BattleZone ram problems.

[MarbleMad]

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.
MarbleMad2013-01-01 12:56:24

 

[guddler]

I'd need to go up to the workshop and grab the schematics to be able to be really helpful, but certainly your description of your process seems fine to me.

You don't mention if pins 5 and 7 of the 139 did indeed behave OK when they were lifted? I'm sure I don't need to tell you this, but if they do, yet when put back in, they go back to their stuck state then something else is pulling them up and I guess that could be /VRAM0, /VRAM1 or something somewhere that is connected to those signals.

Do 5 and 7 both stick high, or is one stuck low and the other high? I need to look at the schems really to see what they're connected to.

With all the VRAM removed, are they still stuck high?

I have BZone problems myself at the moment but I've not looked at them for some months now. To be honest, I think I hate BZ PCBs. No, scrub that, I KNOW I hate BZ PCBs

 

[MarbleMad]

i made a typo on my original post. pins 5 and "6" stick.
Lifted out of circuit pin 5 remains stuck high.
I didn't think of removing the VRAM but with the 139 removed the holes left by pin5 and 6 read nothing on my probe, eg not stuck high.

 

[guddler]

OK, just had a little squint at the schems - they're a bit blurry in the software I'm using - and pulling the RAMs won't help as they're not directly connected, there's what looks like 157's in between multiplexing the RAM with the Stack.

Did you check the 'new' 139 to make sure it was good before you put it in? Yeah, I know, I'm stating the obvious again

, if we're sure that's good on both parts of the chip (given it's a dual encoder it could be shorted internally with something on the other encoder) then I'd probably look at hitting it with a logic analyser or something like that to ensure that the inputs are driving it as you expect. How do the inputs look on a scope? Are they toggling high and low enough to make the difference needed to drive the outputs? Not that there's anything in between the 6502 and the 139 mind

Had pin 5 been behaving when lifted then I'd have then suggested pulling each of the 157's as something would be holding it high, but it doesn't sound like that's the case.

I'd need a better look which probably isn't going to happen today but it looks to me like the 139 is the right place to be fishing...

 

[DanP]

Are you running the board on its own or with the aux board connected? If the latter i'd try it on its own to see if it makes a difference.

Dan

 

[MarbleMad]

Tried disconnecting the AUX board. made no difference.

Beginning
to think there could be some involvement with those LS245 at P2 and P4 I
don't see how they involve the 139 but I was comparing one of my other
dead boards and I noticed its video ram worked until P2 gets very hot
and suddenly the ram stops with exactly the same symptom as the other
boards, eg pin 5 of the LS139 at M2 sticks HIGH.

Going to try replacing those.

 

[MarbleMad]

Replacing LS245 at P2made no difference.