BattleZone Video ram completely inactive?

MarbleMad

New member
Joined
Dec 28, 2012
Messages
16
Reaction score
9
Location
Glasgow, Glasgow, United Kingdom
I,ve got 2 boards with the exact same symptoms.
Ram test fails reporting 10 beeps.
I fact all graphics ram is dead. The same beep code is received if you physically remove every chip. It,s not the ram.

_____. _____
VRAM0. And. VRAM1. Appear stuck high. They should be pulsing, right?

They appear to be derived form
_____
VMEM. Which is also stuck high.

That signal derives from an LS139 at M2 pin 5. Pins 4 and 7 toggle away. Pins 5 and 6 are stuck.
Pins 2 and 3 which connect to the 6502 also toggle happily.

I,ve tried replacing the ls139 and the 6502. I,ve also tried lifting pin 5 to see if it behaved differently unconnected to the rest of the circuit.

am I on the right lines here at all.?
Can anyone offer any more suggestions?
Thanks.
 
Back
Top Bottom