Microcassette tape mechanism interfacing

P

Philip Pemberton

Guest
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.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
philpem@despammed.com (valid address)| ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
No to DRM, software patents and the EUCD! <http://www.ukcdr.org/>
.... Unable to load REALITY.SYS -- Invalid Parameter: /UTOPIA
 
Philip Pemberton <philpem@despammed.com> wrote:

Hi,
I've just salvaged a microcassette mechanism out of an old answering
machine.
[...]
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
Click to expand...
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.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
 
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

Philip Pemberton wrote in message <6a79e66c4d.philpem@dsl.pipex.com>...
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.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB,
6GB,
philpem@despammed.com (valid address)| ViewFinder, 10BaseT Ethernet,
2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
No to DRM, software patents and the EUCD! <http://www.ukcdr.org/
... Unable to load REALITY.SYS -- Invalid Parameter: /UTOPIA
Click to expand...
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top