ULA and clock signal - advice needed

[BleatingSheep]

Hi,

I'm in the process of bringing my old ZX81 back to life. I've so far replaced the electrolytic capacitors and checked there's 5V at each of the chips. I've also managed to get a picture of the output from the oscillator circuit. Using the probe in 10X mode I see on the ULA pin 35:

Which looks good to me. I don't then see a clock signal either going into the Z80A on pin 6 or from the ULA on pin 14. What I see is:

Am I correct in believing this indicates a broken ULA? If so, a couple of questions:

- Is vLA81 the only ULA replacement option nowadays?
- I've also been looking for a composite video mod. A number of the obvious ones are out of stock e.g. at TFW8b. Can anyone recommend one?

Many thanks,

Mark

[1024MAK]

The ULA should always produce a 3.25MHz (or thereabouts) clock on pin 14. However, I do recommend you use manual settings for your ‘scope. Some ‘autoset’ functions get it wrong, and then will show a misleading waveform.

If there is a square-ish waveform at pin 14 on the ULA, TR2 will invert this and extend the switching levels so that it is suitable for pin 6 of the processor. Pin 6 needs to see a rail to rail waveform.

At the moment, yes, I believe the vLA81 is the only replacement currently available. Although a member of this forum is working on their own replacement version.

If you use a vLA81, you don’t need any extra composite modification modules/boards.

If using an original ULA, I actually strongly recommend Pokmon’s ZX8-CCB. His store is here.

There are however various other composite modification modules/boards. Some “clean up” the signal and add the “back porch” part of the video signal (like the ZX8-CCB), some just add the the “back porch”, while others are just electrical buffers/level shifters. You can also copy my design, link here.

Mark

[Lardo Boffin]

Plus 1 for the ZX8-CCB! I have fitted a lot of these over the last few years.

[Moggy]

Also the ZX8-CCB does a pleasing inverse video and border via either a jumper or switch, which three of mine are fitted with.

[BleatingSheep]

Hi,

Thanks very much for your responses.

Mark - I agree about the auto set - that second picture was one of several the trigger latched onto. In reality all were just noise. I’ll invest in a vLA81 and see if I can bring my original ZX81 back to life. Unfortunately my ULA is a soldered one so that’s going to be a challenging afternoon.

Also thanks for the ZX8-CBB recommendations but also note that I won’t need that if I go down the vLA81 route.

Many thanks,

Mark

[1024MAK]

If you are certain that the original ULA is dead, then I recommend cutting each pin. It’s much easier to remove each pin individually once the body of the chip is out the way.

Mark