Sinclair ZX80 / ZX81 / Z88 Forums

Ahh, now that you mention it, it has been reported before that using a CMOS Z80 (and a Z84C0008 and a Z84C0020 are both CMOS) occasionally causes problems. The solution is to fit higher value resistors in positions R1 to R8. Also occasionally if different (more modern) RAM chips are fitted, this may also cause problems. Z80s don’t fail very often (unless the edge-connector is abused), so I have not seen this problem myself.

It’s all to do with the more modern chips being able to more forcefully drive the data bus, but at the same time, requiring less power to drive their inputs.

Glad you got it working

My next suggestion was going to be, “well you changed everything except one thing, change the whole PCB for a working one”

Thanks for letting us know that you got it sorted and for reporting back what the problem was.

Mark

Mark, when you say "The solution is to fit higher value resistors in positions R1 to R8", which value would you then recommand for R1 to R8 ?
I would like to be able to put any Z80 on this "issue 2" motherboard ...
Thanks again !

I’m sorry, I don’t know what value to recommend.

I have not yet had to fit a new Z80 to a 16K or 48K board (I have some NMOS Z80s from some surplus equipment). So I have not experimented with this.

I know CMOS Z80s work fine in the +2A, +2B and +3 machines, plus they work in ZX81s and Harlequin machines.

If I were to guess, I would say try 820Ω to 1.2kΩ... Just don’t hold me to this guess!

Mark

Thanks a lot Mark. In fact I tried a CMOS on a Spectrum with issue 4S and 6A motherboard, it works fine.
I only had problems with my issue 2 motherboard. I will check the R1 to R8 value for 4S and 6A, may be I can try the same value for issue 2 ?

Or may be I will just give up, thinking that I already worked too much on this issue 2, and now that it is working fine, I'm a bit afraid of breaking something by testing some new value with R1 to R8 ...