Mortal Kombat II Sound Issues - Dying Amp?

redshoepaul

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Hey all, could use some help with my MK2 sound board. I've had this cabinet for a few years at this point and up until recently it's been working perfect the few times that it was turned on and played. The other day I fired it up and it was playing fine but felt like the sound was getting lower over time. Then it cut off and never came back. I turned it off, turned it back on and I didn't get the loud tone you'd expect to hear, and the game itself had no sound. I ordered a cap kit and got that done. Plugged the board back in, and it fired up and worked great. I thought that was that.
Next day I went to fire the machine up just to check and same issue all over again. Today I reseated all of the roms and put the board back in. I got the tone at the startup that you would expect, but no in game sounds. I went through some old threads and basically went through all the diagnostics I could find. I checked the power supply and was getting proper 5v and -5v, although I actually haven't checked on the board itself as I'm actually not entirely sure where to check that. Here's where things get a bit weird.

At first, despite the sound and LED turning off, if I ran my fingers on the amps pins, I could hear the static. I noticed if I pulled the Ribbon connector, the LED would turn on. One of the times I pulled the ribon cable and turned it around, only for the sound to suddenly kick on, but it sounded extremely distant. I then pulled the ribbon cable out entirely and was still getting sound, just very distant. Put the ribbon cable in properly (red strip starting at pin 1 on both boards) and again the LED went off. I turned it off for a while, came back to check it out, oriented the cable properly, I got the startup tone, but once the rom checks were done the LED turned off again. I had my friend start up a game when sound would normally play and I reoriented the ribbon cable incorrectly intentionally and suddenly the sound was back on 100%. I stayed back there while my friend played through the game. The LED was flickering and slowly losing power, and as it lost power the sound would continuously get quieter.

I touched the heatsink on the amp and it was scorching hot, so that is 100% overheating. Trying to figure out what the source of this is.
Is this a sign of a dying amp? I've seen a few complaints of very similar symptoms but all of them have different endings. I read one thread about a guy turning his marquee off and that sorted the issue. Another pulled one of the sockets (u11) and replaced it to fix their issues. Anybody have any tips on what to check next? I think the cap kit went well, but are there any specific caps that could be causing the amp to overheat that maybe has a cold joint?

Another odd quirk is I noticed the LED on the switching power supply will switch between saying too much 5v, just right 5v, and too low. That could be normal, just wondering if it could mean something else. Any help is super appreciated!
 
Appreciate it mecha! Would you know if the overheating is a symptom of cold joints? If so which caps specifically should I focus on?
 
no. you just buy a cap kit for it. lol
Sorry I meant is it possible this could be a symptom of poor soldering? I'm going to go over the joints that are newly soldered again to be sure, but checking to see if this is what the symptom would be and if so, which caps specifically should we keep an eye on?
 
Is the speaker wired to 10 and L on the edge connector or is one side of the speaker connected to ground? You cannot ground one side of the speaker on that game.
 
Is the speaker wired to 10 and L on the edge connector or is one side of the speaker connected to ground? You cannot ground one side of the speaker on that game.

Sound has been working fine for years prior - totally not the issue
 
Youve replaced the ribbon cable going to the main board to the sound board? Varmits like chewing on them for some reason think it has to be the plastic coating and they have really thin wires was a common problem in valley dart machines where they had like 5 ribbon cables.
 
Hey all, just a quick update with this since it's been a minute -

Main amp, U37, all new caps, and GAL from another MK2 sound board +new ribbon cable all have been installed. Getting the same issue.
I have a U11 that will go in next and also planning on grabbing a U16 (AD1851 NOS).


Here is a video from startup for people to listen to:

https://drive.google.com/file/d/1oJlRuZJJkmsvqHQ8fN4E9YTW-wvPOzv7/view?usp=sharing

Notice the lingering static-y distortion? Yeah that's the issue.
 
Hey all, just a quick update with this since it's been a minute -

Main amp, U37, all new caps, and GAL from another MK2 sound board +new ribbon cable all have been installed. Getting the same issue.
I have a U11 that will go in next and also planning on grabbing a U16 (AD1851 NOS).


Here is a video from startup for people to listen to:

https://drive.google.com/file/d/1oJlRuZJJkmsvqHQ8fN4E9YTW-wvPOzv7/view?usp=sharing

Notice the lingering static-y distortion? Yeah that's the issue.
there's a TL084 op amp, there's what I believe are tantalum caps (the yellow ones by the amp), or maybe it's the DAC. might be something @zenomorp knows more about.
 
Was eyeing the tantalum caps earlier today as a possibility as we got the values of them in needed to order. Will tap back in once the DAC and U11 are replaced
 
Was eyeing the tantalum caps earlier today as a possibility as we got the values of them in needed to order. Will tap back in once the DAC and U11 are replaced
yeah zeno made a video about a bad TL084 on Mortal Kombat 3, may have sounded like yours. those are way more inexpensive compared to that DAC. lol

I had a Mortal Kombat II sound board where they mixed up the +12/-5V which obliterates the DAC and it had a substitute part number on it. another case of me missing taking a very important picture!
 
Hey all, checking in - is there anywhere to see the silkscreen for the MKII audio board as we suspect a few topside traces are damaged and want to test!
Paging @mecha
 
There are some traces that run under the DAC and can't see exactly what they're going to. They trace back to U17 and U1 and they seemingly connect to the legs of the DAC.
 
There are some traces that run under the DAC and can't see exactly what they're going to. They trace back to U17 and U1 and they seemingly connect to the legs of the DAC.
schematics exist. just beep them out point to point.
 
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