Apr 202018
 

I’ve been sent from Ireland this R-Type II PCB for repair, actually a conversion on Major Title (Irem M82 hardware)

Board played fine but sound was completely absent:

Here is an overview of the sound circuit:

Someone previoulsy tried to fix the lack of sound by replacing many components.The analog part of the circuit was doing his job as putting my fingers on solder side of the amp produced some noise.Probing the Z80 sound CPU revealed  /INT line was stuck LOW.While testing components something strange happened, I got sound fully working :

The IC I was testing with a logic comparator is a 74LS244 @IC13 whose ouptuts are tied to the 8-bit data BUS of the Z80/RAM/ROM/YM2151.Signals didn’t look fine to me:

But when I put the clip of my logic comparator on the TTL they correctly toggled :

Data lines are tristate, this means they can be in high-impedance  state (Hi-Z) in addition to the 0 and 1 logic levels.When outputs are tri-stated, their influence on the rest of the circuit is removed, and the circuit node will be “floating” if no other circuit element determines its state. Usually pull-up or pull-down resistors are used to influence the circuit when the output is tri-stated.This even more when a same line is shared among different devices like in my case.The 8-bit data BUS of the sound circuit is pulled-up by a 10K resistor array:

When I went to measure it I got high resistance on half of its pins :

I pushed the array a little and this happened :

It was actually cracked in a not visible way (this explains why the logic comparator fixed the issue, it acted like a pull-up).I replaced it:

This restored the sound, board 100% fixed

 Posted by at 4:36 pm
Apr 172018
 

Have you ever encountered the following scenario on your beloved arcade PCBs?

Probably sometimes.The cracked component is silkscreened ‘051550’, maybe now the name tells you something :

It’s a custom IC in SIL package that can be found on countless boards manufactured by Konami (from mid 80s to late 90s).Here’s the pinout taken from a TMNT schematics:

As you can see, it performs two functions :

  • master /RESET generation for the whole system and watchdog timer
  • coin counters handling

In the below scan of the “naked” component  I have highlighted the part of circuit that handles the coin counters.The other part is the /RESET generation circuit which is built around a power supply monitor IC like the Fujitsu MB3773 :

You can read its datasheet here:

http://www.jammarcade.net/files/Datasheets/MB3773.pdf

In this project I took into account only the /RESET function since it’s the most important and vital, coin counters can be now considered obsolete hence I intentionally omitted the relevant circuit.Anyway,  for my repro I wanted to keep the dimensions of the original part:

 

Here’s final testing on a Lethal Enforcers PCB:

See you all to my next reproduction project!

 Posted by at 6:39 pm
Apr 162018
 

The ‘PC040DA’ is RGB DAC (Digital to Analog Converter) used on some Taito PCBs from mid 80s like Rastan, Operation Wolf, Tokio, LSA Squad.As other similar color DAC it comes in SIL package:

But,  as you can see from picture above, unlike the “big brother” ‘TC0070RGB’, you will always find three ‘PC040DA’ (one for each color) on a same PCB.Technically speaking the custom is a 5-bit DAC (6-bit actually but the least significant bit is tied to GROUND or VCC ) for a total of 15-bit of palette data received on their inputs.You can see its pinout in this snippet of Bubble Bobble schematics:

This custom has been already analized and successfully reproduced some years ago, you can read more about on this french forums :

https://www.gamoover.net/Forums/index.php?topic=27336.0

But I decided to do my own reproduction taking inspiration from some bootleg boards where the component has been re-engineered with a simpified circuit.The prototype on breadboard worked fine:

So I developed a proper PCB:

This is the final testing on a Rastan PCB where the reproduction has replaced the original part related to RED color channel:

See you all to my next reproduction project!

 Posted by at 7:53 pm
Apr 152018
 

Received a box of faulty PCBs to repair from USA.I started my work on them with this Teenage Mutant Hero Turtles by Konami:

PCB was in very good state and played fine with correct sound too but it had a color issue, the screen was yellowish simptom that the BLUE color was missing:

BLUE, as well as RED and GREEN, is generated by a custom SIL marked ‘052535’ (basically an R2R resistor ladder which acts as DAC , see my reproduction post here for more details)

Probing the  custom revealed activity on all inputs (pin 2-3-4-5-6-7 ) but output pin 8 was stuck low:

So the component was internally faulty.I replaced it with same part taken from a spare board :

This restored the missing color.End of job.

 Posted by at 6:02 pm
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