Sinclair ZX80 / ZX81 / Z88 Forums

Maybe the capacitor value needs increasing slightly? I have a big pack of assorted low value ceramic disc capacitors on the way and three ZX81s to play with now so I'll experiment. Here's a link to the capacitor pack just in case anyone else may be looking for similar... https://www.ebay.co.uk/itm/2...

Very nice!

Internal 16k RAM for ZX81 are found cheap on eBay: https://www.ebay.com/itm/195791643922 That one doesn't include a RAM IC Martin. I'd be more inclined to just source a 28-pin DIP SRAM IC (and socket if required) and homebrew it personally. No-ones going to see the difference once it's inside the c...

It's so much easier reading this as written words than trying to roll it around inside my head. Thank you.

That makes sense. Thanks.

It does indeed look as though commercial producers are following that exact schematic blindly.

This is weird! RudeDogRetros make the same mod I'm using (minus battery backup) as a plug-in module and that has a link that appears to either connect/disconnect that 10K resistor to enable/disable HRG8. This is the second commercial product I've found with the 10K resistor claiming it gives HRG8 ca...

1024MAK wrote: ↑Sat Jun 03, 2023 10:00 pmThe resistor in the other modification is only needed for configurations where you are trying to use battery backup.

I'll correct my saved copy of the schematic then as it says that resistor is for HRG8 and the other one is for battery backup. Thanks.

Yay! Thank YOU! I now know roughly what capacitor value I'm likely to need.

Mike put his across pins 8 and 9 of the empty IC4B space. That's going to remain the same regardless of ZX81 version as the RAM IC pin designations stay the same. EDIT: It looks as though I'll be doing the same as mine runs plenty of other hi-res games properly but not the dr.beep ones I've tried so...

Me too by the looks of it. That schematic states that the 10K resistor relates to HRG8, but I have no idea what that means. As Mike suggests, you may be experiencing a completely different problem altogether. Please keep us updated. EDIT: This mod which is identical (as far as I can tell) to what yo...