Hello Erm, (and welcome to this forum
)
The RAM chip is a new 62256.
Do you happen to know what manufacturer? I don't recognise the logo (which happens to me rarely
). MM62256 turns up little, HM62256
suggests Hitachi but it's not that? I'd like to check datasheet for that part.
You could have saved a wire: I see you changed LK1 to LK2 position. Which would have put A10 on pin 21 of the SRAM. Instead you bent that sideways, connected to D3? (=A12), and connected A10 (from D2) to the other SRAM pin (23). But that wouldn't affect operation. As far as I can tell, all address pins of the SRAM go to
different address lines (A0..A13), and leftover one is on a fixed logic level (pin 2, +5V). No other weirdness. So that looks okay to me, and one would expect normal operation with that.
Other info seems to suggest adding a resistor to make it hi res graphics compatible (..)
That would be a pull-up on the /WR signal most likely (10k or so between pin 27 & 28 of the SRAM). For an external RAM pack that makes sense, as (in the case of RAM pack wobble) it would pull /WR
INactive rather than floating.
A bit less likely to cause a crash or hardware damage.
But for an internal mod it doesn't make sense. The Z80 always drives /WR (apart from BUSREQ which isn't used on the ZX81), so a weak pull-up wouldn't affect timing at all. I don't have such a pull-up on any of my ZX81's, and no problem there. Nor do ZX81's have a pull-up on the /WR line from factory, but are hi-res capable with their internal RAM anyway.
and I've also found mentions of putting caps across pins to rectify display issues.
Most likely a few pF (4.7 ... 22 pF or so) on a control line like /RAMCS or /ROMCS (on the non-ULA side of series resistors in those signals). Either intentional or accidental. Of course you could experiment with that. But should not be needed.
Basically my problem is that when I try to run any of the 1k games by Dr Beep I get odd screen goings-on making most of them unusable.
How about 16K software? Have you tried any? Also: you could check some system variables to see how much RAM is detected. And watch the startup delay before the "K" appears - a ZX81 wit 16K takes longer for this than a 1K machine.
I think you'd probably best check all connections around the IC socket. Both optically (light
through the board + magnifying glass),
and by measuring resistance between connections. And check for shorts too.
What RAM was on the board originally? A pair of 1K*4 bit chips, single 1KB SRAM or 2KB one? And right before the mod? It's possible that during unsoldering a via got ripped out. Normally you wouldn't notice as IC removed -> IC is out-of-circuit. But sometimes an IC pin or via routes a signal from one side of the board to the other. Remove IC -> no issue. Remove IC + damage via -> connection is cut. Same thing if via's aren't plated, so that IC pins connect signals between sides of the board.
Only other thing I can think of is a wonky SRAM (ESD damage or whatever), or a part that
looks standard, but has some uncommon pinout or timing requirement. About the latter: you could try taking /OE (SRAM pin 22) out of the socket, and wire it to GND. But again:
should not make any difference. Or put in another SRAM.