exxos's Atari, Amiga & retro forum

stephen_usher wrote: 20 Jun 2021 14:05 :dualthumbup:

Ok, resistance checks are done, and I did have to swap over the +ve and GND leads to get a reading. Power was off.

They are a long way off 25K though, which was what was suggested they should be over at *.

A0-A12 & A15 all read 400K

A13 read 40K.
A14 read 22 ohms.

Potential CPU fault then?

Oh! 22 ohms of A14. That would be why it's low then!

Are there any other components between the CPU's A14 and the ULA?

To be honest, without the CPU being out of the circuit it's probably impossible to determine where the fault is.

Actually, IC17 (74LS00) and one ROM are on that same bus aren't they? Hmm... I'd still remove the CPU myself so as to try to find the fault. Once it was out I would check the resistances again.

Did you check the resistance between the address lines and VCC so as to see what the pull-ups are?

stephen_usher wrote: 20 Jun 2021 17:30 Did you check the resistance between the address lines and VCC so as to see what the pull-ups are?

I did measure them, yes, and all come in at 56K other than A13 & A14.

A13 reads 40 ohms, A14 I can't get a clear read on it right now as I snipped it after talking to hoglet over on *.

IIRC it as not 56K though, it was much lower. I'm swinging more and more towards just pulling the CPU and dropping in a socket so I can test with a known good CPU.

If that doesn't work then we know it's most likely to be the ULA or the ROM.

Ok, there will be an interlude now I think. We may need to move house a week sooner, which is next week, so I'm packing things up ready to go.

I'll keep you posted.

Good luck with the move!

stephen_usher wrote: 21 Jun 2021 14:56 Good luck with the move!

Thanks, I'm sure it'll go just fine.

Certainly hope it goes with minimal fuss... always lots of stress though!

I might have to find an excuse to put some carpet offcuts on the garage floor and cover up the breeze block walls.
Will follow your updates with interest as you get setup in the new house.

JezC wrote: 21 Jun 2021 15:27 Certainly hope it goes with minimal fuss... always lots of stress though!

I might have to find an excuse to put some carpet offcuts on the garage floor and cover up the breeze block walls.
Will follow your updates with interest as you get setup in the new house.

Thanks, fingers crossed we'll be out of here ASAP as it's getting uncomfortable here now and we want to be gone.

Right, since I've located my de-solder gun, I can get cracking on this little guy again :)

I had a delivery of CPUs the other day too and sockets, so I think I'll just go for replacing that and see what's what.

If it is still dead after that, then there isn't much else it can be other than the ULA. I'm hoping it isn't, as that is a tougher nut to crack.

And the word is: ULA

Sadly after a CPU transplant it still has the exact same pattern of white bars on screen. I haven't got the 'scope out yet (I haven't found it yet, but it's here somewhere!) but pretty much all it can be now is the ULA.

I may try swapping out the last 3 RAM ICs since I am in a much better position for IC removal/replacement now I have my desolder gun again, but it really does look like the ULA, so on the hunt for a replacement.