Sinclair ZX80 / ZX81 / Z88 Forums

That “smudging” of the m and w looks like an artefact caused by the TV enhancement processing system. On some TVs you can turn all the picture enhancements off. Also play with the sharpness control. Note that with some LCD TVs, they still do some image processing even with all the picture enhancemen...

ZX Printer This post details the ports and bit data for the ZX printer. The ZX81 and ZX Spectrum ROMs support three BASIC instructions (LPRINT, LLIST, COPY) which access the external printer hardware by these I/O addresses. Port FBh Read - Printer Status 6D748CB1-A7DB-4ED3-8BFB-477C3E857558.jpeg Po...

Both versions (the original bugged and the fixed) of the Sinclair ZX81 ROM work in ZX80, ZX81 and TS1000 (and clones that use the same circuit) machines. In practice, all later ZX81 and probably all TS1000 machines all have the bug fixed ROM, so if you compare a U.K. ZX81 ROM with a TS1000 ROM, you ...

You should be able to read back the EPROM in you programmer, then compare the read back file and the original on your PC using a suitable application (I use a Hex editor so I can see which bytes are different). I presume you either used an adapter board / socket, or bent the pins out and hardwired t...

A couple of questions, which issue is your board? Can you please upload a photo of your board.
Are you good at soldering? Do you much experience at soldering?

Apart from a multimeter, do you own or have access to any other test equipment (logic probe, logic analyser or oscilloscope etc...).

Mark

Sorry, real life / work has kept me busy... still catching up... Thanks a lot Mark ! In the meantime "Bola_dor" helped me with the voltages measurements. They are all good. But now something new happens : running the same memory test now always end up with "IC25 failure" ... IC25 is a SN74LS157N whi...

You may have already done this, but double check all connections using a multimeter on a resistance range between the EPROM pins and the relevant Z80 CPU / ULA pins (keeping in mind the series resistors).

Mark

The Memotech 16K, 32K and 64K RAM packs all use a fast PROM for address decoding. A hand drawn schematic for the control circuitry for the 16K version is in this thread ;) Note that since posting that, I have become aware that later versions of the 16K unit used a different board and used HM4816AP-7...

I can't found a good proven replacement for TR7 about the loud speaker.. I wonder if the C635 I used to replace TR4 would work? I don't even know if it will last as TR4.. I had to cross its leads but gave me 12V rail up again.. Another question is if this replacement can be the cause of bad quality...

The ZX81 (and ZX Spectrum) ROM code is designed to drive the Sinclair ZX Printer. So to fully understand the system used, you really need to see how the ZX Printer works. The ZX Printer burns the conductive metal coating off the ‘paper’ as a stylus moves across the ‘paper’. The stylus is moved by a ...