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Akai motor run capacitor

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HendoNZ:
Thanks for all the replies.
There is no belt for changing speed. This is a 3 motor deck with direct drive.
 
There is no tape speed problem - just the mentioned rewind and fast forward slow down. (This could also be a feature - but I don't have another one to compare with)

The back tension seems to be achieved by running the supply motor in the opposite direction to the take up reel. (There no adjustments for this. )

The original pinch roller had some small dents on it. I replaced it with one in good condition.

I'll have a look at the suggestions before replacing the motor caps.

Circlotron:

--- Quote from: HendoNZ on July 05, 2021, 09:54:14 pm ---There is no belt for changing speed. This is a 3 motor deck with direct drive.

--- End quote ---
Way back in 1974 a friend of mine had an Akai 4000DS. I seem to remember the capstan shaft had a sleeve you could put over it to increase the diameter for 50Hz usage. Or maybe it was also for changing the tape speed? Can't remember. Is yours like that?

amyk:

--- Quote from: floobydust on July 04, 2021, 07:17:28 pm ---Ouch that sounds terrible  :o
With no cap I'd expect little or no torque and you wouldn't run it for a long period. I wonder if running on 60Hz but tuned for 50Hz would have less smoke.
It seems to be a synchronous (capstan) motor with the servo giving it variable AC voltage for speed control.

If I modify vintage gear, change or delete a feature, I try make it obvious. It's just so a tech or someone that goes in afterwards can find it's a mod and not a component fail. At least a label on the backside etc.

--- End quote ---
It looks like a permanent-split-capacitor (PSC) motor, in which case no cap = no start. Low power ones may be sufficiently impedance-protected to not burn up when stalled, but I wouldn't bet on it. The capacitor's value should be chosen to cause an ideal 90 degree phase shift in the magnetic field.

bdunham7:

--- Quote from: HendoNZ on July 05, 2021, 09:54:14 pm ---There is no belt for changing speed. This is a 3 motor deck with direct drive.

--- End quote ---

Hmmm.  It appears to be an AC synchronous motor, the capacitor values would not actually change the speed of the motor--just the torque characteristics. 

Circlotron:

--- Quote from: amyk on July 06, 2021, 01:34:56 am ---The capacitor's value should be chosen to cause an ideal 90 degree phase shift in the magnetic field.

--- End quote ---
The phase shift would vary with load, so measure it under actual operating conditions, and preferably at full operating temperature.

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