This is an old revision of the document!
The following information is kind contribution of our forum members Lydux, BlueBMW, and RobIvy64.
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 you happen to hear the sound distorted or at a very low volume then first thing you can try is changing all capacitors.
- If that does not help, you could have 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 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 the solder mask. Please test all suspected traces for continuity.
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| MC3403 Op-Amp |
It's 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 test points at IC39 (pin in the right picture can become corroded and leave an open trace. Please check them for continuity.
If replacing the appropriate Op-Amp does not solve your problem then the following IC's could be faulty:
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| 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.
Also, please check for ICL# (pin 7) ! If stuck to a logic 0, it'll keep initializing both channels, so no analog output !
Our forum member Lydux has drawn schematics for fm audio pre-amplification part from an ACE I/O board : Model K5710DE
You could check DAC amplification (the part at the right of YM3012), and channels preamplifications. Relevant components : all ceramic precisions caps (C41, C28 and C29). These are rated at the lowest possible tolerance (B = ± 0.1 pF), this is really important.
Also, Q4 an Q5 may be removed for testing purposes (AND ONLY FOR TESTING !) with the use of AO connector. If the sound is correct then, these transistors may be dead, or the diode D7 could be shorted.