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COMPAQ Power Supply

Are you checking the motherboard or the power supply. I don't see any device listed as CB3 or CB4. Check the pins on the motherboard where the power supply connects. There are four voltages supplied. The service manual shows what voltages should be present as well as the proper ground for each. Put your meter across these pins and see if you get a low resistance. Keep in mind that any capacitor that is connected across the supply voltage could be shorted an it is difficult to determine which one is bad. The ones I found to be bad were the tantalums very close to the power connection on the motherboard. The schematic shows those to be located at I27, J27 and K27 on the gridtrace pages 12 and 41 in the service manual.

I've got commitments all day, so it will be late tonight or tomorrow for me to reply again.
 
Amazing how much an "8" can look like a "B", isn't it?

I discovered that C83 has traces connecting the terminals, but C84 has no excuse.

R22 has no resistance, and C85 just kinda sits at 3 ohms.

I will look at the service manual again.
 
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OK, looking at the schematics:

Test the following at J107. This is the power connector on the motherboard.

Pins 6,7,8,and 9 are ground. Connect the negative lead from your meter to any of these pins.

Now, place the positive lead on the following and check for shorts:

Pins 11,12,13and 14 are the +5V supply and are all tied together. Short here suspect C79

Pin 10 is the -5V supply Short here suspect C81

Pin 5 is the -12v supply Short here suspect C83

Pin 4 is the +12V supply Short here suspect C77

If you find a short on any of these cut one lead on the suspected tantalum in such a way that you can resolder the lead if needed.

Retest to see if the short is gone.
 
It was C83! I'll look through the parts list so I can get a replacement. Is this something I can get at Radio Shack, or is it more of a mail order part?
 
It's a 10uF tantalum. I always go higher on the voltage rating, so I suggest 25v or 35v and suggest a high reliability series. Radio Shock only carries a couple of tantalums and neither have the proper values. There are several places to mail order. I usually use Mouser or Digikey.
 
Well, At a local shop I saw one that's 16v and 10 uF, I don't know about high reliability, but would that be suitable, or should I just go ahead and mail order?
 
Jtterbug said:
Well, At a local shop I saw one that's 16v and 10 uF, I don't know about high reliability, but would that be suitable, or should I just go ahead and mail order?
Click to expand...

It will probably work, but personally, I would order a good quality with a voltage rating of at least 25v. Some of our other members may want to make a recommendation.

When I repaired my portable 1, there were 7 or 8 bad tantalums. So, be prepared to find other bad caps on the other boards. You may be lucky and that is the only one.
 
Jtterbug said:
About how long would the 25 volt version last over the 16 volt one?
Click to expand...

Who knows? As long as the applied voltage is under the rated one, you should be okay, but you won't have much headroom. Temperature is always a factor--a hot box capacitor will age prematurely. One subjected to a high ripple current will age prematurely. A low ESR electrolytic would also probably work just fine in this application and be easier to find.
 
Well, I found the right specs around town, 25V.
I hope to get it tomorrow.
In the meantime, are there to be any empty chip sockets? There are two on my machine?
 
What size? If one is a 40 pin DIP, it's certainly a missing NDP (8087), which the system doesn't need to operate. Can't say about the other one--unless it's a 24/28 pin socket--those are usually for optional BIOS extensions.
 
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