PANG – Mitchell #4

Another MITCHELL PANG board on the bench :

the PCB looks in good condition the usual visual inspection didn’t reveals anything strange, the battery has been removed, so the board should be at least suicided, powering up the game only a solid white screen is displayed.

STAGE 1 – CPU and EPROMs

I replaced the program ROMs with the correct unencrypted ones, removed also R33 resistor, this will turn the Kabuki CPU in a stock Z80 cpu able to run unencrypted code, but no impovements were made, still solid white screen. I checked the Kabuki CPU with the scope for vital signs, and i found almost healty signals but something looked strange so i replaced it and powered up the board again … but once i removed che CPU from the socket it looked like this:

So i fix the broken leg and made an overall clean-up …

by the way, i was greeted with the same white screen, but now healty signals came out from the CPU, so something was going on … in the meanwhile some audio fx came out from the speakers, so the game should be running blind …

… now i should focus on the video section of the board.

STAGE 2 – Video RAMs

Looking at the video RAMs clear signs of oxydation and rust can be easily spotted:

The SONY CXK5814 SRAMs are very prone to failures over the ages, and deeper looking at the board we can see they were the only rusted ICs, all the others chips looked dirty but in good shape. I was confident about the failure, and without testing them in-circuit i removed and replaced both of them:

once replaced both video RAMs, i powered up the board and it played in the right way:

I tried to play the game, but the board cannot be coined up, but the demo is still running, so there should be some damage in the controls.

By pressing the TEST switch i can perform some controls diagnosing and debugging, once in the I/O TEST screen i was greeted with this :

The COIN switch seemed to be pressed all the time …

STAGE 3 – COIN CONTROLS

I followed the path of the coin control from the JAMMA harness up to an 74LS367, passing through an RC network SIP module …

the 74LS367 IC is a 3-state buffer/ line driver, here is its logic :

The coin control input is on pin 6 and its logic unit output is on pin 7 which seem to be stucked at VCC level even if a coin is inserted and the IC is triggered by pushing LOW the enable pin 1, so i replaced it:

This fixed the coin mechanism logic!

STAGE 4 – FM AUDIO

Now the game played fine, but only audio FX were present no FM synthesis at all.

Here is a closeup of the audio section:

I started probing the quad op-amp the M5224P, but it didn’reiceive any FM audio signal on its inputs, so going backward inthe FM audio chain we found the YM2413 an FM synthesis sound chip, and with a close inspection it didn’t seem in a good shape … probing both melody and rythm analogue outputs with the scope, reveals they were totally silent. The IC must be replaced …

Once removed, the chip was totally wrecked on the underside:

Final Stage COMPLETED !

Replacing the YM2413 bringed the FM synthesis back to life, so we can be amazed again with the well known Pang music theme!

Another case closed! 🙂

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