Re: Newbie - ZX81 ULA issue?
Posted: Mon Feb 05, 2024 9:19 pm
I wouldn't have thought the differences in the ULA would make any significant difference. It's possible that if the 6.5MHz resonator circuit has drifted frequency slightly, it may be mixing with another frequency and the result is a frequency near enough to that of the colour sub carrier frequency, hence the TV sees a colour signal.
As jail bars are reported, one of these may help:
The video board circuit has been derived from an existing design, but with alterations. Hence I can only guess what the two presets (adjustable resistors) do. My best guess is that they control the back porch part of the circuit. But that board has mote components than the existing design.
If the OP can take some photos of the top, bottom and sides of the board (so that I can see the labelling on the three legged transistors, I may be able to have a better guess.
Mark
As jail bars are reported, one of these may help:
- Try a different Sinclair PSU, or a suitable alternative 9V DC PSU.
- Adding two 100nF capacitors, one across the +5V and 0V/GND rails for the ULA and one on the power input to the video board.
- If possible, try selecting different colour standards on the TV (PAL/NTSC etc)
The video board circuit has been derived from an existing design, but with alterations. Hence I can only guess what the two presets (adjustable resistors) do. My best guess is that they control the back porch part of the circuit. But that board has mote components than the existing design.
If the OP can take some photos of the top, bottom and sides of the board (so that I can see the labelling on the three legged transistors, I may be able to have a better guess.
Mark