Advice on fixing G8 RAM fault on TMNT board

yaniktheyak · Post by **yaniktheyak** » Sun Aug 20, 2017 6:13 pm

Hi
I have a TMNT board which is poorly.
I was playing it just today, all was good.
I left the room and came back and it was stuck in a reset loop reporting
a bad RAM chip at G8.

The thing I find confusing is how it went from running the attract sequence, to rebooting. I'd have thought it'd just crash. Or more likely, since I think that is pallete RAM, the colour would just be wrong?

Anyhow, when it starts up now it fails the RAM ROM check on the G8 chip.
This is an MCM2018AN45 and I have a replacement I can lift from a donor board.

So I have a few questions:
What sort of tests should I do to verify it is the chip that's faulty?
Is it worth me just piggy backing the donor chip on the 'faulty' one, to see if it works? Or is it more likely to be something else, since it was running ok, right up to it failing?
Am I more likely to just break the donor chip?

Any advice welcome.
Thanks

OMFG · Post by **OMFG** » Sun Aug 20, 2017 11:05 pm

When it resets, are there any abnormal sounds coming from the speakers?

G8 on that board is Sprite Ram. You could try to piggyback the chip, but normally that'll just omit graphics. What loop is it doing? Is it just looping the rom test or does it go to any other screen?

yaniktheyak · Post by **yaniktheyak** » Mon Aug 21, 2017 6:07 am

OMFG wrote:When it resets, are there any abnormal sounds coming from the speakers?

G8 on that board is Sprite Ram. You could try to piggyback the chip, but normally that'll just omit graphics. What loop is it doing? Is it just looping the rom test or does it go to any other screen?

No noise just the ram rom loop - here is a video link of the loop :
https://youtu.be/kXsHjQhQbHQ

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yaniktheyak · Post by **yaniktheyak** » Tue Aug 22, 2017 6:04 pm

Did some testing last night and while probing around on the G8 IC I did intermittently get it to make garbled sounds. Not quite sure how or why. But...

Having looked at the schematic it looks like the G8 IC only connects to the large surface mount chip (graphics processor?)
So I'm guessing if replacing it doesn't fix it, then it's going to be a big job.

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yaniktheyak · Post by **yaniktheyak** » Sun Sep 03, 2017 7:22 am

So I replaced the IC with a socket. Pulled another IC from a completely dead Major Title board. Fired it up, and, exactly the same. So I guess the IC was fine and the fault is either in the only other chip it is connected to, or something that controls that.

I had a look at other areas of the board and nothing looks wrong visually.
I flexed the board a little while it was running to see if I got any different results. Tried probing the IC again to see if I got random sound out of it, but nothing except a bad G8 on a reset loop.

Any ideas?

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