Run-up time for Thorens deck?

[John_BS]

Update.

This is a Thorens TD280 II which usues a two-phase stepper motor driven from a pair of "balanced" amplifiers. The schematic is attached.

I've so far

1. boiled the belt (no change)

2. Lubricated both the main bearing and the motor bearings (no change)
3. Run the deck from a 19V dc regulated supply (ditto)

4. Changed the main smoothing cap to 2,200uF (ditto)
5. Replaced C108 with 220uF (ditto)

DC voltages are correct, the two phases are 90 degrees apart, and the ac drive voltages are slightly low (3.7v, service data says 4v): but that's down to the zener tolerances?

The strange thing is that the current thro R22 indicates that one phase draws about 60% more current than the other under light load. I've measured the DC resistance of both windings (same) and also the inductances (same again). However, if the platter is loaded with a dragging finger, the currents both increase and come into balance. Could this be caused by a small mechanical offset in the rotors? (i.e. they're not precisely 90 degrees).

The turntable actually spends longer running up on 33rpm: in fact it often ends up "ticking" at just below synchronous speed, a condition which shows up in the current waveform as a lower-frequency noise-like higher-amplitude component. I do wonder if there might be a kind of resonance between the inertia of the motor rotor and the compliance of the belt? Once nudged into lock there's plenty of torque. R22 measures spot-on.

John