I just tore apart many boxes of junk that I haven't seen for 20 years, but didn't find my Dictaphone or belts. Sorry. I think asking in the rec.antiques.radio+phono newsgroup might be more productive in finding someone with a working Dictaphone machine.
That explains the strange errors in apparent belt speed. I would have expected South Africa to use the same standards as UK (230V 50Hz).
If that's the situation, you may need to handle a fairly wide range of belt speeds. I suspect that it might be easier to setup a servo controlled motor, external belt tensioner, and stylus attached to a linear actuator. In effect, a belt recorder implimented mostly in software. That should give you control over speed and head position, which should also solve the track skipping problem. It would also allow you to run at some high speed (i.e. 15 ips) and slow down the playback in software.
Good luck. Sounds like an interesting project.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
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I have a working Dictablet machine (in addition to the one I have cannibalised to make the archival player). That is how I know the approximate speeds. What I do not know is the manufacturer's specified speed. For a number of reasons, the speed I have measured on my machine may not be the same as the specified speed.
What I really need is the manufacturer's written specification (which might be found in either an internal works document or a decent workshop manual). Somebody somewhere must have worked on those machines and will know what the speed was supposed to be.
It did, but they were using a machine which had been adapted to run at half speed on 60 c/s - and were running it on 50 c/s. The speed was 17% below half speed.
My experience suggests that it is better to allow the stylus to follow the grooves, rather than being rigidly fixed where the groove ought to be. That way, belts which wander across the faces of the rollers don't cause the stylus to skip. The leadscrew of the machine drives the playback head carriage, but the head swivels freely to adjust itself + or - 1/8" from the central position set by the carriage.
At present I am using the machine's existing motor with the two sets of stator coils driven in quadrature from a pair of high voltage amplifiers by signals from a 4-phase oscillator. As well as providing variable frequency, the system has to control the current through the motor windings so as to gives adequate mechanical torque but not saturate the iron and give rise to excessive hum harmonics in the playback head. The motor will work over a speed range from 20 to 70 c/s if the phasing and currents can be correctly maintained.
It may be possible to put a toothed wheel and magnetic sensor on the drive roller shaft, which would be used to control the motor drive frequency so as to compensate for slippage in the rubber idler and drive belt and the motor slip frequency. That way, the drive roller speed would be accurately controlled and the only discrepancies would come from the very slight slippage of the recorded belt on the roller.
Another way of specifying the speed would be to count the drive roller revolutions. Off-load, it seems to be spot-on 120 rpm, so perhaps that is how the manufacturers laid down their standard.
It is.
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~ Adrian Tuddenham ~
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