Sep 212017

Another board from the box of the “death”: The Simpsons

It immediately appeared to be a “desperate case”, board was heavily “harvested”, many components were missing:

A smashed TEST switch too:

I tried to power up the board and obviously it didn’t work, it was  in an endless loop:

Summing up, these were the missing/broken components : 

  • 6264 RAM @16G
  • 3.579545MHz oscillator @4G
  • ER5911 serial EEPROM @12C
  • MB3722 amplifer plus electrolytic capacitors @C21 (1000uf 16V), C16 (1000uF 16V), C114 (330uF 16V)
  • momentary switch @16A  

I reinstalled the RAM, the oscillator and the EEPROM (temporarily omitting components in audio circuit)  and board showed the self-test reporting bad devices @12C (ER5911 serial EEPROM), 6G (sound Z80 code 27C010 EPROM) and 7G (2k x 8-bit SRAM of sound system)

I cleared the first error by re-initializing the EEPROM (holding down the TEST switch while powering up the board) 

As for the other two errors, I dumped the EPROM @6G and code didn’t match any existing MAME set so I reprogrammed the device with correct code.This cleaared the error but the one regarding the device @7G was still present.Probing the device (a Sony CXK5814, a 2k x 8-bit static RAM compatilbe with 6116) revealed unhealthy signals on data lines:

Once removed, the chip failed the out-of-circuit testing:

Finally the self-test no longer complained so board booted into game but colors of some backgrounds were wrong and sprites were mssing parts:

The MASK ROMs check reported a bad device @16L, this is a 8Mbit one containing sprites data:


I replaced it with a 27C800 EPROM but with no luck so I focused on the background colors issue.The hardware uses an ASIC marked ‘053251’ which is responsible of palette and priority:

The chip outputs 11 bit of palette index called ‘CO0-CO10’ on schematics:

Probing the outputs with a scope revealed bad signal on pin14 ‘CO4’ (healthy signal on the left for comparison)

I removed che custom and replaced it with a spare take on a donor board (Premiere Soccer)

This fixed the color issue but sprites were always wrong :

and device @16L was still reported as bad:

Schematics don’t cover this part of circuit but I knew the design.The two ASICs marked ‘053246’ and ‘053247’ work in tandem, the first generates the address lines to the sprites MASK ROMs and the latter read and process their data:

I checked connections between the ASICs and MASK ROMs, no problem found.Actually the data lines from MASK ROMs are not directly tied to the ‘053247’ ASIC but there are some 74LS374 in the middle (two for each ROM) which latch data bits.Piggybacking the ones @12K and 10K restored partially the sprites.I desoldered the first and it failed the out-of-circuit test:


This improved sprites but not yet perfect :

So I removed also the other 74LS374 @10K, it failed too!

This fixed the sprites issue completely.As for lack of sound it was only matter to fit the missing MB3722 amplifer and electrolytic capacitors  @C114, C21 and C16.

Another board torn from the darkness, long live to it!

 Posted by at 7:36 pm
Sep 172017

Another board from the box of faulty ones, an Irem M92 motherboard (with Undercover Cops license seal)

For my testing I used the ROM board from a working Undercover Cops (but I could use a whatever one).On boot I was greeted by this screen, self-test failed  reporting errors related to some RAMs 

There are two  6116 SRAMs @IC43 and IC44 whose data bus is tied to the two graphics custom ASIC ‘GA22’ and ‘GA21’  :

When I went to piggyback the RAMs, error changed in this:

Probing them with a scope revealed the outputs were dead on both despite the address lines were properly toggling:

Both chips failed when tested out-of-circuit:

With good RAMs the board successfully passed the self-test and entered in game:

But sound was missing at all, I could hear only a slight buzzing noise:

Using my audio probe I was able to verify that both FM sound and PCM samples were properly generated and then pre-amplified/mixed by OP-AMPs but signal got lost somewhere not reaching the input (pin 1) of the MB3730 amplifier.The whole audio section was previously recapped by someone so I ruled out electrolytic capacitors.But a closer inspection revealed heavy corrosion on many 1/8W resistors (probably due leakage of original ELNA capacitors), here are pictures of them taken with a microscope:

Measuring them gave me infinite resistance sign that they were open:

Here in details the bad resistors I replaced:

  • R201 –  1KOhm
  • R203 –  100KOhm
  • R205 –  4.7KOhm
  • R208 – 4.7KOhm
  • R209 – 2.7KOhm
  • R211 – 10KOhm
  • R213 – 10KOhm

This restored full sound, board 100% fixed.End of job.

 Posted by at 11:58 am
Sep 132017

Received some days ago a box of faulty boards for repair.There was a clean Sunset Riders PCB:

Game played fine except for sound since samples were only noise (music was fine instead):

Sound samples are stored in a 8Mbit MASK ROM @1D:

MASK ROMs check reported it as bad:

I was sure device was really bad (Konami ones are well known to be prone to failure) so I removed it :

But it turned out to be good instead since reading it ended up in a good dump and also replacing it with a programmed 27C800 EPROM had no effect .

Sound samples circuit is very simple, the ASIC ‘053260’ (a custom PCM/ADPCM sound chip) addresses the MASK ROM @1D and plays its data:

No other component is direclty involved so most likely the ASIC was the culprit.I removed it :

Took the spare one from a scrap Asterix and soldered it back on my board:

Success!MASK ROMs check reported now a good device @1D hence board 100% working and fixed :

 Posted by at 6:33 pm
Sep 122017

Received this original Bubble Bobble PCB for repair.Set is made of a CPU board :

and a VIDEO one :

Game played blind, no RGB output, only SYNC was present on edge connector pins.Each colors is generated by a single ‘PC040DA’ custom (a 6 bit DAC) 

Probing them revealed that pin 10 was stuck, this is the vertical blanking signal called /VBL on schematics:


Tracing back the signal  lead me to a counter 74LS163 @IC11:

Clock on its pin 2 was present but all outputs were stuck low:

The IC failed the out-of-circuit test:

I replaced it and finally had output on video but all was displayed with the only red color, green and blue were missing at all:

I checked the two involved PC040DA customs @IC85 and IC86 and found that their output pin 3 was clearly dead compared with the active one of red color:

I removed them and installed sockets:

I took the donor parts from a Rastan PCB and installed them:

Colors were restored and board 100% fixed.


‘PC040DA’ DAC could be the next candidate for a reproduction although someone has already did it:

 Posted by at 10:16 am
Sep 072017

Got this Michael Jackson’s MoonWalker PCB (Sega System18) for repair:

Board booted but sound was missing at all and game behaved strangely, you could no longer move your character whenever you pressed the buttons or being hit by enemies :

At first glance I thought the FD1094 custom CPU module was slightly suicided so I swapped it with a good one along with the i8751 MCU but with no luck.I began to think of a correlation between the lack of sound and freeze.Probing the Z80 sound CPU revealed /HALT line was asserted while the /INT was floating.The latter comes from one of the two YM3438 FM sound generator @IC88 :

I removed the chip and tested it on a good motherboard, it was bad.But replacing it made no difference, the shared data bus of the two YM3438 was still inactive.So I pulled the other one @IC87:

It was bad too.Replacing it fixed the board.

 Posted by at 10:22 am