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Compaq Portable Power Supply

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bluethunder

Experienced Member
Joined
Jan 5, 2010
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Calgary, Alberta
I drug my old compaq portable out, which I knew didnt work, and tried some basic troubleshooting on it..

Turned on.. Dead..

Checked for power on the floppy connectors, nothing.

Pulled all the cards, unplugged the mainboard and floppies. Nothing.

Unplugged the monitor power then I had +12 for a few seconds, then nothing again.

Wiggled some of the power plugs on the board, powered up and had 2 pops and the stink of burnt electronics.

After stripping the machine to nothing to get the PS out, I see that 2 tant. caps have exploded C39 & C37. 2.2-25 is written on the side of one of them thats still legigble..

I'm going to go an replace all the 2.2-25, and the 15-25 tant caps.

Is there any others I should be looking out for?
 
Not unless they're causing problems. I generally just let them explode when they want to, then replace them. You might even be able to replace them with electrolytic caps if you can't stand the noise.
 
Well, i replaced those 2 caps, and got the power supply working, screen comes up.. It even boots..

Now it turns out the keytronic keyboard is hosed. The foam pads have disintegrated.

Any one know if the compaq uses a same protocol as a standard xt keyboard?
 
I JUST got another Compaq Portable on Friday. I haven't torn it apart yet, but it looks like our power supplies have/had the same problem.

I looks like the 2 portables I have a re different revisions. The one I just got appears to be newer, and the cards are smaller and less populated. I'm not sure it the newer one is any better internally, but it does have a better case. At any rate I will be combining them into one and making a really nice one. I still gotta tear into it though to get that board fixed.
 
Chuck(G) said:
Not unless they're causing problems. I generally just let them explode when they want to, then replace them. You might even be able to replace them with electrolytic caps if you can't stand the noise.
Click to expand...

Electrolytic caps typically have much higher ESR. This may or may not be a problem, but if you use electrolytics I recommend a .1uF ceramic cap in parallel with each. They're small enough to be tacked on the bottom of the PCB.
 
james1095 said:
Electrolytic caps typically have much higher ESR. This may or may not be a problem, but if you use electrolytics I recommend a .1uF ceramic cap in parallel with each. They're small enough to be tacked on the bottom of the PCB.
Click to expand...

In most of the applications I've seen, the tantalum is used for decoupling, with a 0.1 uF film cap at every IC location for bypass. ESR in this case shouldn't matter--what you really want is storage capability to keep the DC supply stable.

If these were filter caps in the power supply, ESR can matter a lot.
 
Ever since I replaced the caps in the PS, it has been powering up fine..

Too bad about the QUADRAM that was in it, the battery leaked, and the machine won't boot with the card in place. A little text error, and thats it. I might try cleaning it up, and see if I can make it work. I went and desoldered the clock battery in my PS/2 at the same time, before it puked everywheres too.

Sure glad, at sometime in the past, some one replaced the 6000 torx screws with phillips heads. That thing was engineered to survive an atomic strike I think.

Now, to try hacking a connector up to connect a standard XT keyboard to it, until I get around to rebuilding the keytronic keyboard foamies.

Does any one have the SAMS book on the compaq portable? Is it worth getting to repair these machines?
 
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The electrolyte in NiCd cells is alkaline, so a scrub with white vinegar and a toothbrush is a good start, followed by a rinse with warm soapy water. Corrosion can bridge pins or track up into the IC body and rot out bond wires so I usually replace any ICs with significant corrosion on the pins and clean the crud out from under them.

The first thing I do with any "new" old computer, arcade game, pinball, or other item with a backup battery is pull the battery, most of the time if they're still there, they've already leaked and sometimes the damage is extensive. To make matters worse, it alters the solder it gets on, significantly increasing the melting point making it more difficult to remove parts without damage to the pads.
 
Chuckster_in_Jax said:
Does this look familiar?

http://yfrog.com/ht002kpj
Click to expand...

