Greetings everyone, I got another dead MVS that is proving to be annoying to get running. Might have another dead custom IC on my hands, but I'd like a second opinion before I put this in the pile of scrap boards.
MV1FZ Motherboard, when powered up it shows bursts of random lines on the screen, with short intervals where a static screen with random characters appear for less than two seconds before going back to the garbage screen.
SP1 ROM never gets enabled: Pin 20 (/OE) is always high.
I then ran a check on NEO-C1:
- SROMOEL, SROMOEU: Always high
- A17, A18, A19, A20, A21: toggling
- A22I: low
- A23I: high
- LDS, UDS: Toggling
- RW: Toggling, but seems to toggle between HIGH and disconnected...
I moved to check NEO-E0, according to https://wiki.neogeodev.org/index.php?title=NEO-E0 I should have
- Pin 55 (A22I) following pin 53 (A22) if VEC is high
- Pin 56 (A23I) following pin 54 (A23) if VEC is high
What I actually see is:
- A22 and A23: toggling high and low in short busts
- A22I: low
- A23I: high
- VEC: low
So, with VEC low it is expected for A22I and A23I not to change.
VEC is generated by the HC259 latch at U7 (pin 5 / Q1), so checking on that:
- Pin 1,2,3 (the addresses for the latch) are toggling in short busts
- Pin 13 (data for the latch) is toggling
- Pin 15 (/MR, the reset for the latch) is toggling
- Pin 14 (/E, the enable for the latch) is always high. So the latch never gets enabled, thus VEC never goes high
Checking the schematics for the MV1F I see that the enable for the latch is connected to BITW1, pin 100 of the NEO-C1.
I have connectivity, and indeed pin 100 is always high. Sadly I can't see any explanation on how BITW1 signal is generated, so I'm left with my suspect that I have (another) dead NEO-C1 in my hands.
Different ideas/things to check are welcome!
MV1FZ Motherboard, when powered up it shows bursts of random lines on the screen, with short intervals where a static screen with random characters appear for less than two seconds before going back to the garbage screen.
SP1 ROM never gets enabled: Pin 20 (/OE) is always high.
I then ran a check on NEO-C1:
- SROMOEL, SROMOEU: Always high
- A17, A18, A19, A20, A21: toggling
- A22I: low
- A23I: high
- LDS, UDS: Toggling
- RW: Toggling, but seems to toggle between HIGH and disconnected...
I moved to check NEO-E0, according to https://wiki.neogeodev.org/index.php?title=NEO-E0 I should have
- Pin 55 (A22I) following pin 53 (A22) if VEC is high
- Pin 56 (A23I) following pin 54 (A23) if VEC is high
What I actually see is:
- A22 and A23: toggling high and low in short busts
- A22I: low
- A23I: high
- VEC: low
So, with VEC low it is expected for A22I and A23I not to change.
VEC is generated by the HC259 latch at U7 (pin 5 / Q1), so checking on that:
- Pin 1,2,3 (the addresses for the latch) are toggling in short busts
- Pin 13 (data for the latch) is toggling
- Pin 15 (/MR, the reset for the latch) is toggling
- Pin 14 (/E, the enable for the latch) is always high. So the latch never gets enabled, thus VEC never goes high
Checking the schematics for the MV1F I see that the enable for the latch is connected to BITW1, pin 100 of the NEO-C1.
I have connectivity, and indeed pin 100 is always high. Sadly I can't see any explanation on how BITW1 signal is generated, so I'm left with my suspect that I have (another) dead NEO-C1 in my hands.
Different ideas/things to check are welcome!