PowerMac 7100 ATX Adapter - PSU shutting down quickly

[sulley86]

Ok I'm kinda scared to ask as I've seen so many threads about ATX conversions now, but I couldn't yet find a solution to my problem, still I have the feeling this —may— have been discussed at some point of time…

So I beg you upfront to excuse my foolishness!

Here we go: I've got myself a couple of George Rudolf's "OldMacATX" (https://github.com/GeorgeRudolf/OldMacATX) PCBs, added the SMD components and tried to get my Power Mac 7100 running. But the only thing that happens is that the machine is firing up for about a second, and then it shuts down. When powering it on again the machine seems to keep running, but it eventually always shuts down.



Facts:
- Soft power on via keyboard works, so I 'suppose' the single gate inverter is working properly.
- I've tried a couple of ATX power supplies, ranging from a Pico to 200 Watts consumer-grade to 850 Watts server-grade. To no avail.
- The original PSU boots the machine just fine.
- I've resoldered the SMD components six times or so, with fresh components each time as I was not sure if there was a dead solder joint or if I broke one of the components due to heat. The result stays the same.
- I've checked for dead solder joints and things seem to be fine.

My assumptions so far:
- I could have used wrong components, George Rudolf's board is made up of a single gate inverter, two resistors and one capacitor. I made sure to order parts which fit the part list as closely as possible, yet I am not sure if they are 100% correct. I can provide a list of the used parts if that helps.
- The ATX PSU may shut down due to some pin not getting a continous signal for it to stay on.
- Yes, my SMD soldering looks like sh*t (see attached photo) but that's the best I can do currently, I am very new to this and am willing to learn (and I usually remove any remaining flux but not in this experimental stage).

Any help is appreciated. I have the ability to measure pins with a multimeter, and I have very basic electronics skills, but am very glad for any input or proposition.

PS: I will happily send you one or two blank ATX PCBs for valuable help which solves this problem.
(If you're located in Europe. I will also cover the shipping costs, if they are within limits )

 

[cheesestraws]

- The ATX PSU may shut down due to some pin not getting a continous signal for it to stay on.

One thing to think about is whether there's not enough load on the PSU, and it's shutting down because of that. You could stick some power resistors or something across the 3.3V/GND or 5V/GND (depending on how old the PSU is) to draw some more current and see if that helps...

 

[sulley86]

Thanks, I also heard that this can happen. Now where you mention it: for my tests, I did not connect any devices (HDD, CDROM) in case I'd fry the machine. I'll give it a try!

 

[sulley86]

One thing to think about is whether there's not enough load on the PSU, and it's shutting down because of that. You could stick some power resistors or something across the 3.3V/GND or 5V/GND (depending on how old the PSU is) to draw some more current and see if that helps...

Unfortunately more load did not fix it. I'll try to jump start the PSU and see what happens.
If anybody else has an idea, I appreciate any help

 

[demik]

Does the PSU stay on by itself ? (No load / no logic board / nothing)

The other thing to try is to force the PSU on, and see if the Mac keeps running. That may be a fault on the 7100 side (either soft power, or a short, or something). BTW the caps on theses are just falling as bad as 68k ones

 

[sulley86]

Ok it's been some time, but I got good news: the PSU works on a Quadra/Centris 650. It powers on, hard & soft. The machine seems to be running fine. So it has something to do with the 7100, but I don't know what the exact issue is.

I'll do a long-term test of the PSU and will afterwards test it on a Quadra 700 and a Mac IIvx. For completeness here's the (almost) finished PSU, incl. a part list.

• Original Power Mac 7100 PSU. Remove the internals and put them aside, maybe they'll come in handy one day.
• 80 mm fan
• C16 Socket, I've used the one from the old PSU
• 120 Watt Mini ITX PSU
• George Rudolf's "OldMacATX" adapter
• Mean Well LRS-100 PSU, because it fits perfectly into the original PSU case ( could be underpowered for the 7100, but should do the job just fine in a 68k machine.

As the Mean Well PSU does not has a power switch, I will add a separate switch to the other emtpy C16 socket, so the PSU can be switched off when the computer is not used. I'll loose the ability to power on a screen with the Mac, but this is ok for me.

All of this is heavy DIY —no long term testing has been done yet. Also, there's no fuse or safety capacitor visible, as the Mean Well includes all of this, but I'd add at least a separate fuse, just in case.

 

[sulley86]

Stress test has been successful. Over three hours of benchmarks, gaming and DTP. Everything works as it should.

 

[Byrd]

That looks like a really neat solution, well done