H786 power supply help wanted

[Jay Jaeger]

On 3/30/2019 2:25 PM, Charles via cctalk wrote:

> I have a PDP-11/23+ and the power supply (H786) "last ran when parked" a
> year or so ago. But there's no DC output at all today, and the fans are
> running so there is AC power...
> I also have the original H7861 that came with it, which had a blown
> chopper transistor. I couldn't find anything else bad, so I replaced the
> transistor and within a few seconds of running, it blew again. :(
> 
> So I need some help - I've never been good at fixing switching supplies,
> not to mention the high-side hazards.
> The simplest solution would be just to replace it with a working unit.
> Anyone got one to sell, hopefully cheap? :)
> If not, can anyone fix one or both of mine?
> 
> thanks!
> Charles
> 

Sorry, no spares (and certainly no tested spares.  ;) ).

My playbook for these sorts of things goes like this.  Others will
undoubtedly chime in with probably better advice.

1.  Is anything obviously burned / overheated / fuses blow?  Note those,
    and what feeds them.

2.  Check the semiconductors - often I will lift one leg of a diode
    or two legs of a transistor to make sure I get a good test.
    It's a pain, but usually worth it.  Sometimes, depending on
    what the schematic looks like, I'll test without lifting as many
    / any legs.

3.  Specialized regulators / op-amps, etc., require checking their
    voltage while running.  Not usually easy to test them.

4.  Check the capacitors, especially input and output for excessive
    leakage.  I often will reform input and output caps just
    because.   Check their ESR, at least with one of the
    cheap component testers out there.

I use one of these for testing, as well as an ohmmeter, etc.

https://www.ebay.com/itm/All-in-1-LCR-Component-Tester-Transistor-Diode-CapacitanceESR-Meter-InductanceBH/183706764001

I also use a "Blue ESR Meter" of late, as well:

https://anatekinstruments.com/products/anatek-blue-esr-meter-full-kit-for-self-assembly-besr_kit

But before I had these, I just used an ohmmeter to test each
semiconductor junction, and monitored voltage while reforming capacitors
to get an idea of their ESR.