I've borked it

[msknight]

The resistance between the address lines and ground is in the region of 3 to 5 Meg. The resistance between the data lines and ground is more like 15K.

If this is wrong, then while I might not have a dead short somewhere, there could certainly be something a little off, somewhere... although I can't see a single point of failure, unless a chip has failed.

But I don't know if those readings are correct, or not.

[1024MAK]

... but when I measure between the 5v pin and the output pins on resistor pack one, I'm only getting about 5.7K instead of the 10K listed... although I'm aware that this could be affected by it being in-circuit. I can't see an entire resistor pack being affected like that?

But all the resistors in it connect to the data lines. So any of the chips may 'load' each line. A value of 5.7kΩ is not going to cause these issues anyway.

Mark

[msknight]

Well, unless you've got anything else for me to try, then it looks like I'm back down to chip swapping, because I've used a 10x magnifier to look over the board and I'm clueless.

[1024MAK]

You leave me little choice, but to say "you're fired"!

Honestly, I will have to have a think about this.

Mark

[msknight]

Well, here's a little more to add to your thinking.... all chips replaced. ROM socket replaced. By default, earlier on, the RAM socket was replaced when I started working on it.

Scope picture is D0 on the ROM. It looks like there are, "reflections," happening?

Yes, the kynar is connected to D5, the light reflection means you can't see it.

[Lardo Boffin]

Probably a different way of doing it but aren’t there normally 4 wires coming off the RAM for that mod?

[msknight]

Breaking link 1 and making Link 2, activates A10 to the pin, so that saves an extra kynar wire to a diode.

[Lardo Boffin]

Good to know thanks!

[msknight]

For what it's worth, here's D0 on the ULA and it's fairly the same on the Z80 as well. Looks worse that side of the resistors that are between them and the RAM/ROM. My amateur mind is turning to the resistor pack.

Incidentally, when I desoldered the ROM socket, I did find a couple of points where I was thinking, "Where's the pin?"

[msknight]

Well, curiosity got the better of me and I desoldered the resistor pack.

As I expected, being out of circuit, it gave the correct readings. However, what I wasn't expecting is that it is a ten pin pack that they've cut a pin off, to make it 9 pin. I guess they really did use whatever they had to hand