Mac Classic Help! Checkerboard & Wavy!

[eef]

Hello TD! This is my first time working on a Macintosh, but I've done a lot of Arcade game repair.
Ok, So my partner found a Macintosh Classic at an estate sale, and grabbed it. When turning it on for first time, nothing happened. So I took the cover off and looked closely art the boards (Analog and Logic) Everything looked great and the battery was not leaking or bad. No rust anywhere. So I took the Hard Drive out and noticed a fried transistor on the board. I unplugged it, then turned the machine on and BAM! The screen came on! With a very wavvvvvy checkerboard. I did some research, and purchased a BlueSCSI board, along with a full cap kit from Console5, for the Analog and Logic board. I have recapped both, and have the Blue SCSI plugged in and nothing has changed. Not sure next steps, as I have read other threads here and there are different options, but not sure where to go or try next. I also carefully removed the ROM chip, sprayed Deoxit, and reseated. Still checkerboard. Still wavy.
Thanks for reading, and helping a newbie!

 

[Mark_W]

So both analog board and logic board have issues.

I'd figure out the problem with analog board first, wavy pattern is usually a telltale sign of bad capacitors, if you have already recapped it, double check your capacitors. Also check 5v/12v voltage output on hdd power plug, as well as 31v rail that powers crt part. All classic analog boards i've worked on are early revisions, but there's also a late revision iirc.

The logic board seems to be stuck in reset, since the analog board has issue with its voltage output, the logic board might be ok, but if it's still doesn't boot after you fix the analog board, check the /reset signal from sound chip and make sure it's not shorted to ground

 

[SitesOnFire]

You can try adjusting the 5V rail. My own classic has intermittent issues with voltages, where it'll be wobbly, then the 5v rail voltage gets high enough to boot. It'll eventually stabilise and the screen stops shaking. I do need to delve into why mine does this.

I recommend this article for explaining how to measure the voltages from the floppy drive connector

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This Does Not Compute has a nice video showing how to adjust the voltage

I've seen people adjust the voltage while the machine is on, but it requires a very long plastic screw driver, and reaching under the CRT, I couldn't bring myself to do it, so I went through a very slow process of turning it off, discharging, adjusting and repeating.