Hi everyone,
New member here!
I picked up a Macintosh Plus on eBay and it won’t stay on. Video of what happens:
What it does:
This makes me think the supply is shutting itself down, but being a complete newb, it’s hard to diagnose why.
Claude pointed me to the U3 optocoupler, based on the analog board write-up here: https://vintageapple.org/gamba2/images/plus_analog.PDF
“If U3’s LED has aged and dimmed, or something in the feedback path has a bad joint, the switching side gets a weak “output is high” signal and keeps pushing the output up. Once the +12V gets about 15% high, Q8 fires and shorts it, and the supply shuts down. Check: the +12V at the floppy port should climb toward ~13.8V just before shutdown.”
The other possibility it mentioned is a bad connection at J4 cutting power to the logic board.
My plan: I’ll measure the +12V at the external floppy port during the ~3 seconds it stays on:
New member here!
I picked up a Macintosh Plus on eBay and it won’t stay on. Video of what happens:
What it does:
- Power on → single startup beep
- A few seconds later the screen fades up to a uniform blank white raster: no gray pattern, no Happy Mac or “?” disk
- After about 3 seconds of white, the whole machine shuts off with one pop
- Then complete silence: no ticking, no “whup-whup” retrying. It stays off until I power cycle it.
- Same behavior every time
- Nothing connected (no keyboard, mouse, floppy or SCSI)
This makes me think the supply is shutting itself down, but being a complete newb, it’s hard to diagnose why.
Claude pointed me to the U3 optocoupler, based on the analog board write-up here: https://vintageapple.org/gamba2/images/plus_analog.PDF
“If U3’s LED has aged and dimmed, or something in the feedback path has a bad joint, the switching side gets a weak “output is high” signal and keeps pushing the output up. Once the +12V gets about 15% high, Q8 fires and shorts it, and the supply shuts down. Check: the +12V at the floppy port should climb toward ~13.8V just before shutdown.”
The other possibility it mentioned is a bad connection at J4 cutting power to the logic board.
My plan: I’ll measure the +12V at the external floppy port during the ~3 seconds it stays on:
- If the +12V rises toward ~13.8V right before it shuts off, that points to the regulation (U3 or the feedback path) and Q8 firing.
- If it stays normal and then drops, or sags or flickers, I’ll look at J4 and the joints instead.