Discussions about Sinclair ZX80 and ZX81 Hardware
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Mustermann wrote: ↑
Tue Mar 03, 2020 8:26 pm
Let's start with the assumption that a Timex Sinclair 1000 is identical to a ZX81 except of the size of the RAM.
Some TS1000 use issue 3 ZX81 boards... the only differences are the RAM chip(s), the modulator and video circuitry, resistor R30, extra inductors (to reduce the RFI) and metal springs to make contact with the metallic layer on the inside of the case.
To upgrade to 16Kbytes, have a read of the links in this post
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http://web.archive.org/web/201509070007 ... ernal_RAM/
is a quite good how-to to start with internal ram expansion.
It is near to the way i started with two differences:
I did not cut any line on the board but cut the legs of the socket that doesn't fit to the right place. Instead of that I soldered wires to the cut legs and connected them to the right place.
The 28-pin IC socket in IC4 position will now be wired as follows:
Socket need to be soldered a little bit lifted to avoid unwanted connections.
That will give 16k RAM and opportunity to upgrade to 32k WRX enabled RAM in next step.
ZX81 issue 1 near to original state
ZX81 issue 1 56k internal ram with battery backup, UDG, CHR$128 and WRX enabled
Minstrel 3 ZX81 clone 64k with battery backup, UDG, CHR$128 and WRX enabled