Newbie - ZX81 ULA issue?

[1024MAK]

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:

• 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

[Lardo Boffin]

At the bottom of this page you can get a voltage regulator to replace the 7805 with one that won’t generate much heat:

https://www.retroleum.co.uk/spectrum-repairs-mods

I have fitted quite a few of these.

[msknight]

I'll second that voltage regulator. Took my zeddy down from 300 miliamps to just under 200. I'll be doing it to everything now

[Moggy]

The CMOS CPU surprised me. 200 milliamp consumption for the old Z80 and 20 for the new one!

[msknight]

On reading that, I had to swap one out, and went from 158 milliamps down to only 123. Still a drop, but not by that much.

[1024MAK]

Keep in mind that the 200mA for the NMOS version is a maximum rather than a typical figure. I have measured the current of a NMOS Z80 running in a ZX81 but can't remember where I put the results...

Sir Morris (I think) had a ZX81 with as many low power parts as possible.

Mark

[W3bb0]

Long delay for an update - see below!

On top of the original work (replacing the Z80, voltage regulator, RAM, electrolytic caps and PSU) I also reflowed all of the joints (by the state of the soldering I suspect that this was originally a kit) all to no effect.

It took a while but I managed to track down a loan ULA (of the same revision) for testing purposes and this did fix all of my issues so looks like the original ULA is marginal at best.

Always the one thing that's difficult to replace but I'll enjoy the ZX81 while it lasts!

[msknight]

There is the uLA81 although it does cost the best part of £40... but it does have a few tricks up its sleeve

https://www.retrorevivalshop.co.uk/prod ... placement/