IBM 8573-121 (P70?) Faulty power supply

[RizThomas]

I pulled my IBM 8573-121 luggable pc today in the hope that I can restart it again (after a couple of years in storage). I salvaged this from a recycle place back then and was able to get it to boot.

Today, however, the power supply did not even turn-on first. After a few tries, the fan started but stopped so quick after i hear a sort of relay switch clicking. I opened it up hoping and maybe reseat the ram chips , etc.

The power supply started one more time and then quit. I noticed that this P/S supply does not need the motherboard to get it started as i had the fan come on for a few minutes then quit entirely.

I checked for any obvious problem like bulging caps, etc. Everything looks normal although after switiching it for about 20 minutes, I noticed that one part (STR81159-- a 150-256V voltage regulator) is hot. The rest are cold (normal temp).

Has anybody here come across or had a chance to fix one of these power supply? It looks like this unit will end up just for display if I can not fix it. I wish i can make it work though. However, I am not well versed in fixing switching power supplies.

Any tip is much appreciated.

 

[Malvineous]

I'm no expert but having the PSU start for a moment then shut down sounds like it could be a short circuit somewhere. Especially if a regulator gets hot, that's a good indicator of an overcurrent condition which could be caused by a short. In my experience the early power supplies didn't have short circuit protection, they just delivered as much current as they could until something got really hot and went bang - usually a motherboard component in my cases.

I presume you've disconnected the PSU from *everything* (except the mains) and it still won't start?

 

[Stone]

I'd agree with that. Start looking for a shorted cap on the motherboard. Before doing that pull any cards from the motherboard and then power it up to verify that a card doesn't have a short on it. I've seen serial/parallel cards with short a cap cause the same symptoms.

 

[RizThomas]

I believe this is so (maybe a busted capacitor) but i think it is on the PSU itself as I have disconnected the whole unit from any load except the psu fan. I have examined visually the parts but could not find any obvious one. I may have to start changing the capacitors. Somehow, it starts sometimes then stops (again this is without any load).
I will try to source the capacitors locally since the 2 big ones seems to be low value but high voltage type (250V).
Also, I have read somewhere that it has "85-watt automatic voltage-sensing power supply"... whatever that means. I suspect because there maybe a voltage anomaly that it shuts down once detected. This may explains sometimes it will turn on then suddenly shuts off.

Thanks for the hints. I will keep a status update. Thanks

 

[modem7]

I believe this is so (maybe a busted capacitor) but i think it is on the PSU itself as I have disconnected the whole unit from any load except the psu fan.

But what if the PSU design is such that it requires a suitable (external) load in order to reliably start? One example is [here].

I have read somewhere that it has "85-watt automatic voltage-sensing power supply"... whatever that means.

Regarding "automatic voltage-sensing". That sounds to me like a description of some "international" PSUs of the time, fitted to portable computers (e.g. computer possibly taken on overseas business trips). The PSU that I pointed to earlier would automatically switch between 115 VAC and 230 VAC mode; no switch to flick over.

 

[Chuck(G)]

If you change the large 250V capacitors, observe if they're non-polarized caps. Many PSUs used them in the primary circuit. You cannot take a polarized capacitor and use it in the same place. You can, however, take two capacitors, each with twice the capacitance and connect them in series back-to-back (+ to + or - to -) to get an equivalent to a nonpolar capacitor.