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Here are some comments from Peter Spies. They may apply to your player. If you do modify one and 'cook' the board then I have plenty spare. The sound processing program Audacity has a feature to invert the phase of an audio signal and Gary Millard's video2nbtv also has a video invert option. If your NBTV source is battery operated you can simply swap the signal leads over to get the correct phasing. As with both NBTV and SSTV gear when using plug in wall mounted power supplies for display and source you expect that as the earth pin is normally plastic there shouldn't be any electrical connection path between supplies although you can get some interesting results especially if you are using an earthed 'scope.
1. the coupling capacitor from pin 14 (DAC-L) of the GD3200B chip to pin 5 (DAC-L) of the module was only 0.1uF.
As a consequence the cut-off frequency was much too high for NBTV purposes.
I removed it and replaced it with a short, then used a sufficiently large (47-100 uF) external coupling capacitor.
(I believe the capacitor was 10uF in the older, 'original' versions of the module).
A coupling capacitor is needed, as the DAC-L signal from the chip has a considerable DC offset.
2. I removed the 4k7 resistor coupling the DAC-L signal to the (input capacitor of) the 8002 amplifier chip.
The integrated amplifier now only amplifies only the DAC-R (=NBTV audio) signal. It will drive a speaker.
The need for an additional, external audio amplifier chip was thus eliminated.
(I believe these L- and R- channel coupling resistors were 22k in older versions of the module, and presumably not mounted in the same place)