Thank you all for the valuable pointers!
Great explanation of bias. Makes perfect sense.
- Playback does work on the machine. It plays pre-recorded tapes, and erases them with nothing when I try to record over.
- This is indeed a mono machine with 2 tracks per side with a selector switch. The track switch works well during playback, so that's probably not the issue.
The main switch for the record / playback function is a long sliding stick-like device on the PCB, that changes contacts at many points (see S1 - 1 to 9). I gave it a good deoxit, but that may not have been enough. Though, since the signal is present at R47 / C31 my understanding of this schematic (only partial understanding) suggests that S1 - 9 works. (doesn't mean the other contacts all do)
Continuity of the heads, I'm not sure. But now I'm not sure I want to check, I don't have a demagnetizer (aren't those devices often unnecessary and over-hyped? I know nothing, just what I heard)
So... if the audio goes through the bias secondary tap with the bias, and the audio makes it to the head, then I'm guessing the bias does too. Yet it won't record. Signal too weak, bias too weak, both, head not grounded at S1-2...
I will go check the bias on the scope, head grounding, and check the S1 switching.
Cheers!