Mac Classic wavy lines, no boot

[daguil]

Recently picked up a broken mac classic for dirt cheap, turned it on and surprise surprise it was dead. Went through the usual process of recapping the logic board as well as the analog board (which had this weird rotten-smelling green goop that I had to clean up), but nothing appears to have changed. Stuck with a screen with wavy lines that appear to go back to uniformity right after turning the power off. Additionally, every subsequent boot leads to weird glitches on the screen, which lead me to believe it's some sort of ram issue, however removing the memory expansion board does not change anything. Any clue as to what could be the issue?

Here's a video of the issue. The smaller white horizontal lines were fixed by readjusting the knobs on the back of the mac.

 

[Byrd]

Hi daguil,

What voltages are you getting from the floppy port? The goop-like honey, is capacitor goo and highly conductive so you might want to deep clean it once, then again to remove.

 

[daguil]

Meant to post about that. I'm getting 3.85V on the 5V rail (highly inadequate), and...1V on the 12V rail. Bad stuff. Ordered replacement parts that I've gathered from other posts on the Classic: TDA4605, 1N4148, and CNY17.

Edit: Testing it again right now, and it's just fluctuating like crazy.

 

[falen5]

voltages are the first thing you should look at before doing anything

post some pics of the 2 boards

 

[dochilli]

There are a lot of parts on classic AB that can be defect. Most often: DP3 and DP4 (1N4148), TDA4605, CNY17G, IRFBC40. I would replace them all.
5V should be 4.85V for booting.

 

[daguil]

Sorry for disappearing, I had completely forgot to come back to this thread. Replaced DP3 and DP4, as well as TDA4605 and CNY17G (replaced with CNY17-3), and definitely some improvement, but the voltages are still weirdly low. 1V where 12V should be; no clue why. 3.5V on 5V, and -9.8V on -12V. Replaced a couple more diodes that also use 1N4148, but still no dice. At the very least the screen isn't nearly as wavy as it was before, but it still is wavy to an extent. I'll go after IRFBC40 next (wish I had seen that before I bought from Aliexpress, oh well), but any other ideas? Thanks.

 

[bibilit]

Hi, i don't understand how your 12 v line can be so low.... have you cleaned the board thoroughly ? cap juice can do a lot of weird things.

 

[daguil]

Hi, i don't understand how your 12 v line can be so low.... have you cleaned the board thoroughly ? cap juice can do a lot of weird things.

I haven't don't the most thorough job, just swabbing everything down with IPA before removing caps, after all caps were removed, and after new caps were installed. Looking at some schematics of the board, I'm pretty sure the 12V can be explained by my shoddy job at trace repair on the logic board which I didn't verify continuity for. Will fix when I get home.

 

[daguil]

Got a little stumped, and it took until now to get back into it. Indeed one of the traces on the logic board was severed, likely due to the cap leakage, and after a quick repair the 12V line is alive! All voltages are just as low as before, and I'm pretty sure it's something to do with the analog board. Any guide to diagnose the cause of the issue?