Hi Nightmare Tony,
I'm told you're an ex NeoGeo tech? If so, perhaps you can help me?!?
I've got a faulty MV1AX. The original symptom was garbled graphics at powerup - forever. I've traced the fault to the M68K DTACK# line - it's never asserted. If I short it to GND, then it runs OK and I can even play a cart.
Obviously, I would like to find the root of the cause. From what I have traced, the DTACK# is generated from the SNK1B PAL. Obviously without the PAL equations, it's pretty hard to go any further.
There seems to be some pretty weird logic levels floating around the board, eg. both 3.3V and 4V going into the PALs. Most other levels are 5V, though I did measure these when the board was resetting every 270ms.
Any info you can give me or point me to which would allow me to go any further? I'd be very surprised if it was the PAL itself that was the problem - I'd expect to see other problems on the board if that was the case?!? I hadn't gotten much further tracing back from the PAL but I was hoping the DTACK# was heading back to the smaller 74 gates in the same vicinity... would've been easier to fix and a lot more believable!
If is *is* the PAL, I guess I'm stuck with forcing the DTACK# to GND as a permanent fix??? Ugghhh!
TIA
Regards,
I'm told you're an ex NeoGeo tech? If so, perhaps you can help me?!?
I've got a faulty MV1AX. The original symptom was garbled graphics at powerup - forever. I've traced the fault to the M68K DTACK# line - it's never asserted. If I short it to GND, then it runs OK and I can even play a cart.
Obviously, I would like to find the root of the cause. From what I have traced, the DTACK# is generated from the SNK1B PAL. Obviously without the PAL equations, it's pretty hard to go any further.
There seems to be some pretty weird logic levels floating around the board, eg. both 3.3V and 4V going into the PALs. Most other levels are 5V, though I did measure these when the board was resetting every 270ms.
Any info you can give me or point me to which would allow me to go any further? I'd be very surprised if it was the PAL itself that was the problem - I'd expect to see other problems on the board if that was the case?!? I hadn't gotten much further tracing back from the PAL but I was hoping the DTACK# was heading back to the smaller 74 gates in the same vicinity... would've been easier to fix and a lot more believable!
If is *is* the PAL, I guess I'm stuck with forcing the DTACK# to GND as a permanent fix??? Ugghhh!TIA
Regards,