Strange sound glitch on MVS cart

PCB problems and fixes
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Sumez
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Strange sound glitch on MVS cart

Post by Sumez »

Hey forum. I have this Spin Master cart that I bought from a fellow forum member - unfortunately, there seems to be a weird issue with it:

https://www.youtube.com/watch?v=ZBgF8oAKtJg
(warning for headphone users, the sound is extremely high pitched and loud)

As far as I can tell it happens every time it's trying to play sample data (the Neo Geo jingle plays fine which AFAIK is just FM sound)
I've had a ton of different MVS games throughout the years, and I've never seen anything like this before. It's definitely related to the cart, as I tried it on two different systems (a 1 slot and a 4 slot) with exactly the same results.

I've cleaned the pin connecters thoroughly, and also tried opening the cart. The game looks legit, and I don't see any visible issues with it such as burned traces, etc.

Anyone seen a similar issue before with any MVS title, or have ideas for what I should look for? I want to do everything I can to attempt fixing the problem, saving the seller from having to compensate if possible.

I guess the worst case scenario is a damaged sound ROM chip which is not easily fixed. :\
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Stompp
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Re: Strange sound glitch on MVS cart

Post by Stompp »

Curious :eh: Kinda resembles the problem I have with two different MSX carts:
https://youtu.be/WMG_NxVWT3E
https://youtu.be/8f_nfg452f4

Haven't been able to fix any of them, but they're being looked at atm by someone with far superior troubleshooting skill than mine. I'll keep you posted if he finds issue and a solution.
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Re: Strange sound glitch on MVS cart

Post by Sumez »

It sounds like exactly the same thing you're experiencing in the first video!

What's the approximate ETA on the troubleshooting results for your carts? I would love to hear your conclusions.
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Stompp
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Re: Strange sound glitch on MVS cart

Post by Stompp »

Not sure. Might be a while. He's pretty choked and don't think these are at the top of his list. I'll poke him and see how he's doing :)

Seriously. Until I saw your video clip I thought I was the only one with this issue. I think I posted a question about it quite a while back, but never got anywhere :(
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leonardoliveira
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

Very strong possibility that the custom "PCM" IC in the "PROG" board of the cart is faulty. It is composed of a set of latches and multiplexers which "encode" the PCM ROMs into the format the YM2610 requires.

https://wiki.neogeodev.org/index.php?title=PCM
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Stompp
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Re: Strange sound glitch on MVS cart

Post by Stompp »

leonardoliveira wrote:Very strong possibility that the custom "PCM" IC in the "PROG" board of the cart is faulty. It is composed of a set of latches and multiplexers which "encode" the PCM ROMs into the format the YM2610 requires.

https://wiki.neogeodev.org/index.php?title=PCM
Can that be verified somehow and if it indeed is whacked can it be replaced or does custom in this case == dead?

EDIT: Found my old thread: http://forum.arcadeotaku.com/viewtopic.php?f=26&t=22908
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Re: Strange sound glitch on MVS cart

Post by Sumez »

I second that. What's the quickest and dirtyest way to confirm whether this is the issue? :)
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

It can be replaced but you will need a donor cartridge (both AES and MVS carts have that particular part).
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Re: Strange sound glitch on MVS cart

Post by Sumez »

The quickest way to confirm whether the culprit is the PCM chip, is to replace it? It has a billion tiny SMD legs, doing that sounds like a project for the ages :P
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leonardoliveira
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

Someone used to replace those would have no issue replacing it in minutes. Find a donor cart and someone who can replace the chip. Swap the chip from the donor with the chip on the faulty cart. If the donor cart continues working, the fault is something else.
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Re: Strange sound glitch on MVS cart

Post by Asayuki »

To me it sounds like the V1 rom is broken.
Easier than replacing the PCM custom, you can try and desolder the V1 rom altogether. The game will run anyway. If it does not screech anymore, then I suggest you try and replace V1 first, before going for the custom. ;)
My 15kHz cabinet Peplos will never power up, with any item, and I am quite proud of that.
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

But honestly I've repaired a couple carts already with that fault. I used to go diagnose/check first for the rom but on every single case of sound fault it was the custom IC. (three cases so far)
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Re: Strange sound glitch on MVS cart

Post by Sumez »

Sounds the most likely then, thanks a lot. There's a limit to how far I'll go with this though. If fixing the issue requires a donor cart it's pretty much pointless, since it's a somewhat cheap game anyway.
And of course at the end of the day, whether the game works properly is the repsonsibility of the seller, I just didn't want to hold it against him if it were an easy fix, which I wouldn't consider the replacement of an IC with 80 pins to be :P

Curious though, AFAIK the Unibios (which I don't have) has CRC check for the ROM data of known games. Would I be able to use that to tell if the problem is in the V1 ROM, or would the PCM chip cause a false negative?
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

NEO GEO cannot test the ROMs because the CPU cannot see what is going on with the dedicated data buses. Unibios will only test the ROMs the 68000 CPU can see (P1 and P2 sockets on the PROG board).

Other system designs such as several different types of Konami hardware for example may have read back mechanism provision for allowing the game to self check the ROMs but Neo Geo doesn't have that at all.
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Re: Strange sound glitch on MVS cart

Post by Stompp »

leonardoliveira wrote:NEO GEO cannot test the ROMs because the CPU cannot see what is going on with the dedicated data buses. Unibios will only test the ROMs the 68000 CPU can see (P1 and P2 sockets on the PROG board).

Other system designs such as several different types of Konami hardware for example may have read back mechanism provision for allowing the game to self check the ROMs but Neo Geo doesn't have that at all.
Wel, that's a bummer :problem: So basically only thing to do is to swap the PCM IC on the PROG board and hope for the best?
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leonardoliveira
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

It can also be the V ROMs as mentioned by Asayuki. If you have a chip programmer you could simply try to remove them and try dumping them.

Unfortunately the V roms are using an unusual pinout where the chip is a 8-bit ROM in a large 16-bit DIP package (pins go unused because of that) and most chip programmers can't read them properly.
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Re: Strange sound glitch on MVS cart

Post by Asayuki »

As said, there's no need to dump it. V1 can simply be removed. If it does not screech anymore without V1... ;)
Yet, I guess he has enough information already to decide if to send the cart back to the seller.
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Re: Strange sound glitch on MVS cart

Post by leonardoliveira »

ADPCM doesn't work like that. Open bus (all 0s or all 1s) will make the chip produce static.
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