Mac Plus Sudden Death

[Cortland]

I have a Macintosh Plus that I got cheap because it was broken. I recapped the analog board to best of my soldering skills and replacement parts I could find (some caps are unchanged, but look OK). I went through several stages of „flup-flup“ noises until I finally got it to boot.

Happy, I let it sit for a week or so - plugged in, but turned off. When I next turned it on, it did nothing at all. No start sound, no noise, no screen, no sparks, not even smoke.

I have been going through Larry Pina’s Dead Mac Scrolls and Repair Secrets and checked everything I could, but I can find nothing wrong.

The fuse shows continuity, the light bulbs NE1 and NE2 show a resistance reading (are they supposed to glow in the dark? Mine don’t), I have bridged the switch because I suspected it to be unreliable. Today, I have replaced CR20 and CR21 although they looked good, but that changed nothing either.

I have not changed the flyback transformer, but Larry Pina’s diagnostics check out, and the potential replacement part I have available does not look much better.

I am at my wit’s end. Does anybody have an idea what I might be missing, or what I could check next?

 

[obsolete]

My guess would be that there's a bad/cracked solder joint somewhere. I'd start reflowing things. especially connector pins.

 

[Cortland]

My guess would be that there's a bad/cracked solder joint somewhere. I'd start reflowing things. especially connector pins.

Thank you for your reply. I actually re-soldered the connectors during recapping, but I will re-check the joints. Can’t hurt.

 

[Fred1212]

Have you checked the voltages on the board? played with the pot that adjusts voltages? Cleaned the actual male pins and female sockets of all the connectors to the digital board?

 

[Cortland]

Yes, I did that. In fact, I finally found this weekend that it was R55. I replaced it, but now the board is back at the "flup-flup" stage, the voltage control does nothing. I suppose I need to go through the Dead Mac Scrolls all over again....

 

[bibilit]

can be the optoisolator

 

[Cortland]

I replaced that already during the first round. But lacking other clues, I will perhaps replace it again.

 

[wskjinfen]

There is a fuse in the cable connecting the logic board and the analog board. Check if it is blown. Flup-flup can be caused by too small load current or too high output voltage of the switching power supply.

 

[Cortland]

I checked the cable, it was OK. Today, I replaced the very last capacitor that was still original (C30, international), and I replaced U3 (again). One of the last two things did the trick, the Mac booted.
Thank you all for your suggestions!
Now I just have a wobbly display. I will try to re-adjust the voltage.