512k flup flup

[imactheknife]

Picked up some cheap non working macs. One is the 512k that i didn’t have in my collection. Pulled apart, tried logic board in another chassis, worked perfectly. So focused on analog. Blown fuse, so replaced. Nothing. Recapped what i could. Didnt have the 3.9uf 25v or the small axial caps. It now macs a flup flup noise but nothing else. Is the other caps the culprit? Or something else? Is this red cap a 3.9uf cap?

 

[bibilit]

Flup flup sound can be either the PSU trying to kick in and not being able to, or a bad solder joint or the Optoisolator

 

[imactheknife]

Flup flup sound can be either the PSU trying to kick in and not being able to, or a bad solder joint or the Optoisolator

Ok, great. Which area is the optoisolator? Thanks!

 

[desertrout]

Ok, great. Which area is the optoisolator? Thanks!

Right under the speaker - U3

 

[bigmessowires]

From my understanding of it, flupping usually means you have a short-circuit somewhere. The PSU can't start up because it goes into instant overload. See the discussion beginning on page 117 here: https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf