Thread: Zx81
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Old 15th Mar 2020, 8:07 pm   #77
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 7,538
Default Re: Zx81

Nice methodical approach, made possible by having two of everything. For the 'dark picture' problem with the second ZX81 you basically have a number of options:-

-Re-sell it. It will eventually end up in the hands of someone who has a TV which is retro enough to work with it.

-Fit an additional bit of electronic circuitry which reinstates the missing portion of the video signal. This is not a particularly simple circuit so it would be best to buy it as a kit, but looking around, such kits can cost more than a replacement 2C210 ULA and are more complicated to fit which makes no sense, so the next option is :-

-Obtain and fit a genuine replacement 2C210 ULA. Unfortunately these are not particularly cheap - often more expensive than a whole ZX81 which already has the 2C210 ULA.

- At various times enterprising people have produced cheaper reverse-engineered modern versions of the ULA - typically a PCB with the 40-pin footprint of the original IC, but with one or more surface-mounted ICs on it. Unfortunately these tend to be made in fairly small batches and the two examples I found are currently sold out, although more may be made available in future.

For the original ZX81 I am now inclined towards thinking there is a problem with noise on the internal power supplies, the most likely cause of which would be tired capacitors - the more so if the machine still has poor / unstable video output when modified for composite video output.
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