It's 600XL time!

[rubber_jonnie]

So, a very quick bit of lunchtime diagnosis on the 600XL and I have found that there is no address line activity at all on the RAM chips at all. No RAS or CAS activity either.

I see some on the ROMs and Sally, and though I do need to check the rest of the ICs, I need to take a closer look at the logic sitting in front of the RAM.

So that's a bunch of LS logic chips, and those I can test with my TLS866

[rubber_jonnie]

And one is dead according to the TLS866, a 74LS51, quite a key component, and the one of the few 74LS logic chips I don't actually have to hand!!

Hopefully they will arrive soon!

[rubber_jonnie]

And one is dead according to the TLS866, a 74LS51, quite a key component, and the one of the few 74LS logic chips I don't actually have to hand!!

Hopefully they will arrive soon!

EDIT - The remaining 74LS logic chips all test good, and given that the failed 74LS51, U16, is slap bang in the middle of the CAS/RAS circuit:

Screenshot 2022-01-14 at 14.35.23.png (1.82 MiB) Viewed 3225 times

Then I think I definitely need to change it.

[tzok]

Don't fully trust TL866 in terms of TTL testing, especially those less common. There are mistakes in the test vectors, verify them first.

[rubber_jonnie]

Don't fully trust TL866 in terms of TTL testing, especially those less common. There are mistakes in the test vectors, verify them first.

Fair comment, I will have a look.

[rubber_jonnie]

Well the replacement 74LS51 arrived but made no difference.

Better do some proper diagnosis then

[stephen_usher]

More oscilloscope fun!

[rubber_jonnie]

More oscilloscope fun!

Indeed!! My scope has never seen so much action!

I'll also use my TL866 Pro to check the new ICs, but it might be the case that @tzok is correct and the 74LS51 has a bad logic table in the software, so doesn't test right, and I haven't got round to checking the vectors yet.

[rubber_jonnie]

Ok, so checking the new 74LS51s reveals that it looks like the TL866 test vectors are bad, but I have dropped a new IC in anyway.

I'm seeing activity on the address lines, but some just don't look right, and I do see CAS/RAS activity on the RAM during power up, so that's something.

I am concerned that Sally may have expired as she is extremely hot quite soon after power up, and what with the weird address line activity too, it's definitely a concern.

Clocks are all good at Sally too and elsewhere on the board.

I suspect that the GTIA and ANTIC are probably ok as I do get a picture, albeit a wrong one. More digging needed and the 800XL is going to have to be opened up, see if I can borrow any socketed parts for testing, though I have a horrible feeling Sally is socketed in the 800XL. We'll see.

[stephen_usher]

Poor Sally.