exxos's Atari, Amiga & retro forum

What did I say? ;-)

It still could be the ttl in the video circuit the 74c parts can be replaced with 74hc parts

Tom

stephen_usher wrote: 27 Sep 2022 15:47 What did I say? ;-)

To be fair, I did agree :)

Tomswork wrote: 27 Sep 2022 16:43 It still could be the ttl in the video circuit the 74c parts can be replaced with 74hc parts

Tom

Will see what happens once the LS157 is replaced, but you're right, there could be more wrong.

Ok, so, the collective badness award goes to:

Z62 2102 RAM
Z64 74LS157

Both are bad, and I now at least have a normal non duplicated display again.

There is a new problem however, and thats the keyboard.

The 1, Q and A keys don't work, and the number keys all bring up multiple letters. The MEM SIZE? prompt doesn't appear either, but if I hit return I go into basic and ?MEM gives me 15570,. which is correct.

I had a similar issue with a dry joint on the keyboard connections before, so will have a look at that, as if I disconnect the keyboard I get a MEM SIZE? event on screen.

I'll also have a delve into the schematics, as it's not unreasonable to consider another LS chip has gone pop, as it seems to be a 'thing' with this machine.

Well at least you are one step forward @rubber_jonnie

PhilC wrote: 28 Sep 2022 16:02 Well at least you are one step forward @rubber_jonnie

Indeed. I can't help but wonder if the machine had the power connected to the wrong socket, or if the 5V circuit failed and gave an over voltage, which is feasible since the crowbar zener was blown when I got it.

Looks like all the bad keys are on the same column, which suggests that Z3, a 74LS368 may have a fault. I don't think it's totally dead as other keys that it supports are working, so I'll have a probe around and see what the inputs are doing at the IC.

I'd better buzz out my replacement keyboard cable too.

This is an adventure in logic hunting this board


Tom

Tomswork wrote: 28 Sep 2022 17:43 This is an adventure in logic hunting this board


Tom

Yep, that's about the top and bottom of it. The worst board I've had so far for 74LS fails. Once again, they are logic chips I don't have so have had to order :(

At least the cable buzzes out fine from the mainboard to the keyboard.

I guess a bunch of the ICs weren't keen on being revived after such a long period asleep.