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Here is a basic look at them:
One thing to check is the phase of the audio waveforms applied to the stereo generator — they should be wired to be in phase with each other. If they are opposite, then the output of the L-R subchannel modulator can be zero instead of the normal value, and the output signal will be a form of mono, with pilot.
This could happen if a pair of wires was attached to oppposite sides of the audio path in one channel compared to the other. You could try reversing the termination points of the audio cable wires in/at either the L or the R connector plugged into the “bad” transmitter, to see what happens.