[MeerKat]

The Capstan Motor strip.

Before removing the motor I recorded a mains sine wave signal at 3 ¾ and 7 ½ ips. I compared these using Lissajous figures on my Heathkit IO-18U scope. This revealed that at 7 ½ the playback gives a steady but slowly changing phase relationship, at 3 ¾ there is some wow, the trace stays steady for about ½ second then advances a few degrees and steadies again, this repeats over about a 2 second cycle, notwithstanding this I cannot detect wow when listening to music recorded at this speed so it is not that bad. The figures also revealed the speeds are not an exact multiple of each other, which one or maybe both is is out I didn't fathom, I have requested a suitable tape in the 'Wants' section and if some kind person can make me one I will use that at some stage to correct matters.

I have carefully stripped the motor now and no real surprises here, It's an electrically/magnetically and acoustically well shielded permanent magnet DC motor with 3 coils on the armature connected to a 3 segment commutator, it runs in a pair of self aligning plain bearings. At the front end of the motor is a toothed wheel that with a corresponding assembly attached to the magnet provides the motor speed dependant frequency feedback required by the control board. The pulley is a slip fit on the shaft held in place with a smear of Loctite or similar. It had a quantity of black brush/commutator dust as I suspected it would. After a blow out and inspection it is apparent the internals are mostly in fair condition, no apparent wear to shaft/bearings. The exceptions are the brushes/commutator....the non-technical description 'shagged' comes to mind! See the photos. The com OD is 5mm on the unworn section and 4.75mm across the humps, now I'm estimating it will be down to about 4.5 mm in the valleys, how much is gone from the brushes is anybody's guess.

There is some good news though, the lack of spark erosion on the segment edges indicate not a lot of sparking has been going on. In the relaxed state there is about 1.5 mm between the brushes so they won't 'lift off' the com anytime soon. I estimate there is about ½ mm of brush left before the metal cups touch the com so there is some life left in the old dog yet!.

What to do? To refurbish a motor such as this you can skim the com down to get to rid of the grooves and fit a new set of brushes. Skimming this com is problematic – even if you could find somebody to do it, it would be easy to wreck it and as for finding brushes well they are probably unobtainium even on the planet Unobtainia. (Unless you know different, PM me if you do!). In view of this I have decided to not attempt any repairs to the com or present brushes, just carefully lube, reassemble and reinstall the motor.