Outrunners no audio (system 32)

[makedome]

Hello,
I'm trying to repair an Outrunners pcb that has no sound:
The test reports ic21 bad: it's the mb8464, the ram next to z80. I tried to replace it with another one but no change.
The other things I checked so far:
- the amplifier works fine,
- the z80 address and data lines are toggling fine, clock pulsing, reset high, bus rq high, nmi high. Did I miss any pin to check?
- Audio Eprom epr15550 is a 27c040 and checks fine against related rom file. While data, address lines and Output enable toggle correctly, chip enable is low. Is this normal or not? Because I read that both OE and CE should be high.

Any hint would be appreciated.
Thanks

Makedome

 

[Monstermug]

Outrunnets is sega multi32 system iirc. V70 cpu and z80 for sound. The fm is generated by the yamaha and sound effects by the qfp near there. If u have no sound and have confirmed it with an audio probe then its does sound like a digital fault. 27c040 CE is active low. You also need to check the output enable. Did you scope the clock and does the z80 reset from low to high on power on? If the system reports sound ram as dead snd its not then the test mode isnt able to communicate with that ram. Probably best to probe the ram and find clues from there?

 

[makedome]

Outrunnets is sega multi32 system iirc. V70 cpu and z80 for sound.

all right

27c040 CE is active low. You also need to check the output enable.

ok, got it. so eprom looks good because as I wrote ce is low and oe is toggling

Did you scope the clock and does the z80 reset from low to high on power on

the clock on z80 id toggling and I have to verify reset on power on

If the system reports sound ram as dead snd its not then the test mode isnt able to communicate with that ram. Probably best to probe the ram and find clues from there?

Ok, I'll keep you updated
Thanks for now

 

[makedome]

hello,
as for reset signal of the z80 it does turns from low to high at startup.
I also discovered that the pcb starts with the mb8464a ram removed and the test is obviously saying that chip is bad.
So as you said the test is not able to communicate with ram.
Now the question is how it does it? Is there some specific pin I should follow from ram backwards?
there are a bunch of 74xx component next to it and I was going to check those

 

[Monstermug]

Probe the address of the ram and the enable lines. Test voltages on the ram. U should find clues from there.

Once u found the pin that isnt doing what it ahould trace it back to the fault. Address lines shoukd be toggling and enable line. As its sound it may do nothing until the the there sound sectio has been activated by the cpu. Digital sound faults are always a pita as it only works when it suppose too.