NFG Games + GameSX

The following information is kind contribution by our forum members Lydux, BlueBMW and RobIvy64. Thank you guys for another GameSX exclusive article !

For this repair you might want to consider purchasing a logic probe with audio and visual outputs which will make the troubleshooting more easy. You can go with Elenco LP-560 or the more advanced Elenco LP-900. Both of these logic probes are cheap and can be easily found on eBay.

• If the sound is coming out distorted or at a very low volume, the first thing you can try is replacing all capacitors.
• If that does not help, you could have a bad amplifier on Op-Amps (MC3403) or the dual channel DAC (YM3012). It could also be a power regulation problem on amplifier part. You can check this with a voltmeter on op-amp (pin 4 and 11). They should be +12v and -12v. If there is a PSU failure on -12v rail, it will cause distortions.
• Please note that the acid from leaked capacitors can eat up traces above and below the solder mask and traces which look fine can be corroded and broken below it. Please test all suspected traces for continuity.

MC3403 Op-Amp

It is a regular quad op-amp in SMD SO14 package.

• Original is a MC3403
• Compatible devices are TL074 and LM324.

There are 2 Op-Amps:

• IC39 - Responsible for FM synth (Top)
• IC7 - Responsible for DAC (Bottom)

The circled test points in the right picture can become corroded and broken. Please test them for continuity.

If there are no broken traces and replacing the appropriate Op-Amp does not solve your problem then the following IC's could be faulty:

YM2151 FM synth	YM3012 DAC

• There is a RESET pin on YM3012 (pin 7 = ICL#) which should stay to a 1 logic (High) on normal operation. If stuck to a logic 0, it will keep initializing both channels, so no analog output.
• You can also check DAC amplification (the part at the right of YM3012), and channels preamplifications. Relevant components are all ceramic precision capacitors (C41, C28 and C29). These are rated at the lowest possible tolerance (B = ± 0.1 pF), this is very important.
• Also, transistors Q4 an Q5 may be faulty, or the diode D7 could be shorted.

For further troubleshooting please use the following schematics which have been kindly contributed by Lidux and BlueBMW

Keep the scene alive !