HP 1652B scope/logic analyzer power supply repair




Introduction

HP1652B logic analyzer
My HP1652B logic analyzer in use

I had been using my HP 1652B scope/logic analyzer for many years, and it was with sadness one day in 2015 that I found out it would not power up and had a problem.  I could hear a soft ticking sound and smelled a burning smell.  I did some searching and found only this account of someone repairing theirs.  In that case, it appears the capacitors had gone bad.  I hoped mine would be the same issue and started the repair.

With the benefit of hindsight, the best way to access the power supply module is as follows:
HP1652B parts
Exploded view of the HP1652B from the Service Manual.

You should then see the image below.

HP1652B Power Supply
Photo of the component side of the board.

Starting with the red connector in the top left and going roughly clockwise, this is the function of the parts as far as I can tell (note 'left'/'right'/'top' etc refer to the orientation in the above image):
I first decided to try and isolate the fault by disconnecting most of the power diodes from their output heat sinks.  With this process I was able to isolate the problem/short to the bus that is near the bank of 8 identical filter capacitors.  These are 1000uF electrolytics.  The vertical 'fin' near the transformer appears to be one of its output terminals, and four diodes bridge from this fin to two horizontal ones adjacent to the octal cap bank.  I removed the bottom four caps in this octal bank and saw quickly that there was a short between the case of C8 and a trace below.  There was a prominent burn mark on the trace.  See below for an image.

Damage to power supply
The damage/short area on the board is right by C8.

I tested all four of the capacitors from C7 through C10, and found they all had a good value (about 1100uF).  Their ESR all tested to be ~0.05 ohms, and their insulation resistance was in the MegOhm range.  As a result, I judged them all undamaged and installed them back into the board.

On power up, the unloaded power supply board still produced a soft ticking noise, but no burning smell.  I reasoned that the Switched Mode Power Supply may need a minimum load, and was perhaps shutting down due to lack of load.  The voltages I measured on the output connector was as follows.  Top reading is on the left most terminal:
I then powered down and plugged in the scope.  On subsequent power up, success!  The fan spun up and the passed self-test.  I then reinstalled the disk drive and the cover and the test after that the system worked normally.




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