Asynchronous Motor troubles

Hello Guys

I have had two B&O model 1900 turntables I bought years ago 'resting' on my workshop shelves and decided a few weeks ago to clean them up and maybe dispose of them as I have several more turntables in use.

When I started work on them two weeks ago, I found both had seized motors due to solidified lubricant.

I repaired the first one's motor by carefully freeing the rotor, re-lubrication every other component according to literature I have and also replaced 1x 10 ohm .5W resistor and 1 x 10nf/265V capacitor and all functions and speed works correctly.

I then started working on the second 1900 and found one of the oilite bushes chipped and carefully machined the 1.2mm chip away on my lathe. I then turned a nylon spacer to take up the space left by the turned-away bit of oilite bush, ensuring the endplay of the rotor and its position inside the stator laminations was exactly similar to motor no 1.

The motor was carefully assembled and is completely free-spinning. Connecting 235Vac to it via the turntable ac lead, produces a faint hum and the motor runs at only about 35 to 45% of the rotational speed compared to motor no 1.

I have spent two weeks trying to find the cause of this problem and am totally at wits end.

As with the running motor of deck no 1, I also thoroughly cleaned all contacts inside the controlbox and also replaced the resistor and capacitor as with deck no 1.

If one connects ac to the motor without its rotor, one can feel a strong magnetic field in the cavity where the rotor goes.

A few days ago, I did some more testing and found one of the leadout coil wires shorted to the laminations and I thought I found the problem.

I carefully removed the coil together with its former and found that the joint between the (red) leadout coil wire had melted through the coil former so I repaired this problem as well by inserting a piece of very thin phenolic between the laminations and the coil, thus insulating the shorted coil.

The only other clue I can give is that the measured coil impedance differs between the two motors - the working motor's coils measure 332ohms for each coil and the faulty motor's coils measure 525ohms on each.

Having other B&O gear, I suppose I can ascribe this discrepancy to B&O's often spec changes between similar model series equipment.

The term 'asynchronous' appears in B&O literature for the 1900 turntables and the same literature is also giving similar specs for a model 3400 deck.

If one compare the two rotors, I can find no physical difference between the two.

I have already connected the faulty motor to the OK deck's ac supply but the result is similar - slow running and a slight buzz.

I am reluctant to remove the working motor's copper eddy brake disc and its pulley (as I have done with the faulty motor) and try the rotor in the working motor's stator as these parts look like onetime fits and will have to be glued back in position.

Can these rotors become faulty?

Much obliged for any recommendations or advice to solve this mystery.

bulgin
 

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