Mortal Kombat II sound and coin issue

bradp56

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I have a MK II board that when connected everything is fine but two little things. First there is a loud pitch humming when the game is on coming from the speakers, the gameplay sounds come out fine but there is still the hum and I am not sure what to look for.

Second it will keep adding credits, I will sit at the start screen and it will add 2-3 credits at a time every few seconds until it gets to 30 which is where the max is set. I could put this on free play but it will always bother me until I get it working.

The only thing I can say is the the light on the sound board stays lit red but with the main PCB there are two lights one stays lit red and the other constantly flickers like when a computer light shows its doing something.

Thank you;
 
check your coin switches via the test menu to see if one or both are stuck NC instead of NO (lit vs unlit, should be unlit) this is a simple wiring issue and the disconnect is probably on the wrong terminal or the switch could be bad or shorted.

as for your sound issue, pull the chips and clean the legs (note the key mark/orientation), reseat the ribbon cable and power connector.

check for breaks in both of those (but mainly the power connector). get out the manual and meter the voltage at the soundboard. check for damage ect... report back :D
 
Ok the coin up issue is resolved there is a metal piece that holds a switch and a button, the switch was in the wrong location.

The sound issue is still there, Could this have to do at all with the -5? My +5 is at 5.4 and the -5 is at 3.9.
 
The buzz in the speakers is common. I have heard it on multiple board sets. Some do it. Some don't. Although, I wouldn't say that it's not an issue. I'm just saying that it's common. I would adjust your voltage to get as close as possible to 5 and -5vdc.
 
Thats the problem I adjusted the voltage to exactly +5 then the -5 is at -3.0 then the PCB will not boot up and there is no picture on the monitor.
 
Sounds like your power supply is on its way out. Magic 5v number to shoot for is 5.18v
 
So it sounds like the issue is with -5v being too low? One thing that was weird is when I unplug the board to check the voltage at the jamma harness, the power switcher has a loud pitch sound coming from it then when I hook up a Jamma PCB the noise goes away. I will order a new switcher or grab from another game to see if that helps.
 
Brad,
Couple of things……

First, you do not need to remove the pcb to test the voltages. You can, and should, test them while the board is still connected. Power supplies will give you different readings when they are connected and running a game as oppose to not carrying a load.

Second, running a switching power supply without a load (i.e., without a pcb board connected) can actually damage the power supply itself.

I would hook the board back up to the jamma harness and retest it with the game on.
 
Adjust the 5volts at the board. That is the most important as that is what the games chips use. Just wanted to add, my mk 2 makes a humming sound while its on. It never started into I installed my switcher. I installed two fans in my game and I think that only amplified the sound more. It is common. I wished I knew how to fix it, it is a little annoying. Probably a poor ground issue somewhere on the sound board?
 
Yea the humming is annoying because its kinda high pitched almost like when you have a game with 2 sound pots and one needs to be turned all the way down and the other is the one you turn up and down.
 
Adjust the 5volts at the board. That is the most important as that is what the games chips use. Just wanted to add, my mk 2 makes a humming sound while its on. It never started into I installed my switcher. I installed two fans in my game and I think that only amplified the sound more. It is common. I wished I knew how to fix it, it is a little annoying. Probably a poor ground issue somewhere on the sound board?


Yes, definitely check at the board.

(Picture is by Anthony Pietrak of QuartrArcade)
 

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Sweet thank you for the picture I will try that tonight and post back. If need be can I wire a sound pot to turn it down some?
 
The short answer is "yes". But are you sure your cab does not already have one? Is this a dedicated cab? As these guys have said, sometimes MKs just hum.
 
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