I've borked it

[msknight]

The ULA decides on if to select the RAM based on address line A14 and /MREQ. are these lines both okay between the Z80 and the ULA?

What do you get on ULA pin 12. What do you get on RAM pin 20 and 22 (assuming a 28 pin chip)?

Mark

A14 and /MREQ tone out between the ULA and the Z80.

What I get on ULA pin 12 is attached. Next step is to get out the 10x magnifier and check the trace. Even though they don't tone to be shorting to anything... there has to be something wrong somewhere to get a signal like that. Yes?

[msknight]

/MREQ is pulsing healthily at the ULA. A14 on the ULA is having four pulses, then silent for about 8 pulses, then another 4 pulses...and repeat... but throughout all this, I'm getting that same 500mV noise on ULA pin 12.

I take it that the expansion packs don't use the RAM select line?

[msknight]

I used another sockets and took out the pin for ram select, so that I could observe what the ULA was trying to put out on that pin without any potential interference from beyond it. The behaviour was most odd. Didn't even see any activity on A14.

I'm thinking my next move is to replace the ULA socket. Grateful if there are any other thoughts before I do it... because I can't find anything else and I'm grabbing at straws now.

[1024MAK]

I take it that the expansion packs don't use the RAM select line?

They drive it high (on the RAM side of the resistor) to force the internal RAM to stay deselected.

Mark

[msknight]

Socket changed. Fault persists. Ram select still looks the same coming out of the ULA

[msknight]

Replaced with a vLA81 and same outcome, but the ram select was at 5v, so I replaced the EPROM with the original ROM and still the same.

Short of re-replacing the RAM chip I'm not sure where to go next.

[1024MAK]

Do you have any spare resistors with a value between 1k to 4.7k (inclusive)?

Mark

[msknight]

Yes I do. I also have a variable resistor board, so I can jumper whatever resistance I want.

The other thing I have, is I have a signal generator. I can pull the chips and put deliberate signals down pins and then see what pops out the other end... if that's any use.

[msknight]

I've also, hopefully, added the UK modulator to that schematic correctly? It's at the bottom of the page here - https://msknight.com/bbc/zx81-reference.html

[1024MAK]

Connect a resistor (with a value between 1k to 4.7k) between any suitable +5V point (say IC4b, the unused 2114 RAM chip position, pin 18) and IC4b pin 8.

Then see what voltages and 'scope traces you get on ULA pin 12 and on RAM pin 20 and 22 (assuming a 28 pin chip).

Mark