[rescue] Decstation 5000 Power Supply

[Michael Dombrowski]

 I spent some more time with the PSU tonight. I'm not getting voltage anywhere on the secondary side. I pulled out a thermal camera and the whole secondary side is cold, as are the mosfets. I don't see an IC to drive the mosfets, there is another control board with discrete SMD transistors, which I assume provides the same function. Would bad caps cause this behavior? I'm a little worried there is something wrong on the primary side.

mike

    On Friday, May 31, 2024 at 07:24:50 PM CDT, CJ Reha via rescue <rescue at sunhelp.org> wrote:  
 
 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. 

thanksmike
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