There is no right channel, because it is a mono cassette player/recorder. The output is for an earphone, with a stereo headphone connected to it you get only the left channel.
The Sony TCM cassette recorders are Mono, the TCS series are Stereo! BTW, if needed, you can find the service manual for the TCM-10 here: http://freeservicemanuals.info/en/
I just searched my external drives and I bought 1 of this TCM-10 from paragvaymazzy in ebay from Moscow, Russian Federation a long time ago. Sony Recording Walkman TCM-10 (WM-10 + Rec) Rare JDM only Tested, extreamly rare masterpiece of '80s ^ and that is all I know, as I have yet to find this again as I recently moved 20 years of stuff, and, well.