I've borked it

[msknight]

In usage, I seem to have borked the zeddy I'm currently working on.

I see a clock but I'm not sure how to check the actual frequency on my basic scope. I'll try and read up.

Most starts are white screen with no prompt. Some starts see "ordered garbage" or "repeating garbage" on the screen. I have tried changing the ROM and that's failed. The RAM is a 16k unit from MyRetroStore so I'd rather be more confident about thing before I go changing that.

A14 is low and A15 is pulsing.

Pin 16 is pulsing
Pin 17 is High
Pin 18 is Low
Pin 19 is pulsing
Pin 21 is Pulsing
Pin 22 is High
Pin 24 is High
Pin 25 is High
Pin 26 is High
Pin 27 is Pulsing
Pin 28 is pulsing.

My initial conclusion is that Reset, pin 26, is being held high, precluding the system from running ... but that's a guess. As to what could be causing it... now that's the game.

[msknight]

Found out how to check frequency. The readings out of the ULA and into the Z80 are as expected. The clock appears fine.

[msknight]

Processor has been swapped. That's not it either.

I'm left with RAM, unless someone else has any thoughts please.

[msknight]

This is an example when I get garbage on the screen. The capacitor has been removed that was inserted to slow down the inverted OE line.

I'm suspecting RAM at this point.

[msknight]

RAM replaced. It's not that, either.

The screen occasionally goes from white to garbage on moving the board, so it looks like there's something loose somewhere. I've done a quick inspection and re-worked some joints but I'm shooting in the dark and would be grateful for some advice on where to look, given these results.

[msknight]

My suspicion is starting to fall to the ROM. Possibly the socket, or a connection on the ROM, or to one of the other components that use it.

Going to take a break for the night, to lick my wounds.

[msknight]

On the ULA, RAM select is low, ROM select is pulsing. I'm not sure what to make of this.

I "think" that the ULA is attempting to read from ROM, but it's not succeeding, so the system isn't booting.

EDIT - The CS pin on the ROM is also pulsing, so it seems that this part of things is working

[msknight]

I've found a schematic and pin 26 reset on the CPU starts at zero and rises to 5v over a half a second and stays there.

As I've got a good clock, I assume that TR2 is working. That leaves two capacitors, two resistors and the X1.

I'm now at a brick wall. If I changed the X1... and I don't really know what that is yet... then I'd be doing so out of guesswork.

[msknight]

Just for extra information, I have tried one of the vLA81 replacements for the ULA and I've also inspected and re-done a series of joints on the board, and the board itself seems to not glitch when moved now. But obviously I'm left with this issue and I'm pouring through potential replacements for the transistors because I can't find this elusive X1.

[dessony]

msknight,

Which ULA version does your computer have?

How is the video signal ULA pin #16 coming out of your computer with your oscilloscope?

Thank you.
Dessony