By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...
Wednesday, June 7, 2017
Beogram 4000: Restoration and Adjustment of AC Platter Motor
After rebuilding the electronics of the Beogram 4000 that I am restoring right now it was time to look into the AC platter motor. This particular Beogram 4000 has the old style pulley that does not have belt guards integrated, i.e. they added guards on the posts that hold the motor:
These guards are not really needed since the pulley is slightly crested which securely holds the belt on the pulley. The vertical position of the belt can be adjusted via the tilt of the motor.
Similar to the DC motors of the later Beogram 4002 and 4004, the AC motors also develop dry bearings. This motor made this clear via a noticeable 'knocking' noise when driving the platter that could not be adjusted away via the motor voltage trimmer or motor tilt (without platter some 'cogging' can be normal, since the motor is 'bored' without platter, hence the phase lag for smooth operation between rotor position and AC voltage is too small) . So I took the motor out
and disassembled it:
Disassembly requires drilling out the rivets that hold this motor together:
The standoffs (left most parts in the above picture) need to be reused once the motor is reassembled, i.e drilling should be done carefully in small steps to limit damage to the parts. Since they also served as rivets, they needed to be drilled out with a 3 mm bit to allow 25 mm M3 screws all the way through.
Anyway, the bearings cannot be extracted without doing damage, so I usually put the entire motor housing in the oil and under vacuum:
After 24 hrs I reassembled the motor using 3D printed 'nut arresters' that allow the replacement of the rivets with M3 bolts and nuts:
the purpose of these parts is to hold the nuts in place under the motor that the tilt screws can be adjusted when the motor is installed. This is how the parts fit on the motor:
After re-installing and adjusting the vertical position and tilt of the motor, I adjusted the motor voltage. Per manual this is to be done at 33 RPM that the sine wave measured at the connection point between motor coupling and phase capacitors is maximized without showing distortion:
This motor could be adjusted to 6.5V RMS as specified in the manual. At 45 RPM is still had 6.0V:
There is no way to adjust 45 separately, i.e. one has to live with whatever 45 shows when 33 is properly adjusted. But 6.0V is plenty to drive the platter vigorously. This motor seems to be back in business!