Corrado Tomaselli

Megazone repair log #2

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Aug 102017
 

Got this faulty pcb for my collection

 

Game was resetting over and over right after the first stating screen without even the possibility to see the diagnostic screen “RAM ROM OK”

The code was running and the watchdog was triggered by some event which I eventually found in a faulty 2114 SRAM @E15 on the video board

 

Marked in red on the actual pcb:

After changing it, the boot process went on until it displayed RAM and ROM OK but it reset again just before screen with the white grid.

At this time I was complete blind , missing any programming skills I couldn’t check what the code missed to go on.

I had a suspect on the sound part and after checking the clocks on the 8039 MCU I noticed it was completely missing.

The oscillator was working correctly and from the schematics I could see that the clock for the 8039 and the AY is generated using some 4bit binary counters 74ls293, very difficult to find on other boards.

 

Probing pin 9 of 74ls293@A12 it was stuck high while on pin 10 I had the clock.

I proceeded to test it out of circuit but was tested good.

Not sure about the reliability of the tester, I found a Starforce boot which use one 74ls293 and I installed it in a socket on megazone.

Still reset and no clock from pin 9

The output of the IC@A12 goes to pin 8 of a 74ls240, therefore I decided to test pin 8 of IC @D14 against +5V and it gave me a short.

After changing it the game finally booted without any other faults

 

Jul 012017
 

These system boards from Namco has a common problem: SMD capacitors

These caps expecially the ones produced in the 90s are very unreliable and sooner or later will start to leak

On my boards there isn’t one that has the ESR in a good range.

On Namco system NA ( Emeraldia, Tinkle Pit, Super World Court) or NB ( Point Blank, Nebulas Ray), the problems you will face are sound related ( low sound or scratchy ), on ND system ( Namco Classic Collections), the SMD caps are used also for the RGB amplifier and in addition you will get colour problems.

I will take as an example a Namco Classic Collection I just finished to repair and which had all the problems coming in about one week of intensive use after a while in storage.

 

First of all it started to have trails on the RED component of the image.

The system uses a common RGB amplier LM1203 which for example is used on the majority monitors neckboards

Here are the usual circuit taken from the schematics of the LM1203:

As you can see , on the inputs of each colour component you have to place a 10uF cap.

Namco engineers decided to use a 4,7uF but I tested and there are no differences between the two values.

So, if you have colour quality problems on your Namco Classics Collection pcbs, first thing is to recap the RGB input section using commercial

electrolitic caps.

 

I am not a fan of brute force recap, so with my ESR meter I usually check all the caps and change only the ones that have a very out of range value.

As said before, after few days I get no sound for a while and after about 5 minutes, you could hear it coming up but very scratchy.

The amplifier got also really hot after a short time.

The system uses a LA4705 sound amplifier

 

I probed with the ESR meter all the caps and found out the both the small ones 2,2uF placed on  IN1 and IN2 had a value of more than 99 ohms.

The other ones also were really not in the specification range but to get sound back it was enough to change the small ones.

 

As said the sound section of Namco system NA and NB is the same more or less, therefore if you start to have low sound and you don’t have an ESR meter,

change the 2,2uF, 47uF (33uF on the amp schematics) and 100uF one.

Nintendo Vs System repair log #2

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Jun 282017
 

I bought this system in a lot of faulty pcbs.

Game was booting but it had no sync and the graphic was all corrupted.

The OSC 21.477mhz was giving out around 12mhz

 

After changing it, the game was back to life but it was missing the blue component

Looking at the schematics , I could confirm that the RP004 video chip was giving good signal on the BLUE signal pin, therefore I started

to check the LM324 OP AMPS involved in the DAC circuit.

The output of the LM324 for the BLUE component was dead therefore I proceeded to change it but the game was not fixed yet despite now having a good signal

 

By turning the blue pot I could see briefly some “sparks” of blue ,therefore confirming that the blue component

was working again.

In the end it turned out that the blue pot had some cold joints on the ceramic base.

I reflowed the solder and the game was 100% fixed

Satan Of Saturn repair log

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Jun 282017
 

Got this game cheap for my collection because the seller stated that it has very noisy sound

When powered, the game had a constant white noise while music and the other sound effects were working correctly.

The sound problem was not due to the amplifier or bad caps but it was produced by the logic of the game.

To make a long story short, this game has a similar circuit to what Space Invaders use to produce analog sounds.

It uses a SN76477, which is a programmable sound chip which has a white noise function.

On Space Invaders and in this game, the chip continuosly output the white noise which is switched OFF and ON using operational amplifiers and it is modulated with capacitors and resistors to produce different sounds.

I downloaded the schematics from the US version ( called Zarzon ) where it is shown how the chip is connected to the LM3900 OP AMPS

 

 

Instead of trying to understand which of the two chips are broken, I piggybacked a good one on both chips until

the white noise disappeared.

Problem was fixed by changing the LM3900 Nr.47 on the schematics

Burnin’ Rubber repair log

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Jun 282017
 

I bought this game for my collection some months ago.

 

 

I tested briefly and it was OK. After a few weeks I decided to make a good play but the game developed a strange problem:

 

 

 

 

As you can see from the pics above, the screen was very dark and the brightness was not steady.

Also all the objects left trails of different brightness.

The problem was somewhat similar to when you don’t have video GND connected but as you can see

the text HI SCORE was repeated on top and bottom.

Adjusting the horizontal size of the image with the monitor pots ( vertical on the game) I noticed the screen didn’t shrink, it was like it had no borders.

I had immediately remembered that on the CPS1 hardware , on certain screens of some games ( 3 wonders selection screen for example)

the image is very dark and this is due to a flaw in the video circuit of the CPS1 hardware which doesn’t blank properly the video at the edges

leading to some monitor electronics to loose the black reference and behave strangely.

Charles McDonald on his website  http://www.techno-junk.org/ has a fix but right now the link is missing because he is restructuring the page.

 

Anyway back to the topic, I was convinced that my problem was due to video blanking not working properly, so I downloaded the Bump and Jump

schematics available from Bally (  the japanese ones are impossible to find ) and searched for a video blank signal and tested all the TTLs involved until I found this:

 

Output pin 5 of 74ls74@1B was floating and didn’t clear the 74LS273 connected to the video DAC

 

Therefore the video was not blanked properly on the edges.

Changing the TTL@1B  fixed the problem

 

Unfortunately after a couple of plays,  the pcb developed two other problems:

  1. Car didn’t go left
  2. car crash sound was broken

 

The first problem was fixed by replacing the 74LS367@4E on the sound board ( directly connected to the left direction pin)

Second problem was fixed by replacing one AY8910.

The game, for the moment, is working nicely 😉