Weird SE Power Issue (Intermittent Power On [Not Cold Analog Board Joints])

[mooseman]

I've got an SE I'm trying to get working and I'm a bit stumped, would appreciate suggestions.

Problem: from cold the system powers on and runs well. If restarted (via software or via the programmer switch) it boots fine. After running, however, if I switch off the main power and then switch it back on it's completely dead: no bong, no video, no fan. Let it sit for 20 minutes and it works fine again.

What I've done:
- Replaced all electrolytic caps & D2/D3 rectifier diodes on the analog board
- Replaced all electrolytic caps in the power supply
- Reflowed all solder pads for the plastic terminals on the analog board
- Replaced all electrolytic caps on the motherboard (not the root issue but I'm sure it needed it)

The good news is this fixed all the video issues and when it runs it runs great. The bad news is it's still got the flaky power issue. At this point I assume it's gotta be the power supply, right? It's got all new caps, any thoughts on what else goes bad on those or what to test? It has the Astec AA13800.

Normally I'd just try to find a newer replacement, but I'm doing this for a buddy and trying to keep things cheap. Suggestions appreciated.

 

[Builder68]

Have you checked that the AC power is reaching the PSU, just to rule out an issue with the power switch itself?

 

[mooseman]

It seems to be, yes. I'm getting power at the floppy drive port, but it's like 2V on the 12V line. I've tried flipping the switch a bunch of times and that's a no-go, which I wouldn't expect to be the case were it a mechanical issue.

Sure seems like something in the PSU is not happy after it heats up or charges up, but if it's not the caps I'm not sure what.

 

[Byrd]

I'd reflow the back of the PSU PCB, might be a dry joint. A can of electronic freezer spray might help you diagnose here as appears heat related; also check diodes, and exercise/clean the voltage potentiometer to rule this out as well.

 

[Builder68]

It seems to be, yes. I'm getting power at the floppy drive port, but it's like 2V on the 12V line. I've tried flipping the switch a bunch of times and that's a no-go, which I wouldn't expect to be the case were it a mechanical issue.

Sure seems like something in the PSU is not happy after it heats up or charges up, but if it's not the caps I'm not sure what.

To be sure it's not the power switch dragging down the AC voltage, you'll want to measure the AC voltage right at the switch's output pins. Just checking the DC voltage at the 12V rail won't rule out the switch just yet—nor will it rule out bad solder joints on the mains AC side. While you're at it, it's also a good idea to check the solder joints on the input side of the main transformer and measure the AC voltage there, too.

I'd reflow the back of the PSU PCB, might be a dry joint. A can of electronic freezer spray might help you diagnose here as appears heat related; also check diodes, and exercise/clean the voltage potentiometer to rule this out as well.

Agreed, that would also help to rule out issues like rust in the potentiometer, or cold joints on the AC/DC rectifier or individual diodes.

 

[mooseman]

Cool, will do some more checks. Which one is the voltage pot?

The CR2/CR3 rectifier diodes were replaced. I checked the other ones but since it's intermittent who knows.