Sinclair ZX80 / ZX81 / Z88 Forums

Hi,
I have a couple of issue one ZX81’s and one has the early ULA without the back porch signal and early buggy ROM and the other the later ULA/ROM combo. I have fitted some homemade 555 composite mod boards to both and they work great, giving a stable clear picture.
When I fit the vLA81 to either all I get is either a black screen when the settings of MOD and BP are off or a white screen if the are set to on. If I switch to either one or other being off/on all I get is a black screen.
So I went and fitted a Transistor mod instead and removed the 555 mod and with an original ULA I got the “K” cursor back but with the vLA81 fitted it was the same behaviour as above.
I followed proper ESD procedures and it has not been out of the anti static bag since it arrived a couple of months ago.
Am I doing something wrong with this chip? or the settings or which video mod is used?

I suggest you reread the installation instructions. Check that ALL pins of the module are making good contact in the socket. Even better if you can test using a digital multimeter on the continuity range.

The composite mod. boards that use a 555 timer chip are intended only for use with ULAs that don’t generate the back porch part of the video signal. They are not needed when using a vLA81.

Then if you are still having problems, contact Charlie directly via his web site here. He also occasionally pops up here under the username of Bloodnok.

Mark

I had read the manual several times before I posted on here and tried removing the 555 composite mod and fitted a simple 2n3904 transistor only as well. So even with the back porch composite mod removed I’m getting nada.
I fitted new 40 pin sockets before I tried the vLA81 and the fact that they both work with the original ULA’s I have makes me suspect the vLA81 has problems, realistically I’m not going to send it back to NZ, I’ll just stick it away for now and look for an issue 3 PCB to try it on.
I have two working ULA’s for now and I’m building a ZX81+38 so it is not urgent, I just thought I was doing something obviously wrong but it seems not.

There’s no significant difference in the circuitry between an issue one PCB and an issue three PCB.

Mark

1024MAK wrote: ↑Mon Aug 30, 2021 2:16 pm There’s no significant difference in the circuitry between an issue one PCB and an issue three PCB.

Mark

I want to pick one up anyway just to add to my collection. It sounds like it won’t make any difference to the vLA81 but I still want to own one. Hey-ho.