ZX81 - fixed with vLA81 and now completely dead ...

[BleatingSheep]

Hi all,

I asked for help a couple of weeks ago in diagnosing a failed ULA in my ZX81. I bought and fitted a vLA81. I also disconnected the UHF connections and had a beautifully stable composite picture for the first time. All good - I could even load very old tapes happily.

Earlier this week, when I reset the ZX81 while I was using it, it didn't start - no composite signal. So I've taken it apart again and I'm completely confused. While all of the chips are getting +5V and 0V correctly, I can no longer see any clock signal - either on pin 6 of the Z80 or coming out of the vLA81. So instead I looked at pin 35 of the vLA81 today with my scope (in 10X mode). Before I saw a nicely formed signal from the resonator - now I see nothing at all - just a bit of noise.

Is it likely the resonator could have gone after all of these years? Is there any way to test the resonator without replacing it and hoping for the best? Is there any other component that could have gone that could cause this that I should check first?

Many thanks for any suggestions,

Mark

[1024MAK]

Pin 35 normally provides a connection to the 3.5MHz ceramic resonator for the original ULA but as the vLA81 has it’s own oscillator the pin can be used for another function.

I suggest you contact the designer/maker of the vLA81.

Mark