[rescue] Decstation 5000 Power Supply

[CJ Reha]

Hi Mike,

Though I unfortunately can't give advice on the wiring of new power
supplies, I can potentially be useful in bringing back the original. Much
like basically any other switcher from the 80s or 90s, those supplies
suffer from the output capacitors leaking. Even if there doesn't appear to
be any leakage, pull the caps anyway - in my experience many supplies will
have little pools of electrolyte under each cap, or the solder will sizzle
and smell like fish even if it looks OK from the outside. Sometimes it's an
oily "film" not easily seen unless you're really looking for it. I saw that
in your VCFed thread another gentleman posted about his machine with the
same problem, and he had a bunch of caps leaking on both the main board and
the auxiliary control board...so it's likely your supply has the same
ailment. Do check to make sure the fuse is intact - if not, there may be
more serious issues afoot - otherwise, I think a simple recap may get you
back on the road so to speak. What have you done on the supply to this
point?

Regards,
CJ

On Fri, May 31, 2024 at 7:54 PM Michael Dombrowski via rescue <
rescue at sunhelp.org> wrote:

> As I've mentioned on this list, I recently picked up a Decstation 5000/200
> and was mid-way through Ultrix install when the system powered off and
> wouldn't come back on. I opened up the H7878A power supply, but don't see
> any obvious faults. I'm slowly working through it, but I don't have any
> experience repairing switching power supplies, so this will likely take
> some time.
>
> As a temporary measure, I ordered a beefy 5v power supply and a -12/+12v
> buck boost converter. Unfortunately, I'm not able to boot. The power rails
> all are steady, and thermal imaging does not reveal any suspiciously hot
> areas. There are two other connectors from the power supply to the logic
> board: warning and POK. I've tried various combinations tying these to
> ground or +5v without success. Does anyone know the expected behavior of
> these circuits? I can find references for older Vax-en, but not for the
> Decstation.
>
> thanks
> mike
> _______________________________________________
> rescue list - http://sunhelp.org/mailman/listinfo/rescue_sunhelp.org
>
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