Time to go back in time again - ZX80 replica build

[HigashiJun]

too...

[rubber_jonnie]

too...

Postie has just been and I have all the IC's and a crystal

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Hopefully I can do a power on test and see what's what later on. Don't have the sockets yet but that is fine, I can work around it for now.

Note, the crystal is in place of the resonator and is fitted as shown by Grant Searle on his ZX80 pages that I linked to earlier.

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We'll see how well it works. Not long to go @HigashiJun

[HigashiJun]

We'll see how well it works. Not long to go @HigashiJun

[rubber_jonnie]

OK, so I have good power but the clock was rather erratic. C15 is related to the clock and can be anything from 'not installed' to 330pF, so to see what happens I removed C15 and the clock has now stabilised at 3.2Mhz, and I am seeing what looks like normal activity across the address and data pins.

What I'm not getting is a clear video signal, so I need to dive into the schematic to see what's what. Of course my 2732 could be bad, I just don't know, though it did program just fine.

What's interesting is that when the clock was bad, I was getting something i=on the video and it looked like there was a response to keypresses, though super difficult to tell.

Hopefully my modulator will arrive today and I can try that on my period correct TV

[stephen_usher]

Good luck!

[rubber_jonnie]

Good luck!

Thanks!

Address and data lines actually look OK on the RAM, ROM & CPU, it looks like it's running code, so I need to track back to the video generation circuit and figure out what's going on.

[rubber_jonnie]

It lives!!!

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You may not think this shows very much, however it is allowing me to see that keypresses on screen(ish...) and the machine is responding and BASIC is running.

I got to here by swapping the RAM out. I had a lot of spare from the OSI600/610 build, which was handy as the ZX80 uses the same 2114 RAM.

Of course this is going straight out via a capacitor for now, but there is at least a good indication that the rest of the machine is OK.

The modulator arrived today so I will fit that later and try with my old TV.

[rubber_jonnie]

OK, so I got the modulator and realised I'd been picking video off the wrong spot... D'oh.

Modulator is now installed, but I get this:

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But it still behaves as though it is at the basic prompt, so if I hit NEW and NEWLINE (Return) it resets and the screen disappears for a few seconds. Pressing any key causes a flash of the screen, a bit like a ZX81 in fast mode. Also, I see a line at the bottom that looks like the bottom edge of the screen, so it's just a case of figuring out what causes the screen to be all black.

After getting the previous screen picking off the wrong point and kind of being able to half see the text I'm pretty sure the machine is running.

Incidentally, I did try the modulator on a TV and I get exactly the same display, so something to do with the display is off and needs fixing.

[stephen_usher]

Does the circuit generate a backporch? I don't think that the ZX80, like the early ZX81s, did.

Colour TVs and all modern LCDs will fail to show much without a valid backporch.

You'll need a B&W telly to test.

[rubber_jonnie]

Does the circuit generate a backporch? I don't think that the ZX80, like the early ZX81s, did.

Colour TVs and all modern LCDs will fail to show much without a valid backporch.

You'll need a B&W telly to test.

It doesn't AFAIK.

Currently composite and modulator (UHF) on colour TV show the same picture as in my other post. I do have a B&W TV but it doesn't have an aerial socket, just composite in and a telescopic aerial, so I may be able to rig something up with that. God I wish I still had a proper period correct colour TV.

There are some composite adapters that replace the guts of the modulator that include back porch generation, so one of those might fit the bill. I had backporch issues with my ZX81 and it was a bad ULA and I dropped in a VLA81.

Sadly I can't do that here, no ULA!