Mortal Kombat repair help needed

Rftckeeper

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I appreciate anyone willing to take the time to put in their two cents.

I have a y unit rev 2.0 MK pcb. The board boots fine with no cmos errors. After about 30 seconds it goes into the test menu by itself and then selects diagnostics and switch test on its own. The board stays on this screen with test button locked on. I am able to test player 1 and 2 controls for the most part but high kick, low kick, and low punch do not register. Occasionally the service and high kick buttons will register on their own.

After about 30 more seconds in this test menu the board will lock up. I can switch off the power and the cycle repeats itself. Seems that times decrease with subsequent boots.

It seems to me that the pls135 at U40 is getting warm to the touch. I suspect this might be bad as the schematics say this is an address decoder pal.

My main question right now I guess is if this being bad could cause the behavior i described. I would like to know if im barking up the wrong tree.

I am starting to use an oscilloscope to troubleshoot this pcb for the first time so I am inexperienced when it comes to the scope. I do see some lines that are tied high on this pal with the occasional activity when scenes change on the screen. My inexperience prevents me from understanding if this is normal,

I appreciate any input as at this point I feel I'm chasing my tail. I would like to avoid shotgunning chips to fix this board so im begging for any help anyone is willing to give.
 
Just putting this here for anyone that cares. I pulled all the sn74hc541 input buffer chips. I noticed I wasn't getting a full 5v on the input line side on a couple of the pins. I socketed all 4 and replaced 2. I have a working game no resets and full controls. I can only guess that maybe all the inputs that were being triggered overloaded the pal and that's why it was getting warm. Effectively overloading the pcb with button presses making it lock up? Whatever I have a working board now……. Sort of. Now I have to trouble shoot why I have no sound on my sound board. Hoping it's just a quick recap and I'm good.
 
Just putting this here for anyone that cares. I pulled all the sn74hc541 input buffer chips. I noticed I wasn't getting a full 5v on the input line side on a couple of the pins. I socketed all 4 and replaced 2. I have a working game no resets and full controls. I can only guess that maybe all the inputs that were being triggered overloaded the pal and that's why it was getting warm. Effectively overloading the pcb with button presses making it lock up? Whatever I have a working board now……. Sort of. Now I have to trouble shoot why I have no sound on my sound board. Hoping it's just a quick recap and I'm good.
yeah, I had a Super High Impact with ram errors years ago. I just let it run for a few minutes and then I smelled ozone aroma and started touching the 74LS245 chips underneath the rams and one was hot. that signifies there's a transistor short inside, that's why they get hot. replaced the chip and no more ram errors. it can go the other way too, if there's open connections on the transistors inside a chip that's normally warm will be cooler than the others.

I'm glad you saved another one.
 
I have now ruled out anything on the main pcb. I used a known good working sound pcb and everything worked as it should.
On the sound pcb in question I get no. Bong at startup and no game sounds. All there is is a constant horrible static that changes as the attract mode changes.
I have run my finger over both op amps which give me feedback through the speakers as well as running my finger over main amp which does the same.
If I probe pin 12 on u28 which is buff (I'm assuming buffer) the static goes away. If I touch that pin with a finger it gives feedback through speakers louder than the op amps.
I really dont know where to go from here. If anyone could kindly chime in I would appreciate it.
At the very least a suggestion of anyone who fixes these sound pcbs would be appreciated.
 
I have now ruled out anything on the main pcb. I used a known good working sound pcb and everything worked as it should.
On the sound pcb in question I get no. Bong at startup and no game sounds. All there is is a constant horrible static that changes as the attract mode changes.
I have run my finger over both op amps which give me feedback through the speakers as well as running my finger over main amp which does the same.
If I probe pin 12 on u28 which is buff (I'm assuming buffer) the static goes away. If I touch that pin with a finger it gives feedback through speakers louder than the op amps.
I really dont know where to go from here. If anyone could kindly chime in I would appreciate it.
At the very least a suggestion of anyone who fixes these sound pcbs would be appreciated.
Have you done a cap kit? if it still has the bright blue axial caps then it needs done and a good chance it solves your soundboard problem and check for leaky green ooze with a fishy spell and make sure all of it is cleaned up before it eats traces if it hasnt already and inspect your sound board for it everywhere especially by the caps, if they just started leaking might only be under the caps
 
Thank you jukebox for the reply. I did a cap kit first thing. No change but I did fix it.
 
For anyone who cares again. This is now closed.
After pulling a donor ym2151 I replaced the socketed one on the sound pcb in question. This returned the sound and eliminated the feedback. Now if anyone is actually reading this thread the last thing I would like to ask from this forum is for a good place to source the ym2151 and ym3012 ics please. Thank you for the comments here juke and Mecha.
 
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