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Compaq portable 486c PSU issues

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w1ll14m

New Member
Joined
Aug 15, 2011
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Dear fellow vintage computer fanatics!

Recently i've acquired a Compaq portable 486c. Now after turning it on it turned out that the powersupply is likely bad. I see the power led flashing for a short moment, that's all it does. So I decided to open her up and have a look.

Now I've disconnected the PSU and opened it up, no bad caps so no quick fix unfortunately. A closer look revealed that when I apply power to the psu I see the fan running for a short moment (let's just say 100 milliseconds), then after 3 seconds it "retries".
I'm pretty resourceful when it comes to working with electronics but I find it very hard to troubleshoot an SMPS. Also I would be interested in the pinout of the 14pin power connector from the PSU to the power distribution board, I'm dying to see if it's working so a temprary hack would be great.

Best regards,
William
 
Did you try looking for a replacement power supply that you could install in this machine? Power supplies are some pretty dangerous beasts you know - you could easily get zapped by one of them if you didn't know what you were doing.
 
Just make sure you don't touch or short the primary side heatsink or you'll get a nasty surprise. Most PSUs have bleeder resistors so it should be safe a few seconds after you remove power from it. You can tell which ones do by checking the X capacitors and the big line caps to see if there's a resistor in parallel with them.

If there isn't, the PSU can take much longer to discharge completely, as it has to rely on parasitic draw of components to wind down.

Try tossing a load on the PSU and see if it makes a difference. A bunch of hard drives or fans should do the trick.
 
@SoftPCMuseum:
I did have a look for a replacement, however these are pretty rare due to the age of this machine and if there are any (found one) the cost around 100 dollar, which is quite high considering the fact that I don't now the state of the other components.

Thanks for the warnings! I can't say that I know what I'm doing but I do know a bit about electronics. Luckily this PSU has a bleeding resistor which works, I always check the voltage on the main caps before touching anything else. When the power supply is in the machine (so it has a load) it does exactly the same. I see the power led light up for a fraction of a second and the fan near the motherboard does a few turns. This repeat every 3 seconds so I think it's hitting some kind of protection and shuts off.
 
w1ll14m said:
@SoftPCMuseum:
I did have a look for a replacement, however these are pretty rare due to the age of this machine and if there are any (found one) the cost around 100 dollar, which is quite high considering the fact that I don't now the state of the other components.
Click to expand...

where are you located?

I was collecting these for a while in the early 00's, and have about a dozen 486/486c's

I still use one for a prom programmer that interfaces through an ISA card.
 
If the power LED is flashing and Fan starts for a brief moment then it is imho pretty unlikely that the PSU is bad. At first I'd remove all cards, drives, periphals whatever and try again. WIth a multimeter you can also proof if the power supply is providing any power at all - if yes then the error is more likely to be on the main board.
 
Thanks for all the replies! It's greatly appreciated.

@Al Kossow: I'm from the netherlands. If it is not to much trouble I would like to know if the PSU can run without any load attached (mine sure doesn't, not sure if this is by design). All pre ATX power supplies that I've came across all work without any external load attached. Also I would be very interested in the pinout of the PSU.
@kyodai: The fan within the PSU also does a few turns even when it is only connected to the mains with a multimeter attached. I still have the feeling that the PSU might be at fault here. I'm trying to figure out the pinout of the power connector so that I might be able to hook up another power supply to test the machine itself.
 
I am thinking same as @kyodai. Something on the mainboard maybe shorted or near a short. I have run into a bunch of IBM PC's and XT mainboards that had shorted Tantalum capacitors on them as well as a Compaq Portable. Even expansion cards as well. Just a possibility. If you can get access to the mainboard or the expansion boards those would be my prime suspects. So good luck
 
w1ll14m said:
...I see the power led light up for a fraction of a second and the fan near the motherboard does a few turns. This repeat every 3 seconds so I think it's hitting some kind of protection and shuts off.
Click to expand...

Bad caps in the PSU can cause a "power cycle", ie: Flashing LED, PSU attempts to start, The faulty caps won't charge fully and the power goes down and the cycle repeats, Usually a dead short will kill the PSU and not power cycle. I'd check the caps first.
 
