PM 8600 shuts down unexpectedly

[mg.man]

Hi all, hoping someone has seen this before. I picked up a PM 8600/200 a while back, and today dragged it out to sort through a pile of 168pin DIMMs that have been piling up. It worked fine when I bought it, but today, when setting it up, it booted up fine, then just powered off. It would not respond to anything - keyboard / power switch, nothing.

Great, I thought... the PSU has gone. So I unplugged it and went about tinkering with something else. After 10 / 20 mins or so, I thought I'd try it again... huh... it booted up fine - so I shut it down. Pressed the power key on keyboard, started to boot... then... hard shutdown and no further response until unplugged for a while (~10+ mins or so) again.

This seems pretty repeatable. Is something in the PSU causing a thermal shutdown? I've checked, and the fan is running and blowing out fairly cool air. Also, being in the UK, it's not exactly "hot" here... So, what can it be? Anyone seen this before?

 

[Unknown_K]

Check the motherboard for leaking caps in the startup circuit.

 

[mg.man]

Overall it looks very clean. Happy to take a pic or three - but where am I looking? I mainly tinker with 68k stuff, so not that familiar with the 8/9600 'board layout.

 

[mg.man]

A few pics from around the ADB port...


...and one of the power switch...


There might be some goo or something near the power switch, will give that area a good clean in the morning. As you can see, the area around the ADB port is pretty clean. Let me know where else I should look.

 

[Unknown_K]

The power circuit goes through the ADB area and there are plenty of SMT capacitors around there.

 

[mg.man]

The power circuit goes through the ADB area

Did you check the pics I just posted? Posts crossed?

 

[Unknown_K]

I posted before seeing the reply.

 

[Byrd]

The motherboard caps look passable to me, I'd check the voltages coming from the PSU - that'd be my first port of call over recapping the motherboard.

 

[mg.man]

The motherboard caps look passable to me

Yeah, I've seen much worse. Also, the 'phone cam makes the 'board look "shinier" than it is - it's bone dry and sporting very little dust build-up. Of course, dusty 'boards show the cap juice better.

the PSU - that'd be my first port of call

Agreed, I'm more suspicious of the PSU. I do have another 8600 over at my storage unit, I might try a PSU swap and see if the issue moves before tearing into the caps. I just thought I'd post here in case this behaviour struck a chord with anyone...

 

[Bolkonskij]

Out of interest: Has there been any success in recapping 8600 PSUs? I know they are darn hard to work on ...

 

[joshc]

Have you got a component tester? It would be interesting to test those caps and see how far out of tolerance they are. At any rate, the caps are quite old now so I would suspect them even if they look good.

 

[mg.man]

Have you got a component tester? It would be interesting to test those caps

I do, and perhaps it would... but I'd rather not start ripping caps off the 'board if it's the PSU that's the root cause. I'll do a swap and report back.

At any rate, the caps are quite old now

Wow... how ageist of you...

 

[powermax]

Out of interest: Has there been any success in recapping 8600 PSUs? I know they are darn hard to work on ...

Not especially a 8600 PSU, but I successfully recapped the Astec PSU from my Power Macintosh 6100. I got a good guidance at the Badcaps forum: https://www.badcaps.net/forum/showthread.php?t=62455
The most interesting topics discussed there are the choice of the replacement caps as well as building a device for testing the PSU under load.

I'm currently recapping a Delta PSU from my Macintosh G3 Beige desktop. Feel free to check out the corresponding thread @ Badcaps:
https://www.badcaps.net/forum/showthread.php?t=107929

 

[LaPorta]

Let me warn you up front: the 8100/840av and up tower PSU from Delta is an absolute insane PITA to recap. I’ve tried two and I messed up two. I am convinced it is because I didn’t get exactly the right caps for the mains side/switcher area. I am going to try again someday. The main thing is that some revisions have no less than three boards in there soldered at 90 degree angles to each other, and to get to some caps, the entire board has to be desoldered.

The funny thing is that the way I messed mine up, it behaves like yours: it runs for maybe 15 min, then shuts off. Can’t restart it until you wait a while. That was what made me theorize that the caps I have so not discharge fast enough for the rapid rate the switcher needs: the caps “fill up” and can’t discharge fast enough so it shuts down. Then it takes time to discharge passively enough to work again: hence the lag time to it working again.

This is all conjecture on my part, but perhaps your issue is the same, just caused by a bad cap that can no longer discharge fast enough.

 

[mg.man]

...perhaps your issue is the same, just caused by a bad cap that can no longer discharge fast enough.

Thanks for that! It is very strange behaviour, and I'm inclined to believe it's PSU-cap-related.

One thing I didn't mention (I know, I know, but I haven't been able to reproduce) is that on one occasion it shut down when loading up a CD. As I said, haven't been able to reproduce -- but perhaps it's somehow load-related? That, or a dip in the 'feedback' (?) circuitry (due to load?) that causes it to shut down. Time-on does also appear to be a factor - like what you're seeing.

 

[powermax]

Let me warn you up front: the 8100/840av and up tower PSU from Delta is an absolute insane PITA to recap.

I don't mean to sound arrogant but I still can't believe that those units aren't serviceable.
It might be not easy or cheap but they are still standard switching PSUs so it should be possible to repair them.
We may need to make a good repair guide first.
There are really advanced tech guys around at the Badcaps forum that can lend a helping hand.
A couple of damaged 8600 PSUs can be used for reverse-engineering the PCB.
Having a schematic for the unit, one can easily find suitable replacement components.
Granted, it doesn't sounds like a quick and easy solution but it still seems to be doable...

 

[LaPorta]

I have no doubt that trained, expert personnel could pull this off. But for us amateur hobbyists, recapping this PSU is not the same as, say, a Mac Plus analog board. It is just a whole other league. I managed to get them apart and back together multiple times without damage, I am pretty sure that the caps were just not to spec.

 

[joshc]

It does indeed sound power related. I'd be considering a ATX PSU conversion. There aren't many cases where the original PSU is special enough to keep, even if you're trying to keep a machine completely stock/original. A new 80 plus rated PSU has all the modern protections and efficiencies provided by the advancements in PSU design from the past 20+ years. It will run cooler and more efficiently than the old PSU ever did, and probably has stronger rails as well.

 

[Byrd]

@joshc - agree 100% go ATX, if you consider a 110V - 250V PSU to be dicey.

Unless you're a qualified electrician experienced with working with mains voltage, short of a solid recap, clean and repair of obvious damaged components - if it doesn't work, I'd shelve it and look into an ATX replacement. I'd much prefer a considered, neat conversion like this over seeing flames by keeping a suspect OEM PSU up and running.

 

[LaPorta]

If something is available in the same dimensions, with trickle current for soft power on, I’d be game.