Completely stumped with a dedicated MK1 cabinet

TerryMasters

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Hey everyone. Well, I've been in this situation now for about a year, and I've never got around to doing anything about it until now. Here's the story. I have a dedicated MK1 machine, with MK revision 4.0 in it. When I first got the machine, it kept resetting every 5 seconds or so, looping the burn-in test. I checked the 5v rail and found it near 4, so I upped it (accidentally) to 6. Once I discovered how high it was set, I lowered it back down to 5v. I thought that solved the problem, as the board made it passed the burn in test just fine and played as such for quite a while. Then I noticed something strange - no matter what the volume pot is set to, the audio fades in and out - sometimes being damn near completely silent, sometimes being way louder than it should be. For the most part if I keep the pot low, the volume only varies minimally. That's not the main problem, however. I just wanted to state that as how this whole mess began.

The main problem, is that the machine finds itself back at the burn in test after about an hour or so. It'll just restart out of nowhere, and then appear to be fine. A half an hour later, it will reboot once more. Fifteen minutes later, another reboot. Then it does something really strange - it sends what appears to be a black screen with no sync to the monitor. Turning the machine on and off usually fixes it, but it's only a matter of time before it starts resetting again.

Now what makes this particularly strange, is that I've replaced damn near everything there is to replace. The monitor itself is fine, shockingly mint for a K7000, so that's all good to go. I figured it might be the power supply, so I replaced that - it still reset. Then I figured the board was toast, so I replaced that with another revision 4.0 - it still resets. The only thing I didn't replace was sound board, as the extra one I had didn't output any sound for whatever reason (yes, the volume pot was fine). This is where I stand, completely stumped. If it's not the monitor, not the power supply, and not the game board... what else could it possibly be? The only two conclusions I can come to are as follows:

1.) Both 4.0 boards were toast. Why would the seller have had two of the same boards in the first place if something wasn't wrong with one of them?

or

2.) The sound board is somehow causing the entire machine to reboot. I can't even begin to fathom how something like this is possible, but I've seen stranger things.

What do you guys think it could be? I'm moving around a bunch of my machines in my game room, so I figured now's a good time to ask. I want to update to a revision 5.0 board, but I don't want to stick a nice new board in there if this thing's just going to kill it. :(


Thanks everyone!
-Terry
 
well I guess the easiest thing to check is if it's the sound board.

you can unplug it and run the game without it. obviously you just won't have sound.

if that eliminates the problem, then I would advise you try another sound board. if you want to go to the safe route, I know a good seller on ebay that has gobs of MK1 boards. I actually bought one off him. if he says it works in the description, it'll work. if it doesn't, he'll say so. he's reasonable, I'd say.

PM me if you're interested in going that route. and good luck.
 
I have an extra MK1 sound board that is tested working. It has NBA Jam TE roms in it, but it is fully working.

Drop me a PM if you are interested.
 
Other than the sound issues (which sound like a bad pot) your symptoms sound like voltage issues. Have you metered the voltage on the board itself? Have you checked the harness to make sure the pins are making good contact with the board? Have you tried the boards in another machine?
 
Well yes, when I measure my voltages I always do it at the board itself, and those are the numbers I got. It's a solid 5v now. I haven't tried them in another machine as at the moment, I don't have room to turn around any of my other Midway cabinets, and they're the only ones with -5v.
 
I'd almost assuredly say it's the sound board. I've had several mk1's and mk2's, and have had all kinds of issues caused by the sound board. I'm pretty sure I had one that reset from time to time until I switched out the sound board as well. Remember when the game boots it checks the roms on the sound board anyways.
 
I'd almost assuredly say it's the sound board. I've had several mk1's and mk2's, and have had all kinds of issues caused by the sound board. I'm pretty sure I had one that reset from time to time until I switched out the sound board as well. Remember when the game boots it checks the roms on the sound board anyways.

Yeah that's what bothered me, it passes all the rom tests just fine but the volume still fluctuates... I just thought it was a bad pot. I had no idea that little board could cause so much trouble. I'm gonna swap it out in a bit and try to figure out why my other sound board doesn't produce anything at all. Either way, I should see a difference after I remove the one that's in there now. I'll let you guys know how it went!
 
The volume post costs $3.00 at radioshack. Try that first, if ti doesn't work unplug the soundboard like Mecha suggested. If you need a working soundboard I have tested working one you could have cheap. I picked up a spare a while back not knowing why the other didn't work. Figured out I needed a volume pot after purchasing a 2nd working sound board.
 
I would start by replacing the volume pot and maybe recapping the sound board. Bad caps could cause the fading sound.
 
OK I have since switched the sound boards to no avail. I put a new pot on the sound board that had no sound whatsoever, and it still didn't work. The funny thing is, it passes the burn in test, and the sound board test in the diagnostics with flying colors.

I don't even know where to begin to look on how to fix these things. The first sound board I have, which has sound only the volume fluctuates, is probably because the board at one point was under 5v, and at another, near 6 and a half. So that board is probably shot. The other one that outputs no sound at all has me baffled. I cannot find any burnt resistors, bad caps, cut traces or cold solder joints, the roms appear to be good according to the test menu, so I'm lost on this one.

Any ideas on how to fix these things?
 
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