It looks like the main caps are working fine. All the other caps have been replaced, there was one 68uF 35volt cap that looked like it had leaked a bit onto the PCB. I temporarily replaced it with a cap with a bit higher specs as i'm waiting for a few elna caps. But still no-go.

@Chromedome45 But the power supply should work when I disconnect everything right? (take it out of the chassis and connect only mains) and flip the power button, if I do this than the same thing happens. I see the internal PSU fan run for just a fraction of a second and it shuts off again for 3 seconds and it repeats this process.
 
w1ll14m said:
It looks like the main caps are working fine. All the other caps have been replaced, there was one 68uF 35volt cap that looked like it had leaked a bit onto the PCB. I temporarily replaced it with a cap with a bit higher specs as i'm waiting for a few elna caps. But still no-go.

@Chromedome45 But the power supply should work when I disconnect everything right? (take it out of the chassis and connect only mains) and flip the power button, if I do this than the same thing happens. I see the internal PSU fan run for just a fraction of a second and it shuts off again for 3 seconds and it repeats this process.
Click to expand...

fwiw, I've been digging into all of my portable 486's and only two of the power supplies work. Most are completely dead, one had the tries to start up symptom. It is definitely the supplies, swapping
between units moves the problem. Also, most of the color LCDs have gone bad, have low contrast, or have noisy video.
 
Al Kossow said:
fwiw, I've been digging into all of my portable 486's and only two of the power supplies work. Most are completely dead, one had the tries to start up symptom. It is definitely the supplies, swapping
between units moves the problem. Also, most of the color LCDs have gone bad, have low contrast, or have noisy video.
Click to expand...

My reaction is a bit late as i haven't been very active lately. Al Kossow Thanks for testing and letting me know! Don't throw the PSUs away just yet, maybe there's still hope for these supplies. I'll try again soon, must have something to do with a damaged component. I'm still very interested to see what my 486c does when powering on. :)

Thanks,
William
 
Chuckster_in_Jax said:
It would help if schematics were available for these machines. But I haven't been able to find any.
Click to expand...

I shot a whole bunch of pictures in preparation for a selloff of everything I have, which fell through. I've gotten it up to two working 486c/66, a 486c and a 486 with an intermittent screen.
pics at http://bitsavers.org/pdf/compaq/Compaq_Portable_486/pictures/
I also dumped the firmware.

I bought replacement caps, but am rapidly loosing interest in these machines. Originally, I kept one around to use with an EETools Allmax prom programmer, but it was less
hassle just to get a new programmer than trying to keep an ISA luggable running.
 
The only reason I kept mine is that it's one of the few portables with two serial ports, useful for sniffing and investigating comm problems; my screen also goes funny occasionally but so far a sharp rap on the side gets it to behave.
 
I just troubleshot a dead 486C supply.

The problem is the primary was only 1/2 of the correct voltage.
I suspected the 110/220 switching circuit, and these supplies use a Sanken STR 81145 module.
Shorting the triac out (pins 2 and 3) brought the supply up.
 
Al Kossow said:
I just troubleshot a dead 486C supply.

The problem is the primary was only 1/2 of the correct voltage.
I suspected the 110/220 switching circuit, and these supplies use a Sanken STR 81145 module.
Shorting the triac out (pins 2 and 3) brought the supply up.
Click to expand...

Thanks for the update after all those years :) I still have the machine and I'm planning to have a look at the power supply soon. This could be very useful information. Thanks a bunch!
 
Today I gave the power supply a new overhaul, after all I'm more skilled at replacing components then when I started this topic.

I ordered a complete new set of caps (including main caps) and desoldered all, then cleaned the PCB (I desoldered the sub pcb and a heatsink to reach to a few difficult places). Then soldered in all new nichicon caps. The power supply works now! The issue was presumably with the 68uf 35v ones near the STR 81145.

Thanks for all the help and hopefully this helps others with similar problems.

Caps replaced: (List comes from somewhere this forum (FlaminV found it)
C2 - 68µF 35V
C3 - 68µF 35V
C7 - 680µF 250V
C8 - 680µF 250V
C9 - 68µF 35V
C21 - 1000µF 25V
C23 - 470µF 25V
C24 - 68µF 35V
C27 - 1000µF 10V
C28 - 1000µF 10V
C31 - 1000µF 25V
C44 - 1000µF 10V
C63 - 1000µF 10V

Best regards,
William
 
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