[radiotechnician]

If you own an audio oscillator, feed a 1 kHz signal
across the head, with no tape, on play, with a level
of 5 mV.

Then check the signal to noise ratio heard.

If loud and clear, then suspect the head.

You also could cross patch the head from your other working
vacuum tube BSR machine running a tape. This trick works both
ways for diagnostics. Needed is good test screened lead with
small alligator clips.


Years ago my firm sold an inexpensive BSR tape recorder, and
very many BSR record changers.