I'm trying to diagnose a couple of issues but am increasingly beginning to think I may have a ULA (2C184E) issue:
When first booting, I will get a black screen about 33% of the time, removing the power and re-inserting will generally return me to a K prompt and everything seems well (Keyboard works, able to load from tape, run programs etc).
However, after about 15 minutes of use, the screen display starts to become soft with odd wavering rainbow colours and has some instability as can be seen in the attached picture (ignore the moire but the colours are what is seen on screen). Note that the ZX81 still functions in this state.
This seems to be heat related as powering off and on again fixes the problem but only for a short period. After leaving switched off for an hour I get back to the 15 minutes of normal operation.
The steps I have taken so far:
- Installed Composite mod with back porch option (ZXRenew version)
- Cleaned legs / sockets of all chips with contact cleaner (all chips were already socketed)
- Replaced C5 (in case there was a reset line issue with the black screen at startup)
- Replaced 7805 voltage regulator like for like - I did not have a Traco to hand
- Replaced power jack (it was *very* loose)
- Tried a different PSU (from an Atari 2600)
- Tried with and without a RAM pack to eliminate issues with internal memory
- Swapped Z80 with a known working CMOS chip
- Tried several different TV's
Can anyone think of any other troubleshooting I can do (without a scope unfortunately) other than trying to source another ULA?
Many thanks for any tips anyone can provide!