Motherboard model: C070523-001 REV.D
There are no modifications applied to this ST, everything stock.
This is what happened:
- ST didn't want to run colour monitor (SC1224, works great on my STfm) anymore (looks like 70 hz signal going out, screen running vertically, no distinguishable graphics)
- Mono works fine still (SM124)
- Started basic from the language disk
- After a minute the ST froze, no mouse pointer visible but screen still visible
- Power off, power on: nothing happens. Screen stays black (although a single frame can be seen with like 5% of non-blackness, looking distinctively like analogue distortion, nothing digital). The drive light doesn't light up. No digital activity of some sorts can be perceived.
- I always smell first (general life advice) but nothing out of the ordinary.
- Disconnect the disk drive
- Look for anything that could cause a short
- Tried 3 different PSUs which all show decent Vpp readings on the scope when sitting in this ST
- Pushed shifter firmly in its socket
- Reseated glue/mmu chips after cleaning with cleaning alcohol
The last steps were clearly the solution in my case, I already suspected the glue or mmu chips being the culprit here because apparently ram wasn't even accessed.
So while I fixed the problem while writing this post, I still wanted to share it because someone with the same symptoms might find it useful.