Thread: Zx81
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Old 17th Mar 2020, 11:35 pm   #99
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 9,217
Default Re: Zx81

I have two ZX81s, one is my original machine, built from a kit and still in working order.

The other had been in an auction lot bought by a fellow forum member here and I acquired it from him, it is housed in a 'Dktronics' full sized enclosure / keyboard which is in almost as-new condition - clearly very little used by its original owner because when they were used regularly, the glue which secured the stick-on key legends would soften and the key legends would start skating all over the key tops. ( I know this because my original Spectrum spent most of its life in the Spectrum version of the same Dktronics enclosure / keyboard).

Both were updated to 2C210 ULAs when those parts were still readily available and not too expensive, and both are internally fitted with 16K RAM because, despite what I have said elsewhere about originality being key, the original 16K RAM pack is a dreadful abomination which neither looks good nor works well, due to the inherent unreliability of the connections (AKA 'Ram Pack Wobble').

It is a relatively straightforward mod to fit a 62256 SRAM in place of the original 1K RAM and you then get a machine which retains Rick Dickinson's classic design looks combined with a respectable 16K RAM, and nothing dangling off the back. Because the RAM is internally fitted, the connections are secure and solid and the machine can withstand a thump or three without the memory contents getting scrambled. The mod can also be done in such a manner that it can be completely reversed if required.

(You may point out that the 62256 is a 32K RAM, and you'd be right, but the most basic form of the mod only maps 16K of the 32K - there is arguably no point in mapping more of it because there is no mainstream software from that period which needs more than 16K.)
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