exxos's Atari, Amiga & retro forum

stephen_usher wrote: 19 Mar 2024 23:24 :dualthumbup: :yay2: :cheer:

I also get a picture on my Dell LCD too :)

It's not a good picture, but it's more than it did before.

So based on some help over at OSIWeb, it looks like I had my RAM ICs in. the wrong sockets, so I moved them over and got something approaching a normal screen, but with just a '-' somewhere in the middle. It was also suggested the RAM might be bad, so I swapped it over for some fresh and now get this:


This is correct, but I'm not out of the woods yet though, as the display is very flickery and I had to hold down a key to even get this to photograph.

What next, I'm not sure since it looks like a key is stuck down, but that is just a guess ATM, so more checks to be done.

I can say it recognises me holding a key down, as the screen stops flickering, and that sort of suggests that it isn't a stuck key, and it could easily be something else. Reset also works as I can see a reset signal at pin 40 of the CPU when pressing break.

I suppose it could be constantly restarting?

However I am largely positive since it means the video circuit and CHARGEN are good, as is the video RAM and it looks like the 2 RAM ICs in the system RAM sockets are also good. I have spare RAM ICs so I should still have enough to fill the bank up when it comes to it.

I think it also potentially means that the ROMs are all good in order to get this far.

Pretty pleased with my progress in the last few days

That's some good progress.

I've always found that having a break from a frustrating project and then coming back to it is very helpful.

PhilC wrote: 20 Mar 2024 11:35 That's some good progress.

I've always found that having a break from a frustrating project and then coming back to it is very helpful.

It's definitely helped in this case, thats for sure.

Nice progress !

:D

HigashiJun wrote: 20 Mar 2024 12:25 Nice progress !

:D

Thanks :)

Getting there! :-)

stephen_usher wrote: 20 Mar 2024 13:41 Getting there! :-)

Indeed, I at least know it is running code and getting to that point where I can input something, even if I can't input anything!!

OK, so looking at the schematics again, the behaviour is as such:

1. Pressing the break key causes a reset on pin 40 of the CPU, so that circuit is working fine. The on screen message also flickers when the reset happens.
2. The RESET circuit is in fact totally separate from the keyboard, even though it is the BREAK key, and as such has nothing to do with KBD operations.
3. KBD is handled by 4 ICs and a number of resistors/diodes and is formed from 2x LS75s (U2 and U3) for the rows and 2x LS125's (U4 &U5) for the columns.
4. If I remove U5, the on screen message is no longer flickering, it's steady, but of course I then can't type anything in.

Time for some hopping, skipping and jumping through the schematics...

And there in the service manual is what I seem to be getting:


I have enough RAM to swap it out, and I have tried an alternative CPU so I guess that leaves the ROM monitor and BASIC 4 ROM to check first.