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#17
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Dictabelt speed
Jeff Liebermann wrote:
On Fri, 30 Sep 2011 09:10:01 +0100, lid (Adrian Tuddenham) wrote: Any Dictabelt model 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. 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. I am constructing a playback machine for archival work, which needs to be able to cope with any Dictabelt. There was a standard speed which applied to all the normal models and all belts, so as to allow interchangeability of the belts world-wide. The basic machine was designed for 60 c/s mains, but there were adaptations to allow the machines to run at the correct belt speeds on 50 c/s. Some special machines were also designed to run at half speed for logging purposes . I have already encountered some belts which were recorded on unadapted half-speed 60 c/s machines working from 50 c/s supplies - and, no doubt, there will be others which were recorded at different non-standard speeds for various reasons. That explains the strange errors in apparent belt speed. I would have expected South Africa to use the same standards as UK (230V 50Hz). 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. What I need to know is the manufacturer's specification for the standard belt speed which was supposed to allow interchangeability between all the machines. I can then calibrate the speed control on my machine to show the %age speed variation from the norm, so as to deduce information about the circumstances under which each belt was recorded. 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. 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. Good luck. Sounds like an interesting project. It is. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
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