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Replacing a proprietary PSU of an early PC clone (Commodore PC20-III)?

S

Scali

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My Commodore PC20-III died the other day. It was powered on, and at some point, it just went dead. I tried to powercycle it, but not much happened. On a few attempts, the power led came on weakly, but then it went away again.
I took the PSU apart and checked the fuse, but that was not it, fuse seemed to be fine.

Now, this is a completely proprietary form factor with proprietary connectors. What I gathered from the documentation however, is that the power it supplies is quite standard:
+12v
-12v
+5v
POWERGOOD signal

The power switch is just a standard 'hard' 220v switch, which is on the PSU itself, accessible from the back.

So technically, I should be able to retrofit any PC/XT/AT-ish PSU in there, if I just solder the proprietary connector onto it. I just need to find one that can fit inside the housing of the PC20-III's PSU.

So, I was wondering, is there some 'go to' modern replacement PCB for PC/XT/AT systems that I could use?
I have been looking at 'Flex-ATX' PSUs, and I suppose I could make one of those fit, but it's rather overkill, and I'd have to take care of the 'soft' power switch in some way. There may be easier solutions.

The main problem is that the PSU casing is very 'shallow', yet 'wide'. So it won't fit just any standard PC/XT or AT PSU.
You can see it at the back here:
Commodore%20PC20-III%2023.JPG


And a picture from the back, so you can see the switch, power input and fan:
Commodore%20PC20-III%2026.JPG


Any ideas?
 
Did you follow the normal troubleshooting steps of simplifying the configuration? Remove the expansion cards and disconnect everything except the motherboard. If it comes to life, you know the fault probably isn't with the power supply. A shorted tantalum capacitor, for example, can cause the power supply to go into protection, which will make it seem dead.

And even if the power supply is at fault, it can usually be repaired. Open it up and look for any obvious signs of failure such as a blown capacitor or burnt component.
 
As above, I'd try for diagnosis/repair.

But in regards to dealing with a "soft power switch" - the way that usually works is the motherboard just grounds a pin when it wants to be on, and disconnects it when it wants to be off, so all you do is tie it to ground so it's on whenever the power is (power switch ON). When testing ATX supplies it's quite common to just jam a paper clip in the ATX power connector to make them turn on.
 
vwestlife said:
Did you follow the normal troubleshooting steps of simplifying the configuration? Remove the expansion cards and disconnect everything except the motherboard. If it comes to life, you know the fault probably isn't with the power supply. A shorted tantalum capacitor, for example, can cause the power supply to go into protection, which will make it seem dead.
Click to expand...

Yes, I removed all cards, the HDD, floppy drive first... Nothing. Led didn't even come on, fan didn't spin up.
Eventually I disconnected the motherboard as well, but even running the PSU 'solo' didn't make the fan spin up.

vwestlife said:
And even if the power supply is at fault, it can usually be repaired. Open it up and look for any obvious signs of failure such as a blown capacitor or burnt component.
Click to expand...

I didn't see any obvious damage inside. Various parts looked very 'custom-designed' though, no idea how difficult it would be to troubleshoot this, let alone find replacement parts. All I know is that I don't have the skills for that.

By the way, this is a 220v-50Hz machine, so that's the type of PSU I'm looking for (although I do have a power converter that I run my PCjr on.... and there happens to be a US PSU for exactly this machine on Ebay... hmmm).
 
if the PSU turned out totally dead and irrepairable, cut the connector off that goes to the motherboard and keep that so you can wire in a new psu much easier.
 
Scali said:
Yes, I removed all cards, the HDD, floppy drive first... Nothing. Led didn't even come on, fan didn't spin up.
Eventually I disconnected the motherboard as well, but even running the PSU 'solo' didn't make the fan spin up......
Click to expand...
Depending on the Manufacture or revision of your PSU it may or may not run 'Solo', Connect a couple of hard drives up and try again, Did you see the fan 'twitch' when you applied power, What's the part # of your PSU ?.
 
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Seasonic and other manufacturers offer PSUs in all sorts of configurations.

For example, I use this one with a Socket 939 mobo in an HP VL6 case.

tfx_300_c.jpg
 
Scali said:
My Commodore PC20-III died the other day. ...
Click to expand...
Weird, I'm sure I answered yesterday but my reply is missing.

Anyway, I can get you a spare supply. But it is not something I prefer to send by post. Two other options:
- Commodore gebruikersdag in Maarssen coming saturday.
- Get it in Heerlen.
Just email me at ruud apestaartje baltissen puntje org.

Groetjes, Ruud
 
FWIW if you have the 'Liton' PA-3750-1B PSU it might be worth checking the thermistor (RT1) just in front of the fuse, If it has gone OC the PSU will appear dead. Shouldn't be too hard to find a replacement if needed.
 
twistedpneumatic said:
Can you open up the supply and let us take a look?
Click to expand...

I did take it apart, didn't see anything, so I put it back together again to give it another try (it's not the kind of computer where you can just take off the cover and run it like that... very impractical design where it is screwed to the case with the lid.

twistedpneumatic said:
Have you confirmed is is not a mobo issue?
Click to expand...

There's no way I can confirm that without a working PSU obviously.
However, the fact that the fan doesn't even spin up, is a clear indication that the PSU isn't working properly at least (this is 1987 technology, the fan is just connected directly to a voltage rail, so if the PSU delivers any power at all, the fan spins up, simple as that. Same goes for the power led).
 
Malc said:
FWIW if you have the 'Liton' PA-3750-1B PSU it might be worth checking the thermistor (RT1) just in front of the fuse, If it has gone OC the PSU will appear dead. Shouldn't be too hard to find a replacement if needed.
Click to expand...

Yes, I will take the machine apart again, and open up the PSU. I want to repair it, if at all possible, even if I do get a replacement.
 
Scali said:
Yes, I will take the machine apart again, and open up the PSU. I want to repair it, if at all possible, even if I do get a replacement.
Click to expand...
I have one which i removed from a Commodore PC20-III i had and scrapped the rest because of the poor condition and damage, I'm keeping the PSU but can help with measurements if needed, For it's age 1988 It's quite a nice PSU i thought compared to some of the crap some manufactures are turning out today, Mine is a revision B, Ignore the dog poo glue in the pic of mine.
IMG_3922.JPG
 
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Okay, something strange happened... I wanted to take the machine apart, and I tried to power it up one last time... It powered up in one go.
So I put the HDD and VGA card back in there, and it still worked.
Then I left it running for a few minutes, and it just dropped off again for no reason.

Anyway, I took a picture of the PSU while I was at it. Didn't bother to open it up yet, since it seemed to work. But I'm certain it looks very different from yours on the inside.
This is the label on the outside anyway:
WP_20180223_15_54_19_Pro.jpg

I'll open it up now, because it stopped working again.
 
Scali said:
...I'll open it up now, because it stopped working again.
Click to expand...
Yes, Different manufacture, Before you open it up though, I would connect 1 or 2 hard drives up on the bench and see if it works, Just to rule out problems else where in the system.
 
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