HP 54603B power supply voltages wonky

[trjw]

Hi everyone,

I have an HP 54603B oscilloscope that has stopped working and I’m posting here because I see lots of success stories with repairing the 54600 series (unfortunately none of them seem to have the problem I’m having).

I was away for about 6 months and when I got back I found that it wouldn’t turn on. There was no display on the screen and the fan sounded like it was running slowly. It’s been sitting in one spot on my bench so there’s no chance it got dropped or had something fall in it. I followed the troubleshooting instructions in the manual, including building the dummy loads, and I’ve isolated the problem to the power supply. Under load, all the voltages are too low and they fluctuate up and down over a 2-3 second period. I also read a significant AC component:

+5.1V rail: 0.69V - 0.75V (and 0.46VAC)
+15.75 rail: 3.1V - 3.25V (and 1.2VAC)
-15.75rail : -3.73V to -3.9V (and 1.0VAC)

My meter also reads 20Hz on all three rails when set to frequency mode. This is all according to my Fluke DMM (I wish a could put a scope on it, but…).

There’s a chattering/buzzing noise coming from somewhere on the board (at first I thought it was the fan but I hear it even when that’s disconnected). It sounds like it could be at 20 Hz too.

I suspected the electrolytic capacitors - I had to desolder some to get an accurate reading but they’re all within spec except one that measures 25uF (in-circuit) when it should be 12uF. There is a bit of brown stuff on both sides of the board but I think it’s flux.

I’ve also measured the inductors on the DMM resistance setting. They show less than 1Ω of resistance as expected. The dual diodes I could get at seem ok too. Bridge rectifier output is 170VDC.

Short of measuring every component on the board, where should I go from here?

[wlzed279]

Hello TRW,

I realize this thread is over a year old but I just acquired an HP 54600B oscilloscope that is presenting the exact same issue. The voltage reading are the same, the fan spins but it does not seem like it is full speed. I am also getting a faint whirring sound with the fan removed.

Were you able to get to the bottom of this issue?

[Runco990]

I've had many of these scopes.  FIRST, ohm out the damn Tantalums on the main board.  If none are shorted or discolored, than check the capacitors in the power supply.  They do leak underneath, although I have not seen this too much.
You can also unplug the CRT display and see if the rails come up correctly, in which case the problem is there... although very rare.  MOST of my scopes I fixed had a shorted Tantalum on the main board.

Always start with the easy and simple things first.

[wlzed279]

Hello Runco990,

Thank you for the reply. I have restored a few vintage tube and solid state audio components, but this is my first venture into a piece of test equipment. I hope I am not in over my head on this one. I have started checking the electrolytics on the power supply and so far no issues there. I pulled the ribbon cable from the display board and I am still getting low voltages on the rails. I will look at those caps you mentioned on the main board.

[minsik]

Re the caps, I was repairing a Leitch SPG recently and replaced a failed fan. All PS caps measured with ESR meter as perfect and well  within  specs.
However for completeness just decided to replace caps anyway. When desoldering several they gave off an instant pong and there was signs of corrosion/discolouration on the  solder pads underneath, yet the caps top and bottom looked like new.   
Very happy I went through the  exercise of replacement rather than just checking! all test methods can only do so much.  For a few dollars of caps just replace the damm things i reckon.

Minsik.