Crime City – Taito

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!

Pac Land – Namco

Another board on the bench from a friend of Mr.Ozzi, this is another blast from the past, an amazing great game from NAMCO published in 1984 : PAC-LAND !

The board didn’t boot in the game displaying some garbage on the screen, it’s time to bring it back to life! Here is the PCB:

It comes with a custom made JAMMA adapter, when the board was powered up something was displayed on the screen :

Sometime a grid is diplayed like the one above, leaving the game in a stuck state, sometime it turns out in some kind of “garbage” like this:

The usual visual inspection didn’t reveals anything wrong with the board, no cutted traces or jumpered pins. This “garbage” by the way is not a bad sign because it told us the video section is working in the right way, tiles and characters were correctly displayed on the screen. So we must focus our cares in the CPU section of the board because i thought the culprit should relied here somewhere, since the CPU shouldn’t always execute the right program code,  and the grid image told us in some way that the CPU is ok and it shouldn’t be damaged.

Probing around there i found something interesting in a custom chip marked “34 J349X” made by Texas Instruments. Looking at the MAME source code i found some info about this IC when in the driver files notes the programmers talked about Memory Mapping :

Part of the address decoding is done by a custom IC (CUS34A), so the memory map is inferred by program behaviour. The CUS34A also handles internally the main and sub irq, some latches, and a watchdog.

Pressing gently it down somettimes changes the game behaviour randomly going over the grid or garbage screen leaving the game in a loop state but something new is often displayed on the screen:

… interesting! Yes i know it was upside down, but .. who cares! Is one little step ahead! These should be the results of the startup selftests of the game, but they should wrote something different than this …

The suspect custom chip “34” is clearly some kind of address decoder with other extra functions, looking at it i seen its pins turned their colours into black, maybe due to corrosion over the ages, it was socketed so i can easily remove it from the board.

Oh my god! It seemed to be a very fragile chip, all the pins were loose and black coloured, and two of them, one per side, were totally broken due age and oxydation, also the socket is rusted and seriusly damaged by corrosion, so it should be replaced and the broken chip legs rebuilded. So i removed and replaced the bad socket too:

I gently cleaned all the chip legs and fixed the broken ones, and then safely and carefully fitted it in place:

After some internet research, it turns out to be a common issue with these custom chips. By the way it’s time to cross our fingers and be ready for the smoke test …

Bwoooooooooooaaaaaaaaaaa … much better! The game booted up and now it’s time to make a deep ingame testing! ๐Ÿ™‚

The game played fine, so no more cares are needed by the board! Here follows some other board pictures, neither NAMCO or SIDAM logoes on the board, which made its official PCBs with NAMCO authorization … i hope! ๐Ÿ™‚

By the way it’s clearly an original and genuine board .. of course!

Another case closed!