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Comment by 2b3a51 | Hacker News Reader

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

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Could you explain more? I could ask an LLM but I find it friendlier to ask you, ha! If you don't mind of course.

What does a bias oscillator do that would help, and is the ips speed slow ie it's a low "resolution" (so to speak) analog recording?

Inches per second… the larger number is of course better, as higher frequencies require a higher tape speed to faithfully capture. Higher-end reel to reel recorders could do 7.5 i.p.s. and 15… to give you an idea what high end would look like.

Anyway, rather than call it "low resolution", loss of high-frequencies (low pass) is closer to the mark.

Was at work today so late in. To add to JKCalhoun's post, magnetic recording is pretty non-linear with a marked threshold. So a 'bias' signal strong enough to overcome the threshold was added to the current from the amplifier and passed through the recording head. That extra signal could be just a dc current as JKCalhoun mentioned, or a high frequency (mid tens of KHz, well above human hearing) current. See [1] for the details. I think that the 'dc rumble' from the magnetic grain is audible on the trailer recording in the OA, but it could just be noise from the bag the tape recorder was hidden in.

The microphone in the OA pictures looks pretty basic as well, and perhaps intended for voice recording mainly. So we have limited high frequencies from comparatively slow tape speed, rumble from the dc bias, and a microphone that perhaps isn't catching low frequencies.

[1] https://en.wikipedia.org/wiki/Tape_bias

Indeed, no hf oscillator, instead it looks like it uses DC bias via R12. It's also neat to see all the circuitry re-used identically for amplifying the tape in playback, or amplifying the microphone input to record to tape. Was that a standard and obvious plan for the engineers of the time?

I'm also curious if R22, the thermistor, would have been just for thermal protection, or if it was actually used to set the operating point for the output transistors in normal operation? Would it have switched on loud and angry before the transistors warmed up and the bias point cooled down, the opposite of a tube amp?

I am not familiar with this machine but I owned several other Japanese miniature tape recorders in the 1960s. These germanium transistors are very temperature sensitive, and the thermistor was almost certainly there to compensate for this and keep the bias point steady in normal operation as the temperature varied. And yes a single amplifier was used in all these machines.

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