AMI C turntable playing too fast....

[frankb]

I'm working on a friend's C mech (rebuild) and my strobe disc sez it's spinning 4% fast or about 82 rpm.  I have never encountered a AMI TT motor turning too fast!  He supplied me another TT motor and it is exactly the same: 82 rpm.  I also measured the speed with the stopwatch on my phone, got 82 revolutions in 60 secs.  I have a C 40 of my own.  I put the strobe disc on mine and it is almost dead nuts 78 rpm.  I measured the diameter of the drive spools on the end of the TT motor shafts with a micrometer;  mine and the 2 fast running motors all measure from 0.443 to 0.445".  I swapped turntable discs from his too mine, mine spins at 82 rpm on his mech, his spins at 78 rpm on mine.  He was thinking of trying a low value wire wound resistor in series wiring to the TT motor.  Something tells me that won't work on an AC motor.  Any ideas of what to do here?   Thanks in advance!

Frank B

[jukenorman]

Hi Frank,
Is it just an idler wheel between motor and turntable rim? In which case reducing the motor shaft diameter to 0.422" is about the only solution I think of.

Norman.

[frankb]

Yes Norman, I think you're right. I just don't know anyone who could turn it down on a lathe that precisely. I tried chucking it onto a drill press I have and carefully holding sharp flat file against it as carefully as I could. After several minutes of "filing" it, all I manage to do was take it down .001" and slightly round off the flatness of it. Hehe, that friend brought me 3rd TT motor today to try on it. That one turns the turntable backward! ...came off of an E mech with the belts I think he said.

[jukenorman]

Hi Frank,
I had overlooked the hardness of the native shaft. I can recall having to reduce shaft sizes to bring down the speed but thinking about it further - being in the UK and on 50Hz - it would have been adapters or springs that I would have been dealing with.

Norman.