398216 Usunięty wrote: adam313890 wrote: Well, unfortunately, I have to correct you.
But – why ‘unfortunately’? It’s great that you’re correcting me; the tape recorder is such an antique that I was bound to be mistaken, and your correction is very valuable. Thank you for that.
Thanks, I’m really very pleased that I can pass on a bit of knowledge
These are very old designs, and I’d call them ‘exemplary’. Why? Firstly, they were designed by engineers, not accountants; secondly, they have practically no bells and whistles, and their design is based on pure mechanics. Back in the day, when I was repairing a valve-based SJ 100, I was absolutely delighted with the device. Everything about it was spot on. And it was precisely this tape recorder that served as the knowledge base for building my reel-to-reel.
I’ll refer to this:
kris8888 wrote: 398216 Usunięty wrote: The only synchronous motors (or at least those which deserve this name due to their design) are motors with a permanent magnet rotor – such as those used, for example, in most low-speed AC motors
The rotor in a synchronous motor does not necessarily have to be a magnet; it can be made of special steel. This synchronous motor from the Melodia tape recorder was quite a curiosity, as the rotor was located on the outside of the stator.
Both of you are right here. But you’ve both forgotten about the concept of ‘salient-pole’. A synchronous motor with a starting cage operates like an asynchronous motor during start-up, and its operation is then synchronised without slip (s) by means of, in this case, the rotor’s salient poles. So what are these salient poles? Either permanent magnets (the tiny motors that drive the turntable in a microwave), or bars with a magnetic field excited by the field winding (all commutator motors), or, last but not least, the ones we’re interested in (namely the squirrel-cage synchronous motor) – a specially designed laminated core with localised magnetic field concentration – ‘protruding’ teeth (these are hidden between the laminations). No additional magnetic field is required there – whether artificial, from a winding or a magnet – as the physics of the magnetic armature itself does the job. Just as in a relay – we pull in the armature, so here we pull the protruding tooth into the rotating magnetic field, closing the gap; if it’s a fraction of a second too late, the rotation breaks down and we have a restart from the cage, but this only happens under heavy loads. That’s a very simplified explanation.
The motor from Melodia is the basic motor to consider when building your own tape recorder. I’ve used it myself and I’m pleased with it. It’s stable; it acts as a flywheel straight away, so there’s no need for belt drives or additional pulleys. It has two speeds. Controlling it is fairly straightforward. The tape speed is constant – so there’s no need for adjustment. It’s quiet and has minimal vibration. Drawbacks? The rotor is outside the stator – you need to encase it with an extra sheet of metal, as it might get something caught in it. I should mention that studio tape recorders, particularly the older ones, very often used motors of this type
All kinds of asynchronous motors can be used as rewind motors (platters): Studer, Revox, Akai. It’s worth taking a look at the STM mechlab schematics to see how the tensioning of the take-up reel is handled – an interesting capacitive solution. Other options include the Braun TG1000 or the ASC AS6002. It is much easier to build a three-motor tape recorder than a single-motor one. It may seem strange, but this eliminates mechanical complexities such as belts, friction wheels, cams and so on. You only need to look at the mechanics of, for example, the ARIA and the Teac 4010 and compare them.
