[Fixed/Closed] HP 6632A blowing fuses and shorted caps

[dermitza]

Hi all,

I (sort of) recently aquired a 6632A with a broken fuseholder thinking I got myself a semi cheap PSU with one part from Digi-key (Well.. lol). In fact, only the fuse drawer was missing.

Once replacing it, the problem is obvious.. the PSU is eating fuses like candy. I went through the service manual and traced the closest steps I could find to this problem but nothing looks bad in that respect.

Just today I thought I'd probe all the caps for continuity (note: the PCB was disconnected from everything and bare). I found C101 and C103 (two of the large output caps) shorting out. I'm not so convinced however.. What would you recommend the next step be? Desolder and check continuity again? Check some other part first? Any help is greatly appreciated. 

[singapol]

One important thing to remember is if this or any electronics keep blowing fuses is that something is shorted and to protect it from further damage the fuse is designed to blow or open. Basic troubleshooting begins with
measuring power supply voltages but in trhis case the whole or part of the unit is down. The best way is to have the service manual which have a troubleshooting flowchart that tells you where you should begin and follow the steps whether yes or no branching out to possible results. Most  of us will start with a visual inspection and a smell test yes your nose can be helpful for burned/scortched parts like blackened pcb
and components. Then a resistance test ( power off!) but don't mistake a charging capacitor with low resistance and going up again as shorted. If it is really shorted it will stay shorted but of course to check if it is a ground point or not ( false measurement). Next is testing the power transistors with resistance or diode measurements. Also look for cold solder joints and bad connectors or broken wires and coper tracks. That's a start, good luck.

[Kleinstein]

Broken output caps alone should not cause fuses to blow, as the supply is supposed to be short circuit proof / current limited. So there is something else wrong.

One thing to check would be the rectifier - this usually works on board.

[PTR_1275]

Looking at the service manual, there is a fuse at most of the transformer outputs. It might be best to disconnect the transformer outputs and see if it is still blowing fuses, if it is then disconnect the inputs to the transformer. If it stops blowing fuses then you have a shorted transformer. If it still blows fuses then there is a issue with the input side (Very unlikely as it is pretty much a on / off switch and the voltage selection switches).

If when you disconnect the outputs of the transformer it stops blowing fuses, reconnect one section of the power supply at a time and see which one starts to blow the fuses. Process of elimination will give you a very good starting point of what area to investigate.

Have you got any photos of the power supply board that you can put up here?

[dermitza]

Thanks for all the help so far. There is no sign of burned/faulty components or traces (visual & smell test). The rectifier diodes all test ok. I did not clarify that only the input fuse blows repeatedly. All other fuses test OK.

If when you disconnect the outputs of the transformer it stops blowing fuses, reconnect one section of the power supply at a time and see which one starts to blow the fuses. Process of elimination will give you a very good starting point of what area to investigate.

Thanks for that! I tested in this way, both secondary supplies run OK, the unit, fan and display power up. So the problem must be after the primary transformer output(s).

C101 and C103 are still showing short-circuit. So I'm guessing it can be either of the components between XTIP2-1 and XTIP2-3 (see snip). I think it now makes more sense to start desoldering the caps and checking that the short goes away, what do you think?

Photos (sort of crappy) of the PCB are attached.

Thanks for all the help!

[Seekonk]

Use a 100W lamp in series with the AC power for testing.  Then you can see power on results without damaging much.  I would suspect the diodes.  I haven't seen many capacitors dead short.

[dermitza]

On a further look, the shorted rails, XTIP2-1, XTIP2-3 also short to the -OUT. This is probably due to R188, so is OK.

[pinyoro]

I once saw 2 shorted smoothing capacitors on an amplifier so it I possible to have a shorted capacitor. Best to remove and test out of circuit to make sure.

[dermitza]

Did some desoldering, C101 was shorted and had leaked out (moneyshots below). After removal the short has disappeared. If thats the only problem then easy fix .

On a second note, I cant help but think that C134 has little bubbles suspended in liquid (electrolyte?) under the plastic top cover. Tried to capture it in pictures, but I dont know how noticeable it is. Should I replace that one too just in case?

Thanks for all your help

[PTR_1275]

Awesome news that you have found a problem. Fingers crossed that was the only problem. If in doubt, I would replace the other capacitor. If you've had one go leaky and short circuit, I personally wouldn't trust many of the others.

[Macbeth]

Definitely replace both caps. The partner in crime has surely been stressed by his friends ill health before his death. Indeed C103 may be harbouring a grudge at all the extra work he has been put to and decide to suicide bomb all over the PCB any time soon. 

(Really while you are at it you should just replace all the Electrolytics on such an old piece of equipment as a matter of course. It's peanuts compared to what happens when they leak and destroy the PCB. Just check some of the Keithley 2001 threads...)

[dermitza]

It was indeed that cap making a short. Having replaced all electrolytics (as a precaution) the power supply works a treat. Thanks everyone