[time-nuts] HP 5328 PSU nightmare... Or stupid engineer, you decide...

Mark Sims holrum at hotmail.com
Thu Sep 10 04:20:09 UTC 2009

Probably 90% of power supply overload problems are shorted dipped tantalum caps.  Start there...  check the resistance across each supply.   The other 90% are shorted pass transistors/base drivers  and/or zener diodes.

You mentioned that you have more than one of these units.  Compare reading between units.

To isolate a shorted tantalum cap when there are lots of them on a board,  I use a Valhalla 4650A digital igniter tester.  It is a fancy-pants 4-wire ohm meter that can read out down to micro-ohms,  but any good Kelvin connection ohm meter should work.  Check across each cap,  lowest reading wins...

I just fixed a Tek SC504 scope with a bad tantalum.  I checked all the caps on the shorted supply (I first eliminated those on the horizontal amp card by unplugging the card).  Found and replaced the shorted cap,  put it back together,  no more short,  powered up,  no joy.  

The previously good cap on the horizontal amp card went short after the first cap was replaced.  Sigh...  cracked open the Unabridged Dictionary of Drunken Russian Sailor's Curse Words and Phrases...  The once nice thing about bad tantalum caps is that their failure mode is seldom subtle.


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