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Mac SE power supply cycling...

channelmaniac

channelmaniac

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Joined
Oct 10, 2008
Messages
813
Location
Dallas, TX Metromess
Hey everyone,

Have a Mac SE here that when powered up, the supply goes on/off/on/off in a repetitive cycle. It's an Astec supply. Any common issues to check? I've been through the analog board and can find no shorts. The logic board and drive have no shorted diodes or transistors and no open resistors. It has been partially recapped.

Replaced a couple of caps in the power supply, but the problem remains. I don't see any shorted diodes. I'll finish recapping it, but in the meantime, any thoughts on what else to check?
 
You replaced the capacitors no the PSU's secondary side, correct? Are you able to put a lighter load on it?
Inrush on some supplies will cause the PSU to cycle on/off/on/off if the output capacitors are weak.
 
Thanks, I don't have all the caps on hand so I ordered a kit from Console5. Easier than taking the time to measure the physical size on the small caps and ordering from Mouser.

I desoldered the +12v Sweep pin on the power supply connector on the analog board and it still had trouble starting up the power supply, but it finally did after 4 up/downs. All the voltages were spot on. I opened it up and redid a few suspect joints and am waiting on the cap kit. One of the main filter cap pair is reading 170uf instead of 220 and I can't find where I stashed my ESR meter to check the others I hadn't replaced so it's time to just cap kit it.
 
Recapped it and it ran for about 20 seconds then started shutting down and restarting over and over.

I'm going to take my Tektronix bench supply and run the three channels at 5v, 12v, and 12v for sweep and power it up to see what it does from it. I can set current limits at the max levels the power supply puts out so I'll be able to see if the analog board is pulling too much power.
 
Dug through the Dead Mac Scrolls which suggested replacing the IR830 switching FETs... I have some on order from Anchor Electronics as Mouser didn't have them in TO-220 packaging, but Anchor did. (And I trust them to not sell me fakes)
 
If replacing those FETs doesn't work, I would bust out the FLIR camera and find out which components are getting super hot prior to the shutdown. Sounds like a thermal shutdown scenario. You'll usually (but not always) see the culprit component get hot, along with a few incidental components also get hot such as volt regulators.
 
Replaced the 2 TO-220 cased high speed rectifier diodes and it started right up with zero hesitation and I let it run for 10 minutes which is about 5x longer than it has ever run. Now to wait on the floppy eject motor gears to get here so I can have a way to boot the computer.

I was wondering about those diodes as the +5v line had a lot of noise on it and I was seeing some really oddball signals on my scope when the supply output was bouncing. Diode test on the multimeter started at ..140v and kept dropping and dropping and I took the meter leads off as it hit .085v. Really odd low reading and odd that it kept decreasing. It was OL in the opposite direction like it should be.
 
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