Well, this post got me motivated to work on my Compaq Portable. Replaced the blown tantalums and it still wouldn't power up. Noticed that the neck of the CTR glowed for a brief moment then turned off. Unplugged the power connections to the video board and the motherboard and the power supply started working. Checked the mobo for shorts and found continuity between +12v and ground and -12v and ground. There were tantalums across those supplies so I removed them and the short is now gone. Hooked the mobo and video back up and I get 2 beeps from the speaker. Looks like the unit is running again so I put the other cards back in and power it up. I get a loud snap and a burned smell. Did I drop a screw during reassembly? Turned the computer upside down and a small wrench falls out.

WTF!!!!! Now the thing is dead again.

Don't think it's going to be so easy to repair this time.
 
Oh damn!

Hopefully its another obvious repair, with some really cooked components. Where did the wrench fall out of?

Don't forget to check the fuse that's in the 3 pronged AC plug. Might have gotten lucky, and only blown that.
 
Took another look at my Compaq Portable today. Since I got it I found the following shorted tantalums.

2 in the power supply
2 on the floppy controller(one burst into flames when I powered the unit)
2 on the main board
1 on the Video Adapter

That's seems like an awful lot of bad tantalums.

The unit will power up, but halts with a screen full of characters and a continuous beep from the speaker. The service manual says the continuous beep indicates a bad power supply.

I took voltage readings with my multimeter from one of the ISA sockets on the main board and got this:

+5v = +5.07v
-5v = -4.97v
+12v = +12.36v
-12v = -12.05v

Those look OK to me. The -5 was a little low. Don't know if that would be a problem.

Tomorrow I'll do some more troubleshooting on the MoBO. See if there is a problem with RAM. The first 2 banks are soldered in and the other 2 are socketed.
 
I have a couple of questions before I continue troubleshooting.

1) Given the voltages I read off the main board and the fact that I get a continuous beep from the speaker several seconds after the computer is turned on, could I assume that the CPU is up and running.?

2) Is it possible that the diagnostics are running and just get to a point before the video ram is initialized and trigger the speaker to beep. Or is it more possible that there is some part of the circuitry that will cause the speaker to beep if it doesn't get a response from the CPU?
 
Is there a Power Good line from the power supply? Most PC power supplies have that, it goes high once the PSU detects that the voltages are stable, which tells the motherboard to release reset and start up. If the PG circuit is bad, the machine will detect a bad PSU. It's been years since I've been inside a Compaq portable and Compaq machines tend to be a bit odd in a lot of ways.
 
All a continuous beep from the speaker informs you of is that your speaker is working, and that at least some of the circuitry that generates tones for the speaker is working.

On either an IBM 5150 or 5160 motherboard (could be both), simply removing the BIOS ROM will result in a continuous beep from the speaker.
 
Chuckster_in_Jax said:
Took another look at my Compaq Portable today. Since I got it I found the following shorted tantalums.

2 in the power supply
2 on the floppy controller(one burst into flames when I powered the unit)
2 on the main board
1 on the Video Adapter

That's seems like an awful lot of bad tantalums.

The unit will power up, but halts with a screen full of characters and a continuous beep from the speaker. The service manual says the continuous beep indicates a bad power supply.

I took voltage readings with my multimeter from one of the ISA sockets on the main board and got this:

+5v = +5.07v
-5v = -4.97v
+12v = +12.36v
-12v = -12.05v

Those look OK to me. The -5 was a little low. Don't know if that would be a problem.

Tomorrow I'll do some more troubleshooting on the MoBO. See if there is a problem with RAM. The first 2 banks are soldered in and the other 2 are socketed.
Click to expand...

As far as voltages go, those all look ok.

What area did the wrench fall out of, it sounds like a chip got cooked when it shorted.

If you can find an ISA POST card, they are the gateway to fixing all kinds of problems. If you get a landmark or similar type of card that has ROM based diagnostics, they are even better.

At the very least, they will tell you if the CPU is running, and what point the computer fails the POSTs. Or if its running at all.
 
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