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

Quadra 650 dead...but PSU works


Well-known member
I recently got my hands on a Quadra 650, not in the best shape, but tantalum capacitors on the motherboard and no battery leakage! I replaced the PSU capacitors, and all was well, but then I left it unplugged for a couple of days and now it won't power on at all. I can get the PSU to start by bridging the requisite pins and measure good voltages, but pressing any combination of buttons (power switch, power key, reset/interrupt) does nothing. I even tried a new battery with no change.

I've taken this down to a bare board + PSU to make sure nothing else could affect it.

I did find a few capacitors with low in-circuit resistance, but upon lifting them with a soldering iron, they tested out fine so I put them back, the line they're on does still show 10 ohms, though, so I'm wondering if that's the issue.

Does anyone have schmatics they can share with me for this machine? I'm guessing it's the same as most other Wombat machines (aside from some ID resistors and headers) or perhaps have some other theory as to what's going on, all ideas welcome!

Thanks :)


Well-known member
OK so I managed to get it to power on by shorting the two pins on the PSU to get it to start. At this point I get a bong and a flashing question mark (although nothing is plugged into it apart from a monitor and a PSU so fine)

What's interesting though, is that I can then power it down with the power switch - so if there's an issue somewhere it's a transistor or resistor or something in the path between the "soft power on" circuit and the PSU.

The thick plottens, as it were!


Well-known member
Certainly on some of the later II series(es) that circuit is known to be a bit troublesome, so I wouldn't be totally surprised if it were on the Quadra, either. I've never had to do this so I will spare you my opinions, since they're worthless, but it might be worth searching for power on circuit fixes in the IIs on the forum, there have been several threads.


Well-known member
It doesn't, sadly - although I have made a little progress.

Whilst I don't have a full schematic, looking at the various startup circuit schematics for II series Macs (IIcx/IIci/IIfx seem similar in operation) I have determined that the signal for powering on the Mac does make it out of the 74LS132 chip near the power LED. For the sake of testing I added a bodge wire from that point (after the resistor) to the "D4" (on the IIfx circuit) diode of the 'top' circuit (this part seems closest on the IIfx circuit - although I can't seem to work out what's feeding that diode on the 650) and this now makes the computer operable, but it immediately powers on when I plug it in (after that I can power it off and it acts normally). This leads me to believe that either that diode is bad (although it tests out ok) or something which is feeding that diode is bad. Note that the voltage output form the 74LS132 is absolutely fine, to the 5v continuous feed is making it to a good proportion of the board.

Certainly something to investigate further. Most likely culprit at the moment is probably the "2A" transistor.

edit: schematics


New member
There is a known issue with the 5V trickle charge. I have a post on this. But I would love to know what pins you shorted to get yours to start!