HP 6825A Bipolar Power Supply - repair advice

[tony t]

Unit description: single +/-20V 2A power supply / amplifier

Bought a HP 6825A on eBay.  Unit mostly works.  Load regulation and current limit works as expected (LED turns on).  Problem I discovered is that positive output can only deliver about 1A before voltage regulation stops and voltages does not increase.  The Current Limit LED does not turn on, indicating that the "current limit circuit" is trigging and limiting output at 1A (I think). When I dial in negative volts, the unit can deliver over -2A as expected. Load regulation is good for negative load.
Input filter caps are good…  All power rails are at correct voltage when loaded.  Since this unit is 24 years old, it’s possible that some transistors or opamps are not working not 100% but have not completely failed.  Could one of the output transistor pairs be open?  How about weak driver transistors?  Don't want to tear the whole thing apart if not necessary.  Heavy transformer, etc...

Perhaps other 6825A owners can chime in and share their repair findings. Looking forward to any advice.

[rastro]

Hello Tony,

I have an HP 6826A +/- 50V Bipolar PWS (similar units).  While doing some functional tests it appeared to function OK .  However, while straightening out a bend frame it received in shipping I noticed a charred part on the bottom PCB.  This was caused by R60 a 1-ohm 3W resistor that had changed value  (reading close to 1.5-ohm) and heated up.  As you look at the front of the unit this resistor is on the left side of the bottom PCB near a large relay.  I replaced it to prevent further damage to the PCB.

Anyway, I may not be an issue on your system but probably worth checking. 

[Alex Eisenhut]

Manual's free

http://www.excaliburengineering.com/media/datasheets/Agilent_6825A_usm.pdf

more of a 40 year old design. Should be relatively simple to poke around in there, also old carbon comp resistors can drift in value (mostly up it seems).