MVS 4 Slot Repair Help

[ArcadeBattler]

Hello all,

I have a mvs mvh4 that is watchdogging. I can't find any broken traces on it, and I have tried to install the diag bios into it but it continues to watch dog with it installed.

Any suggestion that I could get as to how I should proceed would be appreciated!

 

[stj]

tools?
scope?
logic probe?
eprom programmer that can test ram?

 

[ArcadeBattler]

tools?
scope?
logic probe?
eprom programmer that can test ram?

Hey,

I have a logic probe, soldering tools and scope available to me.

 

[stj]

check the address and data on the eprom and the ram chips

 

[Rom1]

Mv4 battery location is crap and some critical lines are next to it. For sure they are rotted.

You should get a diag bios. It'll be a great help.

 

[ArcadeBattler]

Mv4 battery location is crap and some critical lines are next to it. For sure they are rotted.

You should get a diag bios. It'll be a great help.

Hey,

I installed a diag bios and its still watchdoging so unfortunately its not of much help atm.

 

[Rom1]

- Check for continuity between 68k, bios and work ram.
- Bridge the watchdog disable so you can check signals.
- Replace work RAM.

 

[channelmaniac]

A few things come to mind...

Bad Backup or Work RAM can cause this.

Battery leak damage? If so, check the traces around the 74HC32 for continuity. If you have stuck low conditions on the Backup RAM you'll end up with data bus collisions.

Sometimes the 74ALS245 chips on the data lines going to the connector by the battery get shorted. This is usually done when unplugging the top board with the power on. If that happens you'll be stuck in watchdog. Check to see if they are getting very hot. That's a big clue they're shorted.

If the enable line for the BIOS chip is stuck, replace the chip that buffers it. IIRC, it's the same chip that also handles sync and can get popped if you have connected the boardset to a bad monitor.

Good luck!

 

[channelmaniac]

Oh, and try flipping all the DIPs on. Sometimes when you have stuck enable lines or bad Backup / Work RAM, it will help the error messages to pop up. It starts a diagnostic routine on the standard BIOS.

 

[ArcadeBattler]

- Check for continuity between 68k, bios and work ram.
- Bridge the watchdog disable so you can check signals.
- Replace work RAM.

A few things come to mind...

Bad Backup or Work RAM can cause this.

Battery leak damage? If so, check the traces around the 74HC32 for continuity. If you have stuck low conditions on the Backup RAM you'll end up with data bus collisions.

Sometimes the 74ALS245 chips on the data lines going to the connector by the battery get shorted. This is usually done when unplugging the top board with the power on. If that happens you'll be stuck in watchdog. Check to see if they are getting very hot. That's a big clue they're shorted.

If the enable line for the BIOS chip is stuck, replace the chip that buffers it. IIRC, it's the same chip that also handles sync and can get popped if you have connected the boardset to a bad monitor.

Good luck!

I've replaced all of the backup and work ram chips on the board, there is slightly less garbage on the screen now, but still watchdogging. I'll have to do some more probing, but I'm not seeing any obviously broken traces.