Nov 272016

Got this original Juno First (by Konami) for a repair :


Board played fine but some sounds were missing, the most noticeable was the laser shot:

Here is a MAME record for a comparison:

All the sounds are generated by the AY-3-8910 chip.Probing it revealed that three I/O (pin 19-20-21) were stuck high, these should exchange data with same number of I/0 lines of the Intel 8039 MCU like shown in schematics:


The 8039  was dead,  clock was missing on pin 2 and 3.I replaced the 8MHz quartz and the two 10pF capacitors with no change.This MCU has no internal ROM so it needs an external one :


Using a dummy ROM file in MAME reproduced exactly the issue, some sounds were missing like in my board.This lead me to think the MCU was bad so I removed it:


The 8039 is not widely used on arcade system but I was luck and found one on a Gyruss bootleg.Installed a socket and chip:


and all mssing sounds were back.End of job.

 Posted by at 10:47 pm

Truxton repair log

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Nov 062016

I got this Truxton PCB (export version of Tatsujin) in a recent operator raid:


Once powered it up board showed GFX issue, the sprites were blocky :

Besides, there was some kind of video interference, this is a common issue on this board and usually it’s due capacitors in the audio section, indeed I found a 470uF 16V one with bad capacitance :

100_9275 Back to the sprites issue, relevant data are stored in four 1Mbit 28 pin MASK ROMs :


I went to dump them and found a faulty one:


Replacing it with one from a donor board didn’t fix the issue therefore I went on in my troubleshooting.Each data bit of the sprites MASK ROMd is fed into inputs of four 8-bit 74LS166 register.I probed them with my logic comparator and led on output pin 13 lit for two of them:


I desoldered the ICs:


They both failed the out-out-circuit testing (obviously they were from Fujitsu) :


Sprites were fully restored :


The last issue I had to fix was that Player 1 Start input didn’t work.The inputs circuitry was previously reworked by someone (note the replaced 74LS240 and some patched traces)


I traced Player 1 Start back from pin 17 parts side of JAMMA connector to an end of a 220 Ohm resistor @R17 whose other end was tied to pin 13 of the 74SL240 @21K :


Signal on pin 17 of the JAMMA connector and on first end of the resistor was high and correctly toggling when pressing the button while was totally missing on pin 13 of the 74LS240.Therefore the only culprit had to be the resistor.Indeed I got no reading measuring it on circuit:


I removed  and tested the resistor out-of-circuit having confirm it was opened:



Job done.

 Posted by at 12:09 am

WWF Wrestlefest repair log

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Oct 312016

When switching ON the game, sometimes it had very fast tempo music.

It happened 1 one out of 10 but it was very annoying.

So I used my frequency counter when it happened and I noticed that the 3.58mhz quartz ran at about 10.58mhz!

I changed it with a know good one but again it happened very often.

I was sure it was something connected to the quartz oscillator circuit and given that the resistances are difficult to be faulty I decided to change the cap C47  (33pF) with known good one.

After testing about 30 cycles of switching on and off the game didn’t develop again the problem

Game fixed.



Liquid Kids repair log

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Oct 312016

This game had two problem:

  • Glitchy and noisy sounds
  • Blocky background images



Given the high unreliablity of Taito maskroms I immediately tested the ones marked in red which with no surprice some pins not connected internally




C49-04 restored the glitchy sound and C49-02 fixed the blocky gfx.

Replacements to be used are 27c400 (4mbit eproms with maskroms pinout)

Game 100% fixed


Toki repair log

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Oct 312016

Got this game for a repair which had all screen with washed out colours



This game has a SEIBU custom named UEC51 which is used as a DAC to output the RGB colours on the monitor.

The input pins were good, but a couple of output pins were fixed, thus creating the images as they had less shades of colours.

I took a good one from another Toki which had other faults and the problem was fixed 100%