mrthreeplates
Experienced Member
- Joined
- Jul 22, 2018
- Messages
- 77
Hi everyone,
I have a M1 that I've been trying to get working a bit more reliably. The latest issue I'm chasing is a flaky boot issue. The M1 boots intermittently.
It is a rev-G board, with relatively few mods.
What I've noticed is that when it fails to boot, I get garbage on the screen (sometimes with the video sync'ed, sometimes not). When it fails to boot, hitting reset doesn't do anything. When it does boot, it stays running (reset works as it should). When it runs, I'm able to pass memory tests. I also have an EI, but that doesn't seem to affect the boot issue.
I've checked the voltages, and they are all good. (5v, 12v, -5v) - even when it fails to boot.
So, I chased the problem down to the cpu clock. Interestingly, when it fails to boot, the cpu clock is ~5Mhz(!) (should be 1.7). Chasing this back to the crystal, I found it was oscillating at 30Mhz (which makes sense as there is a divide by 6 feeding the cpu).
Obviously, when it boots correctly, the cpu clock is normal (per my scope).
I checked the resistors and load capacitor around the 10.6445Mhz crystal, and they are fine. Resistors measure nearly spot on at 910ohms, and the capacitor measures (out of circuit) 44pF (should be 47pF, but I assume this is close enough). I haven't checked the LS04 around the crystal, that is next on my list.
I know very little about crystals, but does it seem plausible that a bad crystal would mysteriously vibrate at a 3x higher frequency - only occasionally?
If anyone knows of ways to test crystals out of circuit for this, I'd love to actually prove the crystal is bad. Or perhaps I need to modify the values of the passive components to make the crystal more stable?
Otherwise, if it is a bad crystal, I'll have to hunt for a new one.
Thanks!
George
I have a M1 that I've been trying to get working a bit more reliably. The latest issue I'm chasing is a flaky boot issue. The M1 boots intermittently.
It is a rev-G board, with relatively few mods.
What I've noticed is that when it fails to boot, I get garbage on the screen (sometimes with the video sync'ed, sometimes not). When it fails to boot, hitting reset doesn't do anything. When it does boot, it stays running (reset works as it should). When it runs, I'm able to pass memory tests. I also have an EI, but that doesn't seem to affect the boot issue.
I've checked the voltages, and they are all good. (5v, 12v, -5v) - even when it fails to boot.
So, I chased the problem down to the cpu clock. Interestingly, when it fails to boot, the cpu clock is ~5Mhz(!) (should be 1.7). Chasing this back to the crystal, I found it was oscillating at 30Mhz (which makes sense as there is a divide by 6 feeding the cpu).
Obviously, when it boots correctly, the cpu clock is normal (per my scope).
I checked the resistors and load capacitor around the 10.6445Mhz crystal, and they are fine. Resistors measure nearly spot on at 910ohms, and the capacitor measures (out of circuit) 44pF (should be 47pF, but I assume this is close enough). I haven't checked the LS04 around the crystal, that is next on my list.
I know very little about crystals, but does it seem plausible that a bad crystal would mysteriously vibrate at a 3x higher frequency - only occasionally?
If anyone knows of ways to test crystals out of circuit for this, I'd love to actually prove the crystal is bad. Or perhaps I need to modify the values of the passive components to make the crystal more stable?
Otherwise, if it is a bad crystal, I'll have to hunt for a new one.
Thanks!
George