Mac 128K Logic Board (Fault Finding)

[Stephen]

I purchased this Mac 128k logic board knowing it wasn't working. I've poked at it a little over the last 6 months, which includes:

• Building a 4164 DRAM tester and testing the original DRAM. I found 2 were faulty but ended up replacing them all.
• Swapping the LS166 @ F14 & F15 (example board image for reference)
• Swapping in known good ROMs from a 512k and Plus, as well as swapping these ROMs to a Plus

Currently the logic board is booted from an external ATX power supply and video is captured with a RGBtoHDMI.

I can get the following pattern:

This gif is on loop and demonstrates a manual "power on" and "power off" cycle by providing power through the PSU.


I also confirmed in a compact mac that this logic board produces the same result in its corporeal form:

Does anyone have any suggestions? I checked the oscillator and it looks like it's within range.

 

[RetroViator]

I'm curious about the processor. Perhaps if you poke around with your oscilloscope, you can see if it's running correctly compared to another 68k. If a pin that should have data (such as from the address bus) is stuck high or low, it could be a problem with the processor or whatever is feeding that pin. It might give you some more information to track down.

 

[Stephen]

Does this 1000 foot view show anything unusual?

Note

• For pins with a voltage range (e.g., BG) the values represent the lower and upper fluctuating voltage.
• For pins with a frequency range (e.g., D1) the values represent fluctuations typical fluctuations observed over a ~5 second window.

I thought it was a bit unusual that A21 and A20 are not showing fluctuations, like they're pulled high. I'm not sure if this or any of the other records are normal — please be sure to call out anything that seems strange!