04-06-2022, 02:04 PM
I seem to have fixed it :-D
Yesterday, in a fit of enthusiasm, I fitted my core board risers and started probing around the CPU, taking careful notes of all the "important" lines. After checking all the "inverted" pins (which tend to be the important ones) I found NMI was being held low.
OK why? To the schematics Batman! (Thank god for the iPad Pro work lent me)
Tracing back, NMI quickly jumps from the Main board to the Core board and through to IC 49 (pins 5&6) which was correctly inverting.
IC41 (pins 4,5 output 6) looked like a culprit. It was getting a high on pin 5 and a "low" on pin 4 so should be putting out a low (to be inverted by IC49).
I tested IC41 in my TL866 (Not a guarantee I know) and it passed.
Looking closer at the signal on pin 4 and it's hovering up at around 1.5-1.6v which is right in the "undefined" voltage level for an LS part, from memory.
Further back in the chain, we're back to IC 49 again (Pins 12&13). I popped it out and stuck it in the TL866.
Guess what? It fails on Pins 12 and 13!
Went out and bought a replacement and initially it didn't work (While NMI was now high, RESET was being held low) until I remembered to bend out pin 8.
After that I went through some more troubleshooting until I finally realised my video connector was being a bit "pants" and twiddling it I got the solid white "looking for floppy" cursor.
Plug in the Gotek and it boots. It actually works again.
Whew! I got there in the end :-D
Thank you to everyone who helped with suggestions, things to check and offers of help. I'd nearly got to the point of giving up.
(If I hadn't run out of my "mad" money this fortnight, I probably would have already posted it to Ewan. Don't worry though, Ewan. You'll be getting ALL my $$$ when the new Prem Plus gets released :-D)
Cheers all!
Chesh
Yesterday, in a fit of enthusiasm, I fitted my core board risers and started probing around the CPU, taking careful notes of all the "important" lines. After checking all the "inverted" pins (which tend to be the important ones) I found NMI was being held low.
OK why? To the schematics Batman! (Thank god for the iPad Pro work lent me)
Tracing back, NMI quickly jumps from the Main board to the Core board and through to IC 49 (pins 5&6) which was correctly inverting.
IC41 (pins 4,5 output 6) looked like a culprit. It was getting a high on pin 5 and a "low" on pin 4 so should be putting out a low (to be inverted by IC49).
I tested IC41 in my TL866 (Not a guarantee I know) and it passed.
Looking closer at the signal on pin 4 and it's hovering up at around 1.5-1.6v which is right in the "undefined" voltage level for an LS part, from memory.
Further back in the chain, we're back to IC 49 again (Pins 12&13). I popped it out and stuck it in the TL866.
Guess what? It fails on Pins 12 and 13!
Went out and bought a replacement and initially it didn't work (While NMI was now high, RESET was being held low) until I remembered to bend out pin 8.
After that I went through some more troubleshooting until I finally realised my video connector was being a bit "pants" and twiddling it I got the solid white "looking for floppy" cursor.
Plug in the Gotek and it boots. It actually works again.
Whew! I got there in the end :-D
Thank you to everyone who helped with suggestions, things to check and offers of help. I'd nearly got to the point of giving up.
(If I hadn't run out of my "mad" money this fortnight, I probably would have already posted it to Ewan. Don't worry though, Ewan. You'll be getting ALL my $$$ when the new Prem Plus gets released :-D)
Cheers all!
Chesh
John "CheshireNoir" Parker
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One Dark Little Kitten
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One Dark Little Kitten