64th Street – A Detective Story repair log

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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

Psychic 5 repair log#2

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Apr 102018
 

My beloved original pcb developed once again problems after the last repair log you can read here:

Psychic 5 repair log

This time, the overall colour palette was pinked and all text was missing

After some troubleshooting I found again a problem with a Fujitsu ttl 74ls08, same type and week/year of manufacturing of the one I changed on the old repair log!

 

After changing it, the game worked fine for some minutes and then it developed a new problem

 

Game lost sync and became bluish

Again I decided to concentrate on the only Fujitsu  74ls08 ttls which were present on the board until I found the faulty one.

Same week/year of manufacturing and same fault: dead outputs

In addition, I found another couple of Fujitsu 74ls08 which had dead outputs  and I decided to change all of them with good ones.

Now the game is 100% fixed but there are 3 more Fujitsu ttl waiting to be replaced. I ran out of parts otherwise I would have changed them immediately.

Avenging Spirit Repair log

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Oct 312016
 

Received this game for a repair.

The game had messed background images

avenging2

Shorting some data pins rams I found the one responsible for the tiles arrangement.

I replaced the one marked in red below and it was infact faulty but it didn’t fix completely the problem.

I noticed that the 74ls745 above it was pressed down badly like if it was hit but something.

I decided to unsolder it and test the continuity of all the pads and infact I found three of them (marked in red) that visually were OK but they had no continuity

with the other chips.

That meant that the internal pad was broken so bad it detached internally

avenging1

 

Making some ugly flying wiring with something I had at hand I tested the game which was fully restored

 

avenging4

 

avenging3

May 192016
 

Last year after a lot of research I succeded to buy at a good price an original board of Psychic 5 , one of my favourite game of all time, which was really in mint conditions.

 

psychic5

 

Some days ago I picked it up for a play and after some minutes the game developed this problem during attract mode:

psy5

psy3

psy2

 

After some more time the game was total yellowish:

psy4

 

Game hasn’t any schematics available and there are not any games known to me which have similar hardware and schematics available.

I could only go blind and try to find the reason of the fault.

So I started to short some signals of srams looking for palette changings and I found a couple of them at pos.6N and 6M, 16kbits.

The one at pos.6N had the first 4 data pins low and I was pretty sure either it was faulty or the 74ls245 connected to the data bits.

After desoldering, the sram 2018 was tested good in the programmer. That meant that the programmers didn’t use all data pins of the palette rams. The 74ls245 was good because it could drive correctly the signals without the ram installed.

I tested some TTLs nearby looking for strange signals but found nothing.

Decided to go brute force using my logic comparator and testing everything on the video board but starting with FUJITSU parts which are known to be very unreliable.

The logic comparator found a 74ls08 with all the outputs faulty

psy6

 

I double checked the outputs with a logic probe and they were all in the grey area (no good logic signal).

After changing it I fix the problem 100%

 

psy8

psy1

So FUJITSU brand proves once again to be really unreliable!