Repair Log: Scramble (bootleg)

DarrenF

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A Scramble bootleg PCB labeled "Mu-1A". Has standard Konami classic pinout.

Starting point: Game plays fine, but no sound whatsoever.

First of all, there was a broken leg on the sound CPU. Swapped it out, but no change.

Next I checked (with audio probe and scope) from the AY-3-8910s thru the volume pot and all the way to the input of the amp, and audio IS present--but nothing from the speaker. Now, at this point, some might assume if there's input to the amp, but no output (i.e. no sound from speaker) that the amp must be bad. Luckily I happened to look at the amp's output on scope too... I didn't find the "flat line" I expected. I had to adjust the V-div from 2 to 5, and crank up the horizontal speed quite a bit (well beyond where you'd normally see audio frequency signals)... and I saw a waveform that as ~12V peak-to-peak, and very high frequency: the amp was oscillating.

Well, what causes amps to oscillate is usually a problem in the feedback. This particular amp (a M51516L), had a cap between pins 4 & 5 to filter the feedback. I checked the cap (a 47µF 25V) and it was clearly bad. It had a very high ESR (over 30 ohms) and very low capacitance (about 1µF). Replaced that cap, now everything sounds great.

Lessons learned:
1) just because you don't hear anything from the speaker doesn't mean there's no output from the amp. In this case, the output was rail-to-rail, but at a frequency outside of the speakers (and the human ear's) range.
2) when an amplifier is in oscillation, check the feedback circuitry.
 
Another good repair. Thanks for posting. Seems like there have been a few more repair logs showing up and hope to add some myself down the road.
 
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