[Compaq Portable III] Can I replace a 1uf capacitor with a 2.2uf?

[Coder]

I'm taking a break from working on the CRT board/power supply on my VP by working on another project I have. It's a Compaq Portable III and similar to my VP, has a no-video problem. I've traced it down to the board on the back of the plasma display. It has a very obviously bad 1uf 250v capacitor. I don't have any 1uf 250v, but I have a cracked screen from a Compaq 386 that has a 2.2uf 250 capacitor. I'd like to know if in this case it would be close enough to work without damaging things.

I can order the cap, but I'll probably wait until I have other components to buy as the shipping for a single cap isn't really worth it. In the mean time, I'd like to see if the screen is good or not.

I was able to test all the other components (motherboard, video board, PSU, etc) because I have another with a good screen. But I don't want to risk damaging that screen to swap controller boards. The ribbons are very fragile feeling.

 

[DeltaDon]

Why not clip the cap's leads near the cap and then use jumpers to your test 2.2 to see if that fixes the issue. Then decide if ordering a new cap is worth the trouble.

 

[pkhoury]

Glad I saw this. I just ordered some caps to redo a Compaq Portable III (I have 3 or 4 of them, none that power up). When I applied power, you hear a click, the power LED briefly turns on and then off.

That being said - to the OP, have you recapped or needed to recap the PSU at all? I have yet to take mine apart, but have a big Mouser capacitor order coming tomorrow (to recap a bunch of other things). I'd really like to get at least one working with all the parts I have.

 

[Coder]

I did on a Sharp PC-7100. In my case one of the big filter caps was bad, so I replaced both of them. The power board also had some cracked traces near where the AC power connector was. I repaired those by bridging the cracks with some wire. It isn't very pretty, but it got it working.

The method I used to fault find was to first test if I had the expected low voltages on the output side. When I didn't have those, I started checking continuity of the circuit from where the A/C power connector was. It didn't take very long to find a problem. I didn't even have continuity from the connector to the other end of the first trace. The trace itself looked completely fine at first glance, but multiple double checks verified that it couldn't be connected, so I looked closer and could see a very fine crack around the solder joint. Once I bridged the crack with some wire, the PSU would power on, but would make a low pitched, yet very audible, buzzing sound. I suspected the big filter cap since the sound came from that area. Touching it verified that something was wrong with it. I think it was arcing internally. I opted to replace it, and that restored the PSU to working condition.

All that to say; replacing the caps won't hurt as long as your confident in your component removal/installation skills, but I recommend trying to narrow down the issue first. These computers were moved a lot, so it's not uncommon for them to have cracked traces or solder that is no longer bonded to the pins. Especially on PSUs and other parts that go through repeated hot/cold cycles.

Good luck! I hope you figure it out. They're great little machines. If I can help in any way, just say so.

 

[Twospruces]

I know i would certainly give it a try. My vote would be that 2.2 vs 1 uF is a wash. 250V is a must though.

 

[Coder]

I know i would certainly give it a try. My vote would be that 2.2 vs 1 uF is a wash. 250V is a must though.

I did end up replacing it with the 2.2uf 250v. Unfortunately it didn't make a difference and the screen still doesn't work. I bought my first oscilloscope a couple weeks ago, but life's been busy, as it likes to do. So my toys have been sidelined for a while. Thanks for the input though, it makes me feel better about my decision.

 

[pkhoury]

All that to say; replacing the caps won't hurt as long as your confident in your component removal/installation skills, but I recommend trying to narrow down the issue first. These computers were moved a lot, so it's not uncommon for them to have cracked traces or solder that is no longer bonded to the pins. Especially on PSUs and other parts that go through repeated hot/cold cycles.

Good luck! I hope you figure it out. They're great little machines. If I can help in any way, just say so.

Did you replace tantalums on the PSU daughter boards? I finally took apart my Portable III within the past hour. I somehow forgot to order tantalums in yesterday's Mouser order, so I gotta wait until tomorrow to replace 'em on the motherboard, as well as the PSU.

Better yet - did you replace ALL the caps on the PSU? And what's that silver looking cap in the center of the PSU?

 

[Coder]

Sorry, the PSU I repaired was for a Sharp PC-7100, not a Compaq Portable III. I would check the tatalums for shorts. When tantalum caps fail, they short (and tend to pop or explode). That may be why yours seems to power for a second and then go out. I only replaced the large filter capacitors on mine, but replacing all the electrolytic caps wouldn't be a bad idea. As far as the silver looking cap, I'd have to see what you're talking about. Can you post a picture? I don't currently have mine apart.