Diagnosing a faulty CoCo2
Posted: Wed Apr 21, 2021 6:12 pm
Hello everyone!
I'm trying to diagnose a 16Kb Korean made CoCo2 I got from a friend long ago, and as him before me, I'm quite stuck
So, here's the patient: And here's the symptom: The screen is grey because it's a NTSC machine on a PAL tv.
What do I know?
1) The machine boots up, and is able to load software. I type CLOAD, hear the relay clicking and when the program is finished loading, I hear the relay click again.
2) The SAM is working. I put it in a Dragon 32 and it works as expected.
3) The signals I poked with an oscilloscope looked nominal according to the service manual: https://colorcomputerarchive.com/repo/D ... Tandy).pdf
My only doubt is the clock signal from the MC1372 on the modulator, which looks like this: 4) No chips seem to be hotter than usual. 5) I tried to run a small basic program to switch to graphics mode and measure the |A/G pin, to determine if it's the PIA's fault or the 6847's, but it turns out that the 'R' key could be not working so I'm not able to run the program.
Has anyone encountered a similar fault, or knows hot to diagnose it?
My bet is on the 6847, because all |A/G, |A/S, GM0, GM1 and GM2 signals are low according to the multimeter, which is correct for the text mode screen, but as you can see on the tv picture, there is no border, which would indicate that the VDG is in graphics mode.
Cheers,
Rob
I'm trying to diagnose a 16Kb Korean made CoCo2 I got from a friend long ago, and as him before me, I'm quite stuck
So, here's the patient: And here's the symptom: The screen is grey because it's a NTSC machine on a PAL tv.
What do I know?
1) The machine boots up, and is able to load software. I type CLOAD, hear the relay clicking and when the program is finished loading, I hear the relay click again.
2) The SAM is working. I put it in a Dragon 32 and it works as expected.
3) The signals I poked with an oscilloscope looked nominal according to the service manual: https://colorcomputerarchive.com/repo/D ... Tandy).pdf
My only doubt is the clock signal from the MC1372 on the modulator, which looks like this: 4) No chips seem to be hotter than usual. 5) I tried to run a small basic program to switch to graphics mode and measure the |A/G pin, to determine if it's the PIA's fault or the 6847's, but it turns out that the 'R' key could be not working so I'm not able to run the program.
Has anyone encountered a similar fault, or knows hot to diagnose it?
My bet is on the 6847, because all |A/G, |A/S, GM0, GM1 and GM2 signals are low according to the multimeter, which is correct for the text mode screen, but as you can see on the tv picture, there is no border, which would indicate that the VDG is in graphics mode.
Cheers,
Rob