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! 🙂

Another PCB on the bench from my friend Sasha: Taito Crime City. It’s a run and gun arcade game, that was released by Taito Corporation, in 1989; it runs on Taito B-System hardware, and it is a spin-off from Chase HQ, which had been released in the previous year.

The PCB was in very good conditions, clean and with no apparently damage, once the game has been powered up, we were been greeted with this …

An unstable and rolling image all over the screen, it’a a clear sign of a sync related issue, the game seemed to respond in the right way, it can be coined up, the audio seems working, also the players controls seems to work, i say “seems” because trying to play the game in this way produces some kind of “seasickness” … LOL.

… and what’s happening inside the game wasn’t so clear! But it’s time to investigate.

I looked at the sync signal on JAMMA harness and as we suspect it was completely stucked at ground level, an healty sync signal should tick instead. I traced back the sync signal through the PCB up to a pullup resistor connected to VCC (+5v) and then up to an output pin of an IC, an HEX buffer with open collector outputs, a 74LS07.

You can see the sync signal path marked in yellow colour, the signal is fed into the IC into pin 1 and then it comes out from pin 2, here is the logic diagram of the IC:

So before making any assuption about the possible causes of the issue we should point the scope over the input and the output of the IC to see what’s going on … the input is in BLUE (pin 1) and the output is in YELLOW (pin 2)

As you can se the input is correctly ticking at the right frequency (15.7Khz) while the output is stucked at ground level. In the output signal path the only discrete components were a pullup resistor, a ferrite bead and a capacitor and even if one of these components were damaged the sync signal shouldn’t be stucked at ground level, so the IC should be damaged and short circuited its output pin internally to GND (ground). Confident the issue was related to the IC malfunctioning i promptly remove it.

Testing it out of circuit confirm our suspects, the interested logic unit was failing:

and then socketed and replaced …

Powering up the board ends up with a perfect and stable image displayed on the screen! The sync signal now is ticking and healty as it should!

Case closed!