Low Voltage Mac Classic When Cold

[SitesOnFire]

Hi, I picked up a Mac Classic with the aims of restoring it, to write some programs with Think C. I replaced the caps on the analog and logic boards, as well as the octocoupler. After these repairs, the voltage from the 5v pin of the floppy drive, after adjusting PP1, at the very best was around 4.7v. If I left the machine on for 10-15 minutes, the voltage would approach 5v and it would begin to click and bong a few times, before I'd hear the HDD spin up, then it would happily boot and be usable. I have read/watched loads of different resources as references, but this article https://retrorepairsandrefurbs.com/2023/06/22/1991-apple-macintosh-classic-i-repair-restoration/ showed a classic with very similar issues. Following this article, I replaced DP3 and DP4 yesterday, after I noticed the voltage is lower, adjusting PP1 seems to only range be between 4v and 4.4v.

If I apply heat to the back of the analog board, specifically the right hand side (viewed from the bottom of the analog board) of the octocoupler (see attached video), the voltage will climb up to 5v, and it will boot. As the area cools, the voltage drops, and the mac will shutdown.

I'm a total newbie to this, I learned to solder in high school, and haven't done much since. I've scanned through this forum, read Larry Pina's books, watched everything I can on YouTube, but I am at a loss as to how to debug and diagnose this I'm super keen to get coding on this machine and I'd appreciate any suggestions on what applying heat to this area indicates as broken. Thanks in advance.

 

[muse]

Unfortunately, having trouble seeing the videos.

My guess is, since this machine was working before, it’s possible that a trace or solder pad was damaged or cracked in the process of removing capacitors resulting in poor electrical connectivity. When the machine warms up, thermal expansion of the copper can temporarily restore connectivity, which explains why it eventually bongs and becomes usable. I would carefully inspect the work...use a magnifier, phone camera, or ideally a microscope to check for any broken traces or lifted pads. You might need to remove components to see any damaged traces or pads.

Another option. Use freeze spray on the affected area to see if you can reproduce the fault and that will give you a good place to zero in on.