ZX81 essential maintenance?

Discussions about Sinclair ZX80 and ZX81 Hardware
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Re: ZX81 essential maintenance?

Post by 1024MAK » Tue Sep 11, 2018 9:09 pm

You can still open the case without disturbing the membrane tails. Then if you have or can borrow/beg/steal a small makeup mirror or a small inspection mirror, and by also using a torch, you may just be able to see enough of the chip to read the details...

None of the ULAs (or add-ons) generate the front porch part of the video signal. So if the line (horizontal) sync is not a simple rectangular pulse, but has a step on the trailing edge, that's the 'back porch' part of the video signal. It is this level (black level) that colour TVs sample to set their internal reference to 'black level'. Hence the importance of this part of the signal. So if you get such a signal, you must have the last ULA version (2C210E).


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Re: ZX81 essential maintenance?

Post by hjalfi » Tue Sep 11, 2018 9:53 pm

Red PCB, checksum is 855106, back porch present, so it's indisputably the new ULA and the new ROM.

Re black level: yes, you're right, I misread the spec. So about the only thing I can think of which the HDMI upscaler is upset about could be that the sync pulse is at 0V rather than -0.4V? But it's supposed to be decoupled anyway, so the exact voltage shouldn't matter... and it works everywhere else.

So I'm calling it: I think this is now done. My ZX81 is in full working order, with new regulator and composite video and everything. I have a setup I can take to demo to people (although using the video capture device rather than the upscaler), or if I'm really dedicated I can use the 8kg Sony PVM. Shame about the mono TV, but it's adequate in inverse video mode.

So, thanks for the help, everyone! There should be a Youtube video coming along in a couple of weeks...

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