Author Topic: HP 6113A repair -- quick note  (Read 1004 times)

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Offline TimFox

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HP 6113A repair -- quick note
« on: July 03, 2015, 04:46:16 pm »
I recently obtained an -hp- 6113A 10 V DC precision power supply:  a good deal on eBay in "used, untested" condition.
The first problem was that the meter had been pushed out of the front panel (possibly in shipment), which broke the flimsy plastic retainer (not repairable).  I fixed that with judicious use of Elmer's Stix-All cement, after zeroing the movement.
At first, it looked like the supply was working, but out of calibration.  After maybe 30 minutes, however, the voltage fell drastically and finally went negative.  Sometimes, the supply would re-start with a positive voltage, sometimes not.
I traced the problem to VR2, the 9 V zener used for auxiliary bias (including the current limit circuit).
Interesting clue:  the latest rev on the errata/manual changes list included (Change 13) this cryptic note:
"Change zener diode VR2 to 9 V, 0.005% TC, HP Part no. 1902-0785.  Note that the old zener diode (1902-0763) should not be used for replacement at any time."
The voltage across VR2 was wrong (roughly 2.5 V);  removing it and testing it with a simple voltage supply and resistor indicated that the diode was at fault.  Replacing it (on the main board) with a 40-year-old unused 1N936 seems to have fixed the problem.
The old diode cross-referenced to 1N2163A, and the new one cross-referenced to 1N936.  This is interesting, since the 1N2163A (0.5 W power rating, DO-13 metal hermetic package) looks like it should be more robust than the 1N936 (0.4 W, DO-7 glass package).
Try not to disassemble the oven:  there are no instructions in the manual and I hope I didn't mess it up too badly looking for VR2.  VR1 is inside the oven, along with the precision matched dual transistor Q1 and other parts.
 


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