Decapod wiring problems

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Hi:  my Bachmann Russian Decapod had some problems with the sound.

After disasembled the tender I removed the screws retaining the decoder and I found a loose red wire

This wire runs from a little PC board to  the other one below the PCB. The wire was unsoldered and I can't identify wich one is the point to solder it.

Can you help me with some explanation or  wiring diagram??


Thanks