• Hello, Guest! Welcome back, and be sure to check out this post for more info about the recent service interruption and migration.

Power Mac 7600 PSU clicks but doesn't start up

EvieSigma

Young ThinkPad Apprentice
I just acquired a Power Mac 7600/120 and it seems to have a power issue, but I'm not entirely sure what the issue is. When you press the power switch on the motherboard, the power LED lights up green for just a second and there's a click from the power supply, but the system doesn't start up. Where should I start with troubleshooting this?
 

EvieSigma

Young ThinkPad Apprentice
So somehow me blasting 10 tons of dust out of the 7600 with an air compressor fixed the power supply? I guess something was shorting!
 

Franklinstein

Well-known member
Your PS's capacitors may also be getting old. Sometimes if left plugged in long enough they'll charge up to the point where they'll work, but it can be unreliable. I have a hard power supply for a computer and I have to flick the switch and let it sit there before the caps charge enough to power it up. I really need to recap it but power supplies generally have a larger variety of capacitors than the average logic board, so it's kind of a pain to catalog them all and then put in an order somewhere that has all of them in stock.
 

macuserman

Well-known member
Those power supplies are quite suspect at this point. I have a small collection of dead 7xxx supplies at this point.
 

k24a1

Well-known member
Damn, I had a 7300 power supply around but ended up getting rid of it. Looking back at stuff you got rid of is difficult to think of.
 

EvieSigma

Young ThinkPad Apprentice
Those power supplies are quite suspect at this point. I have a small collection of dead 7xxx supplies at this point.
That's no good...at least I know I have a working machine for now, even if the power supply decides to drop dead.
 

k24a1

Well-known member
Now that I think of it I'm lucky mine seemed to have worked. Unfortunately I no longer have it...
 

joshc

Well-known member
Count yourself lucky, I had one which literally blew up - sparks and a dramatic bang when power was applied. Like most old Mac power supplies, they were underspec'd when new and add 20-30 years of use/storage on top of that, and you end up with a pretty flakey PSU. ATX conversion is an option and what I'd go for if you want ultimate reliablity.
 

CC_333

Well-known member
ATX conversion is an option and what I'd go for if you want ultimate reliablity.
I was going to make the same suggestion.

I had one which literally blew up - sparks and a dramatic bang when power was applied.
That happened to me once too, but it was self-inflicted. I had a power supply (I forget if it was AT or ATX) which blew its fuse. Like a dunce, instead of figuring out why the fuse blew, I bypassed it by soldering in a nice, big chunk of copper wire to both ends of the fuse holder and plugged it in. The fireworks display that resulted scared the daylights out of me and stunk up my work shed for quite a while.

c
 

k24a1

Well-known member
Ooooh, that's not good. I've had some electronic screwups before, I blew out a UPS while adapting an eMac's power socket to a standard ATX power cord for a case mod I did back when I was 14.
 

EvieSigma

Young ThinkPad Apprentice
Count yourself lucky, I had one which literally blew up - sparks and a dramatic bang when power was applied. Like most old Mac power supplies, they were underspec'd when new and add 20-30 years of use/storage on top of that, and you end up with a pretty flakey PSU. ATX conversion is an option and what I'd go for if you want ultimate reliablity.
Is there a thread/blog post about what specific kind of PSU I would use to do that?
 

k24a1

Well-known member
Is there a thread/blog post about what specific kind of PSU I would use to do that?
Unfortunately it does not seem like someone has documented using an ATX power supply for the outrigger macs, but someone has managed to come up with a pinout:

As for a power supply you could use, maybe you could try taking the guts from an old ATX power supply and cramming it into the case of the 7600 one? You might also be able to hack in an SFX power supply...
 
Last edited:

joshc

Well-known member
Be careful with the power pinouts for 7xxx machines - there has been a lot of confusion about them, as they are not standardised as far as I know (i.e. a 7200 pinout's is different to a 7600?). I can't remember what the thread was called but I think @mg.man was involved so maybe he can remember better than me.

Basically, you must verify the correct pinout before attaching a different PSU so that you don't fry your 7600 board.

As for which ATX PSU to choose for a conversion, one that fits inside the original PSU would be where I'd start. You want something with strong a 12V rail, but anything modern will have all the protection/safety features that you'll want. For my IIci conversion (similar size case, a bit smaller I think) I used a Seasonic 250W unit. See the IIci conversion thread here for an idea of how to do it https://68kmla.org/bb/index.php?threads/stealth-macintosh-iicx-iivx-style-sfx-psu-rebuild.38439/
 
Top