Sinclair ZX80 / ZX81 / Z88 Forums

It's a 16k pack, but if the chips are being run out of spec I may just turn the juice up.

I only use this pack on my forth machine but as I have found a way to use the xpand+ with h4th I may switch permanently to that, which I run at 7.5v as the 2 regs on the rear of the xpand glow in the dark when used with the Sinclair pack but are luke warm at 7.5v.

So Ive already bid on and won an offical zx81 rampack before reading your post :-/ ...lets hope its ok.

Ive tested the requested pins now 3 times powering off the zx81 inbetween each test. Test 1 & 3 produced the same readings Test 2 produced diffent on 4 of the pins i think.

ZX80 Chip Test 1:

Pin 4 - A14 Pin 5 - A15 Pin 16 - INT Pin 17 - NMI Pin 18 - HALT

Pin 19 - MREQ Pin 21 - RD Pin 22 - WR Pin 24 - WAIT Pin 25 - BUSRQ

Pin 26 - RESET Pin 27 - M1 Pin 28 - RFSH

TEST 2 produced same on all other pins apart from the ones bellow

Pin 4 - A14 Pin 5 - A15 Pin 17 - NMI Pin 18 - HALT

I’ve had a brief look at the waveforms you posted. Apart from the signals on A14 and A15, the Z80 looks like it may be okay.

Mark

Kind of good news then i guess. I did think the A14 & A15 lines looked a bit strange. Ill assume the Z80 is good and wait for the ram pack to test further.

So the Ram Pack arrived and is now fitted. The screen now shows more than just a white screen as pictured below, although sometimes it still just shows the white screen. It looks almost like its showing the cursor but its just repeated and scrabled.

Update - I have managed to buy another working issue 3 zx81. I have tested the rampack on this and this also appears to work correctly giving a longer white screen before the cursor appears when it is connected and powered on.

So im guessing since the rampack didnt fix my issue the memory is good & from the Z80 traces that i took with the scope that this is good.

I did check the capacitors on the new board and compared to the non working board. I noticed Capacitor C5 on the non working has a much higher esr reading than the good board. Could this cause my issue?

There are two electrolytic capacitors on a ZX81 / TS1000 board. One is across the nominal 9V line. As long as the 9V PSU has a good electrolytic capacitor, this capacitor is not essential.

The second electrolytic capacitor is part of the reset circuit. If the /RESET pin on the Z80 CPU goes low to high at power on (it’s a very quick transition), it’s fine.

Mark