Oct 302021

I’ve worked on a few of these boards before so I knew straight away that I should be able to access the components side on both PCB’s when it is all screwed together but on this board set the video PCB was showing solder side out.
This doesn’t necessarily cause damage if powered up but its obviously not going to work either.
I switched it back around and when I did I noticed that the crystal on the video PCB was completely missing.

I had a spare 12 MHz crystal so I fitted that. Without that crystal fitted you would get a blank screen.
I tested the game and it started it self test routine but failed on the 63701 with an “Error” and had some graphical issue too.

I had to hope the 63701 wasn’t actually at fault because that is an MCU with an internal ROM which I cannot replace easily.
The way the self test works is the MCU does its own self test and when it is complete it puts a value of 0x84 into the shared RAM at location IC22. The main CPU them reads this value when it is ready to and compares it to 0x84. If this doesn’t match then we get an error.
Looking at the schematics we see where the RAM is

I desoldered the RAM at IC22 and it failed an out of circuit test.

Replacing it cleared this error and allowed the game to boot.

Straight away I could see that sometimes there was a problem with the characters.

When they glitched so did the game title graphic too. It looked like they were being drawn twice overlapping each other. This led me to look for a counter issue, usually a 74LS161.

Following the VPOS signal on the schematics I came to some 74LS161’s. Found this one that has some excess solder on the legs. Touching this while the game was running cleared the issue so I just replaced it and the issue was fixed.

Now on to the remaining sprite issue.

Found 3 dead ROM’s on the video board which sorted all the graphics out when programmed up with the correct data.

 Posted by at 5:24 pm
Sep 292021

This version installs in 11J

Why a different version ?

  • Bootlegs have a space at at 14J but no socket, so this version simplifies testing.
  • The version I wrote for 14J won’t boot if 11J is bad, if that version fails to boot then you may have a bad rom at 11J

Diagnostic mode is still available via Player 2 Start + Reset


  • This version is only used for testing and won’t boot into the game once power up tests are complete.

Download link


Operation Wolf repair log #6

 PCB Repair Logs, Repair Logs  Comments Off on Operation Wolf repair log #6
Jul 252021

Got a familiar Operation Wolf PCB that I offered to look at for a friend.
I’ve had this board several time over its life and this new fault was “no sound”. The sound actually stopped working when the owner hit the start button but the game could still be played.
I found the fault really quickly. Starting at the CPU I checked all the signals on the Z80.
Everything was doing what it should be doing except for the /NMI pin (pin 17) which was stuck high all the time.

The NMI is used to signal to the sound CPU to start playing a sound. The signal is generated by the custom PC060 chip nearby on pin 12

I transplanted the custom from my Rainbow Islands PCB to test and the sound all came right back.
Here you can see and hear the /NMI pulse as a new sound plays

So now I’ve confirmed the device at fault what do I do for a replacement. I have no scrap boards with one available. Furrtek has recreated the operation of this device and Caius has made a CPLD replacement that uses this code.
I’ve recapped the sound section and Caius is sending me a replacement but for now I’m happy that this is working again.

 Posted by at 6:24 pm
Mar 132021

Lately I’ve gone through all the faulty/untested PCBs I never looked at (tons of them…) to check if there was something that was worth a repair.I found this Sega System 16B motherboard :

I plugged in a Golden Axe (conversion) ROM board and powered up the set.Board booted up, game was playable with good graphics but sound was missing :

The audio hardware is made of Z80 CPU plus a Yamaha YM2151 for music (and a NEC uPD7759 for speeches) :

Looking at schematics the serial data from the YM2151 (pin 21) is sent to the YM3102 DAC (pin 4) which then routes the analog signal to the TL084 OP-AMP :

Probing with the oscilloscope revealed the YM2151 serial data were present on the input of the YM3012 (pin 4) but  the output (pin 11) tied to the OP-AMP was silent :

The DAC was likely fault so I replaced it :

This brought the sound back but it was faint and corrupted (except the speeches that was fine) :

The TL084 OP-AMP is the only part between the DAC and the final sound output :

It’s a well known prone to failure part so I went straight to replace it :

The sound was completely restored and board 100% working.End of job.



 Posted by at 4:10 pm

Passing Shot (Sega System 16B) repair log

 PCB Repair Logs  Comments Off on Passing Shot (Sega System 16B) repair log
Oct 122020

Got an original Passing Shot PCB for repair, as you may know it’s a tennis game released for System 16B hardware by Sega in 1988.

Board had its ‘FD1094’ battery-backed custom CPU module which was still alive since the board booted but sound was missing at all:

These are the times when an audio probe really comes in handy in diagnosing the fault helping you to figure out the nature of the lack of sound (if digital or analog).So I fired up this tool and started to listem to various points of the audio circuit :

For first I probed the outputs of the YM3812 DAC and I got sound from them, this meant the fault was in the analog circuit.Looking at schematics I followed the path, the sound was still present on the input of the volume potentiometer :

But then it was silent on the output which gives the signal to the Fujitsu MB3733 amplifier :

Schematics shows there is nothing between the output of the potentiometer and the input of the amplifier (apart from a 10uF electrolytic capacitor that was tested as good)

Metering the amplifier I found that pin 1 ( the input which takes the signal from the potentiometer) was almost shorted to pin 11 (+12V supply), there was only 12.4 Ohm of resistance :

The amplifier was likely bad so I removed it :

And replaced it putting also some thermal silicon grease for a better heat dissipation :

I powered the board up again and the sound was back.No other issue found hence I could declare this board 100% fixed and working.


 Posted by at 3:51 pm