must be the wrong forum.
> Advice please: where is the best place to get some troubleshooting tips on debugging a DIGITAL H7202B power supply?
> I have 2 supplies that are giving me trouble.  I am competent at digital work, much less so on switch-mode and analogue
> (555 is an analogue in my books!).  I have variacs  scopes, logic analysers, voltmeters but no skill ;-(
> Any advice?

H7202 sounds like PDP-11/84 or VAX-11/730. Either way the schematics are
available on bitsavers:


I have repaired one of these before but there's no guarantee you have
the same fault. For me it was C30 at location D5 on sheet 2. This silver
mica capacitor had become leaky which caused fusible resistor R49 to
fail. This took out (what I'm going to call) the 'standby' power supply
and without that nothing works.

I would start by checking this 'standby' supply. With AC power applied
you should get +14V and -14V as shown at location D2 on sheet 2 even
when the main power supply outputs are off.


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