exxos's Atari, Amiga & retro forum

I brought it down from the loft to give it a check-out before the 40th anniversary in March, plugged it in and switched it on without the RAM pack and it worked for a few seconds and then the output signal disappeared.

Swapping the vLA81 ULA replacement out and putting the original ULA at least got a picture, but without an end of frame so the TV couldn't get vertical hold.

I'm going to have to get the oscilloscope down now to try to work out what's happening, but I think the vLA81 must be toast. I burnt my finger on one component just "north" of the CPLD. I'm not sure if that's a MOSFET. It was extremely hot.

Mine needed a new CPU as well as well as a vLA81, but at least there isn't much to troubleshoot on the ZX81!

I got one of these for composite video: https://www.sellmyretro.com/offer/details/2338

Bit expensive, but I figured I might as well go the whole hog, and the picture is exceptionally clear.

I also added one of these: https://www.sellmyretro.com/offer/details/40565

I've probably spent a good deal more on it than it cost me to buy, but with a new keyboard membrane as well, I'm really pleased how the whole thing turned out.

Good luck fixing it.

Well, I didn't want to gut the modulator. Anyway, there's a simpler solution, a 100uF capacitor connecting the composite feed from the ULA to the photo socket, which is fully reversable. It works a treat on my Spectrums and is a tad cheaper. ;-)

My next step is to get a new socket. The original ULA looks to work so it does seem to be the vLA81 itself which has died for some reason.

The upgrade I did is fully reversible too, the ZX81 modulator came apart very easily and I have retained all of the components. Expensive granted but the picture quality is superb.

I do have a dead Spectrum though and I am going to to go for the cheaper route to composite on that, well once I work out why it's dead.

The ULA and CPU sockets on my 81 were dire single wipe types, so they all got replaced with newer double wipe sockets.

Have you thought about contacting Charles re the dead vla81? He might be able to offer some assistance and was really helpful when mine took an age to arrive from New Zealand.

I've ordered a new socket (turned pin) which should arrive tomorrow. I'll fit the original ULA first.

I've done a quick once-over with the oscilloscope and nothing looks terrible. With the original ULA I am getting a 'K' cursor but it doesn't seem to be reading the keyboard, which suggests bad connection on the ULA end.

I've not 'scoped the machine with the vLA81 installed.

I've not contacted Charles as I've not really had time. I'll probably get the machine running with the original ULA first so as to verify the main machine is working before trying to troubleshoot the vLA81.

OK, I've got the machine working with the original ULA.

When I reseated the CPU last night I didn't notice that the /RD pin wasn't actually in the socket!

The vLA81 still doesn't work, so definitely toast. I'll have to talk to Charles.

Unfortunately the keyboard membrane has finally failed too much to trim, so I'll have to find the new one I've stashed somewhere Safe(tm).

somewhere Safe(tm) = never to be found again ;)

Almost... I had a brain wave and found it with my insurance documents and passport... see, safe!

Anyway, I managed to have a chat with Charles Ingley and did a diagnostic and the vLA81's CPLD is as dead as a dodo.

However, the original ULA still works, but only on B&W TVs.

Nice, that's good progress 😁