CheezyBroccoli wrote on 2024-04-29, 02:52:
All I’ve done is a visual for solder, capacitors, etc
Usually the electrolytics get better when heated, not worse, so it's a bit odd that the system seems to work fine only when cold. So the logic board is in the same case with the monitor? Could be some noise from the CRT system is upsetting the logic.
Resolder the flyback and perhaps the transformer and storage coil if the PSU is of the switching type. Test or replace electrolytic caps on the PSU and logic board(s) - don't skip the little ones, in fact these can dry out without any external signs. Be suspicious of any semiconductors that get hot but are not cooled (esp. if the PCB got discolored). I've seen zener diodes go bad in weird ways, and sometimes even the rectifier diodes go open.
If nothing seems to help it might be one of the logic chips being very marginal. CMOS ones usually work or not (that is - it's not temperature related). I don't have much experience with marginals NMOS/PMOS but TTLs can and will degrade over time. Especially the early ones (no letters or S series).