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Microcassette tape mechanism interfacing

Hi, I've just salvaged a microcassette mechanism out of an old answering machine. Reverse engineering the motor segment was easy enough - run the motor in reverse and the mechanism switches into a fast-rewind mode, run it the other way and the mechanism switches into record/playback mode.

I've also identified what appears to be an optoreflector on the controller board. It's situated under one of the tape drive pulleys, and has three leads. Two appear to be the LED anode and cathode, the third seems to be an output of some description. The sensor is SMD, in a clear case. I suspect all I need to do is feed it 20mA across the LED, then take the signal from the output pin, but I'm not sure if the output is open-emitter, open-collector or totem-pole (I suspect it'll be o/c).

The big problem is interfacing the magnetic head to something. I don't have the foggiest clue what I need to do to record on the tape. I know there's supposed to be an AC bias signal fed to the record head and the erase head, but I can't find any concrete info on how to determine the frequency and amplitude of that signal. The same applies to playback circuitry - no info on output levels or required amplification.

Does anyone have any schematics lying around that I could get copies of? Most of the stuff I've seen consists of one chip, the heads and a few passives. Something that's based on common ICs (standard opamps, transistors and passives) would be great.

Thanks.

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Phil.                                | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
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Philip Pemberton
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Record and playback amplifiers have equalization circuits. On playback the treble is turned down and on record it is turned up. Standard Eq is 120 microsecond or 70 microsecond for the high end and 3180 microsecond for the low end. Bias frequency is not criticle. It is usually 10 times or more higher than the higest audio frequency to be recorded. The voltage is usually quite high, 20 volts or more, but this really depends on the record head impedance. Higher bias frequencies require higher bias voltages to achieve the same bias current. The bias current value is determined by minimum recorded distortion. Unless you have the specs for the head and the tape type, you will have to measure this. Are you up for this? bg

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bg

[...]

For the level of speech quality in a T.A.M., it probably used DC bias: a small current through a resistor from the supply rail with the AC signal superimposed on it via a coupling capacitor. The results will be horrible and sound just like a telephone answering machine.

Don't ever do this to any high-quality tape head or you will need to demagnetise it afterwards.

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~ Adrian Tuddenham ~
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Adrian Tuddenham

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