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

64th Street – A Detective Story repair log #1

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Oct 292018
 

This PCB from my collection had a subtle problem after some minutes of being turned on and with voltages more than 5.1V

Some lines of the sprites started to be shifted toward the left

 

If I lowered the voltage to exactly 5V the sprites were good.

Being a perfectionist I started to find the culprint and eventually I found the circuit involved in the problem.

If I put the oscilloscope tip on pin 2 of 74LS365@IC55, the problem got worse, more lines were shifted to the left.

The chip was good so I changed also all the components which were in the chain, a 74LS245@IC56 and a couple of rams @IC57 and IC58

It must be said that all these components had healthy a good looking signals on the oscilloscope and they were tested good out of circuit but I decided nevertheless to change them with other brands to rule out every possibility

Still same issues after some minutes powered on

At this time I really thought it was either the sprite maskrom or the custom chip which was the source signal of pin 2 of 74ls365 until I noticed that the same signal was also connected to pin8 74LS273@IC94 which was pretty far from the other chips in the chain.

I decided to make a piggyback with a good chip and the problem desappeared completely at all voltages.

I changed it without further checking and still the problem was there!

With the tip of the oscilloscope I touched pin 8 and the problem disappeared immediately.

The oscilloscope probe has some capacitance which delayed somewhat the signal and fixed the problem.

So I installed a ceramic cap between GND and pin 8 and the problem was 100%

 

I think that this board had always a timing issue that no one addressed and preferred to run it with exactly 5V

Pacland repair log #2

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Oct 282018
 

Received a pacland board for a repair which was stuck with a  0 2 0 error at boot.

Normally the middle digit means it’s an program rom error and infact the second prg rom @8D had no sticker  and I immediately checked it.

 

Romident showed it was a Pacland midway rom, but the other prg roms belonged to the japanes new version.

After burning the correct 2nd rom from the jap set I was welcomed with this screen which is the service one.

The dipswitches were all set OFF and after verifying they were working correctly I decided to change all the FUJITSU 74LS257 near them which had dead outputs infact.

In the end all the Fujistsu chips has been changed because faulty: @1L, 2L and 2J!

 

With good ones changed the game finally booted but the palette was very tinted and the sprites were corrupted:

 

Verifying the sprites roms 8,9,10 and 11 these were a mess of different revisions, so using a broken pacland I put the matching roms from the new japanese set

Still the sprites were corrupted and eventually I found a bad 74ls86 ( again Fujitsu) @10F

 

With good one installed I got finally good sprites but still the palette was tinted:

 

I proceeded on the colour prom section and found that the chip PL1-2 had some outputs dead

 

Using one from another Pacland board fixed the game 100%

 

 

 

NBA Jam repair log

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Sep 042018
 

Bought this game for personal collection with a problem on the RED colour which was completely missing.

Checking the schematics there are some 74541 chips which are sending the colour bits to a resistor network for Dac conversion. The one on the red colour @UC4 was correctly working so I checked the 74ls07@UA4  involved in the blanking circuit.

Pin 2 was always below 0.5V  and muted the total red colour

 

After changing it I restored the red colour

Rastan repair log #6

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Sep 042018
 

Got another Rastan for a repair.

This one had some vertical black lines and some text was wrong

Piggybacking sram IC2 and IC4 with a good ram brought some better changings so I decided to desolder them and test out of circuit

Both were reported bad

Game was 100% fixed

Mag Max repair log

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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

 

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