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I bought a used MTH Rail King F3 diesel with protosounds 1.0 (1999). It's got a new 8.4 V battery. Engine sounds, squeaking break sounds, passenger chatter, horn, bell all sound fine. But when I increase the voltage I begin to hear an electrical hum (noise) which increases in volume as I increase voltage. This is not coming from the engine, it's from the speaker. It happens whether the loco is moving or in neutral. At low enough voltage it's gone, but certainly annoying at higher voltages required to run the engine. This happens regardless of the volume I set. Any ideas? Is the board shot?
 

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Caps are likely dying on the bottom board and allowing AC hum into the amplifiers. Of course, it could also be a problem with the volume control, check the wiring and shoot some contact cleaner into the pot and run it back and forth a few times.
 
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