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Astec Power Supply for 8100 - How did I mess this up?


Well-known member
I will make a brief summation, since I have a tendency to be verbose.

Simply put: I re-capped a perviously working Astec power supply from my Power Mac 8100, and now it is not reliably functional.

More in depth: all I set out to do was replace the big electrolytics on this thing. There are three boards on this beast, one being a relatively tiny one that has two caps on it, and two other main boards. The smaller board needed to be completely desoldered to replace the caps, as these things are stuffed in there at a 90 degree angle to the other components on the main board. Below is exactly what happened:

1. Took out the supply, disassembled it, desoldered the existing caps, and soldered in new caps with the same ratings as those there previously.

2. Reassembled the supply. Hooked it up. Started the machine. It started up normally. I started up a Game of Marathon, and right as it opened the application, the entire machine just shut down. I tried to start it again, I hear the power supply click, and then off. If I do that a few more times, there isn't even a click.

3. Waited a while. After waiting 15-20 minutes, I was able to turn the machine on again. However, part way through startup, it immediately shut down again. It seems like capacitors bleeding off stored charge somehow helps it be able to start up.

4. Took the supply out, and jump-started it from the trickle-voltage pin to the start signal pin with a resistor. While it was running, I checked voltages. All grounds have no voltage, all +5s read 5, the +12 is 12, and the -12 is -12. So the appropriate voltages appear with it not connected with no load.

5. Got another power supply and tested it with the machine to make sure it was not an issue with the machine otherwise. The machine functions normally with the other supply.

6. Opened up the supply, looked over all my work, desoldered some cap leads and resoldered, made sure they are very clean. Touched up some ground pad areas. Reassembled with the other supply as a reference to make sure things were going where they should. Re-tested....and the same issue persists.

At this point, I'm at a loss to explain it. Though it seems a long shot, I had an issue like this once with the sound circuit on a Quadra 630, where the sound would play initially, but then would fail because there was no path for one of the sound caps to ground. However, this is so complex, I am not sure what goes where, and to my knowledge there is no schematic floating around. It is bothersome because it worked fine before, and I am not sure what I did to cause this.

Roping in @Bolle, @techknight, and @cheesestraws because, well...you guys are always helpful; and have good ideas! I would appreciate any suggestions from anyone for me to try.



Well-known member
I'm afraid I don't have much for you here; I don't know enough about PSUs to have any advice at all, let alone any safe advice.



Well-known member
I know this doesn't specifically deal with whatever issue is causing the behaviour on your 8100 PSU, but I have read at least a couple of people mention that these PSUs can become unreliable with age (granted, your issue only started happening after recap so is assumably related to those caps) but have you considered a retrofit of an ATX PSU?



Well-known member
I’ve seen a few posts on that. I may have to try that sometime, but I do like to keep the machines as original as possible, if possible.



Well-known member
you have a bad connection somewhere in there. 

the thing with power supplies, they either work, or they dont. 

The only time things become intermittent is either with bad connections, or bad capacitors. (which you have already changed). 

If the protection circuitry drifts a bit, it could sense an overcurrent condition and trip it out, then it will cycle, so you cant rule out resistors that have drifted out of tolerance. But if this is random, you have a bad connection. 

Next thing could be a bad optoisolator that is dropping out on you. This is common. Also, was there any cap corrosion? If so, check the nearby transistors and ICs for green crusties. Epoxy isnt perfect, and the corrosion will walk itself up into the chip and into the die causing problems. 

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