Corrado Tomaselli

No background in electronics. Learned everything by reading pdf books and expecially Video game logic Vol 1 by Atari and in general early Atari and Williams arcade manuals

Jul 112018
 

This game had been in my collection for about 14 years. I decided to make a play but the game displayed a RAM NG text and didn’t boot

 

After checking the schematics I decided to prove rams and some devices but the signals were healthy

Thanks to Kale from mame team, I discovered that the game does a simple check but decrementing some values in ram and read them back.

Therefore I decided to desolder the program srams to check if they were good. The game has only 2x 2016 toshiba rams.

This brand is known to be not really reliable in comparison to others

After checking the first one I got confirmation that it was bad

I resoldered it and this time the game booted with RAM NG briefly shown and behaved strangely. You couldn’t shoot and parts of the scrolling was repeating.

I decided to desolder also the second one since the game still complained about the ram.

Also the second one was bad!

After changing also this one, the game was 100% fixed

 

Jul 112018
 

Got this game for a repair.

At boot it displayed the following:

Using Mame source code with the memory map I could narrow down that RAM0044Eo20 was the colour BLUE ram

Desoldering it I could confirm it was bad

After replacing it I still got:

30TIP, 31TIP and 33TIP error

After discussing with Charles MacDonald he pointed me to the DUAL PORT RAM.

It checks if the last entry in RAM (460FFF) is zero, if so that’s an error (should be non-zero)
Then it checks if 460FF contains byte 6B, if it doesn’t that’s an error too

I started to probe around the dual port ram

Until I found a stuck bit on the Left side of the Dual port ram which came from a 74HC175.

After changing it, the game booted but still with TIP33 error which could be erased by pushing the start button.

TIP33 is the error related to the EEPROM which is not inizialized correctly or contain some corrupted data. Normally you inizialize it only one time but everytime

started the pcb, I got this error and offcourse all the dipswitch settings couldn’t be saved correctly.

To cut a long story short, some smart guy changed the original EEPROM with a normal 8k SRAM!

I took a spare from a faulty final lap 3  and I could finally fix 100% the game

 

 

World Rally 2 repair log

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May 052018
 

I got a dead World Rally 2 which upon boot showed garbage but I could hear that the monitor was hissing, meaning that there was no proper sync.

 

I immediately checked the two OSC on board and found the main one, 26mhz which is driving the 68k and pixel clock dead.

The 26mhz is not very common but I managed to found it on another Gaelco board and replaced it.

Game still didn’t start and was out of sync, but the 26mhz signal now was present

Near the OSC there is a 74ls74@IC29  which divide by 2 the clock for the 68k which runs at 13mhz.

The output of the flip flops was completely dead.

Replacing it I got this time a black screen with no activity.

First thing I did without much hope was to check the program eproms , normally it’s very rare that they fail but to my surprise both eproms had pin 31 not connected internally!

Replacing eproms with good ones fixed the game completely.

Last think I did, was to change the DALLAS MCU battery since it was at 2.9V , very near to suicide.

TumblePop repair log

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Apr 242018
 

I got this game for a repair:

 

The game had some background problems ( unfortunately I forgot to take a pic ) and a sprite problem:

 

The background problem was fixed reflushing the custom chip 56.

As for the sprite problem, after some time checking the part of the circuit responsible for the sprites, I found a couple of 6116 sram @D5 and D6.

One of them had some weak signals on the data lines and tested out of circuit it was confirmed as bad

 

After replacing the sram the game was fixed

Black Tiger repair log #2

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Apr 142018
 

Got a Black Tiger for a repair which had a colour problem on some graphics

It lacked some shades of colour

I decided to look around the palette rams @6C and 6D until I found a fixed address line

Tracing back I found an 74ls157 @5D with an output stuck low.

After checking the inputs I found one of them stuck low and according to its truth table, the output is always low.

Tracing back I found a 74ls273 witth Q3 output low and D3 input oscillating.

Piggybacking a good one restored the correct colours.

After changing it , the game was completely fixed