Dead HP 6632B

[ndp]

Hi all,

I purchased two HP system DC power supplies (a 6632A and a 6632B) at my local electronics surplus place. Paid $20USD for them as they were both marked "no display, no output".

Upon inspection, both units were set to use 240V operating voltage. On the 6632A I just moved the internal switches to 120V, and it fired right up. No problems at all.

The 6632B has proven to be more of a challenge. I changed the transformer wiring as stated in the service manual to 120V. Now when I switch on I get a low-level transformer hum, but nothing else. No display, no output, etc.

The service manual isn't too helpful for this specific issue - in fact, I can't even seem to find the first test point it refers to. Could anyone advise on the first place to start checking voltages etc? Everything on the boards looks perfect, no sign of blown tantalum caps or the like.

Thanks in advance.

[TERRA Operative]

I'd check the transformer isn't faulty first. Do you get any voltage out of the transformer, and are any rectifiers testing ok (not shorted)?
Google will have the specifics of testing transformers.

[Dr. Frank]

Hello,
obviously you already read my repair report about the blown tantalum capacitors:
https://www.eevblog.com/forum/repair/agilent-6632b-fault-repair-and-several-design-flaws/

This defect might not be so obvious on first sight, so I recommend to first check the voltages at the fuses, as indicated.
The +/-15V supply and +5V also feeds the analogue side of the controller PCB, so that's the 2nd item to check.

Nearly everything  is inside the service manual, apart from the controller PCB.

As there is no display, the '+ 5V interface bias supply'  to this controller PCB should be checked.
This might also feed the display PCB.

Frank 

[jchw4]

Check that VFD is not broken.
Unfortunately most of the units with "no display" actually mean "broken VFD". And display is unobtainable.