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Has This Kind of Tonearm Been Made?

I was looking at my son’s 3D printers and observing the gantry that carries the print head. Microprocessor controlled, very precisely, with a stepper motor and a toothed belt. A wire loom made like a bicycle chain curls up to hold wiring out of the way as the head moves back and forth.

So I started to think: what if such a gantry was mounted across the centre of a turntable? It could be hinged where it attached to the far side of the turntable, and on the playing side it could be located by dowel pins in holes as it is lowered. A catch could click into place and hold it at precisely the right height over the turntable. The cartridge can be mounted to the traveller on the underside of the gantry, such that its stylus is exactly over a line passing through the record spindle, and it will be at a perfect tangent all across the record, and without the play associated with the stubby tonearm of a linear tracker. How to adjust VTF? I suppose the two cartridge screws could be made such that by twisting both, the cartridge lowers or rises, placing more or less pressure on the cantilever (and a gauge can sit in place of the disk to set this). Twisting only one of those screws could adjust azimuth. Or in a more expensive version a strain gauge could be incorporated and allow adjustment to be quantified that way. I expect an Arduino could be programmed to control the movement across the record at the correct rate for outer to inner grooves.

To play you lift the gantry, hinging it upwards to a near vertical position. A cam and sprung follower can easily hold it elevated. A record is placed on the table and rotation started. The gantry is lowered and it clicks into place as the stylus rests on the disk. At the end of the side either the turntable motor is switched off by the microprocessor controlling motion of the cartridge carriage, or the catch holding the gantry down can release allowing the gantry to spring up slightly, just enough to lift the stylus off the record. Perhaps instead of a mechanical catch it could be electromagnetic and the current is cut as the stylus enters the deadwax, again allowing the gantry to lift a little.

I imagine someone has thought of this before, but I haven’t seen one. Has it been done, or shall we patent it?!

dogberry
lewm
13,066 posts
 

The Goldmund T8F (I think that is the correct alphanumeric designation) was a Rabco copy but perhaps with better execution.  Also, there are a bunch of hobbyist fans of the Rabco, some of whom have made some improvements to the original design.  Modern switches and quieter modern motors could make it much better.

larryi
3,994 posts
 

There is also the Revox B 790 turntable.  

curlyhifi
157 posts
 

Good Eye dogberry !

dogberry OP
656 posts
 

It strikes me that this is the kind of thing an obsessed hobbyist could make for himself. The only difficulty being getting the gantry rigid - a triangular section box structure might be best, and to make the hinge have no play in it. Possibly even cannibalize a 3D printer or a laser cutter using the same mechanism. Much to the horror of everyone I suspect a lot of it could even be made on a 3D printer to prove the concept, even if plastic isn't the ideal for the best longevity.

So, next question, is there a good reason why this design hasn't caught on? I imagine it is the lack of forgiveness in the motor driving the cartridge traveller along the gantry. As I understand it, a linear tracker has a little freedom in it's little tonearm to move left and right to compensate for imperfections in the arm drive (but in so doing, it loses some of its vaunted tracking accuracy). If there were a motor driving the cartridge, it would not only have to adjust the rate of travel as it crossed, it would have to detect and speed up for the spaces between tracks. That means an optical sensor riding just inboard of the cartridge mounting.

lewm
13,066 posts
 

The original Rabco actually makes tiny arcs as it moves across the LP surface. That was necessary to activate the micro switch that triggered the motor to move the pivot toward the spindle. So it isn’t a pure linear tracker. Also, the motor could be a source of noise. With modern parts, the idea might work better. Rabinow, the inventor, lived right here in Bethesda, MD. While also, Herb Papier, designer and builder of the Triplanar, lived a few miles away in Wheaton, MD. I’m sure they knew each other.

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