The tape recorder's manual is available with circuit diagrams [1] and more pictures [2]. I'm not seeing an hf bias oscillator in the circuit unless the manufacturers are being clever. It was half track mono with two speeds, 3.5 ips and 1.875 ips.

Someone did a lot of work to restore these recordings!

[1] https://manualmachine.com/concertone/tr100/8544644-schematic...

[2] https://www.radiomuseum.org/r/transicord_tr_100.html