ZX81 First line of display distorted

Discussions about Sinclair ZX80 and ZX81 Hardware
daveyj67
Posts: 14
Joined: Tue Oct 31, 2023 10:43 pm

Re: ZX81 First line of display distorted

Post by daveyj67 »

Hi Mark

The Traco was added because the 7805 wasn't working, the capacitors haven't been changed.

I'm getting the same readings whether the RAM pack is fitted or not:
12.02v at power connector and Traco input
5.02v at Tracto out, on ULA, ROM, Z80 and the 1K RAM chip.
Also 5.02v power to the composite mod with 3.38v input and 2.6v output from the composite PCB.

Since installing the borrowed ULA I have had another play around with the tuning pots on the composite mod, I have been able to get a stable top line with both 1K and 16K. However with the 16K RAM pack the contrast on the screen is poor. The K prompt appears a washed out grey rather than black.

Thanks

Dave
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