Akai motor run capacitor

[HendoNZ]

Hi all,

I'm in the process of restoring a Akai reel to reel. I'm looking to replace the motor run caps in this unit. I just wanted to check to see if I'm correct in thinking that for the 60/50hz switch - the capacitance value on 50hz is 5uf (3.5+1.5 ). Am I wrong? See attached schematic

[bdunham7]

No, it is apparently a multi-section capacitor (2 in one package).  You can replace it with two capacitors if you like.

[floobydust]

What model number? To me it looks like a dual section cap, using 5uF for 50Hz, 3.5uF for 60Hz. But the same motor SCM2-24KJ in GX630DB is 5uF for 50Hz, 1.5uF for 60Hz so one of the schematics must have a typo or a different circuit?
A modern film cap would be best, instead of (non-polar) electrolytic. Like CBB61 or CBB60 motor run cap used in small appliances.

[HendoNZ]

Thanks for the replies.

Yeah, it's a dual section cap 3.5uf and 1.5 uf in a single can.
I have two nichicon eec film capacitors. One is 3uf and the other 2uf. My plan was to use the 3uf to take the place of the  3.5uf and the 2uf for the 1.5uf position.
In the end it equals 5uf, and I'm in a 50hz mains country.

So my question was. This would be fine, and I was reading the schematic correctly?

My deck is an Akai gx-265d

Cheers

[andy3055]

Your reasoning was correct.

[floobydust]

That will work, or just wire both caps to the 50Hz switch position and leave 60Hz not connected, in case the unit ever gets used on 60Hz, someone can figure out what is going on.
Based on math, 60Hz 3.5uF equals 5uF at 50Hz by assuming the motor's inductance is constant.

[HendoNZ]

Thanks everyone for the replies 👍

[Circlotron]

or just wire both caps to the 50Hz switch position and leave 60Hz not connected, in case the unit ever gets used on 60Hz, someone can figure out what is going on.

The smoke from the stalled motor because no caps were connected in the 60Hz position should be a good starting point.

[floobydust]

Ouch that sounds terrible 
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.

[HendoNZ]

So I shouldn't replace the old can with two similar size (uF) caps? If it ever got switched to 60hz then there would be a 3uf cap there instead of the 3.5uf. I would have thought that all it'll do is run slower.

[bdunham7]

So I shouldn't replace the old can with two similar size (uF) caps? If it ever got switched to 60hz then there would be a 3uf cap there instead of the 3.5uf. I would have thought that all it'll do is run slower.

If you use the 3uF and 2uF caps wired just as the original, it will work perfectly at 50Hz and might have slightly less starting torque on the 60Hz setting.  I think this will be fine.  What parts of the deck does this motor drive?

[HendoNZ]

It drives the capstan - I'm replacing a 4uf X 2 cap that drives the reel tables (one for each motor) to try to eliminate a back tension problem.

I reckon the reel tables capacitor is losing torque and not providing enough back tension to the tape causing the left channel to drop out. (How I came to this conclusion - when the channel drops out; if I put some drag on the supply reel the channel comes back. Also the find and fast forward start fast then get progressively slower)

And so if I was going to change the reel cap, I might as well change the capstan cap.

A bit more info then was asked for.

[bdunham7]

It drives the capstan

Is there a belt that is repositioned when you change from 50 to 60Hz?

[andy3055]

That left channel drop out may be because the pinch roller rubber has become hard. In a deck that is this old, that will be the most common issue. When the rubber becomes hard, the capstan will not be able to push the tape fast enough. That will be also contributing to a lack of apparent speed. It is easier to suspect the motor caps but they last much longer than the rubber pinch roller that gets hard with time. You cannot use any sort of rubber rejuvenater as that will get on the tape and ruin it. Best solution is to replace it. Check all the tape guides as well and make sure they are clean.

If you replace the caps with anything other than what is specified in the circuit, you are bound to have speed variations/problems which will be hard to figure out and correct.

One more thing to check would be the felt pad (if they ere there) clutch mechanisms on the feed in and take up reels

[floobydust]

You can look at the tape travel, it's not supposed to move up or down with different back-tension unless something is crooked or out of alignment. One channel should not just get quieter or cut out completely.

Note different resistors R2+diode D1, R4 for 240V as well, this affects the motor braking and I haven't figured out how they are doing back-tension here.
The GX-630D has a 50/60Hz switch for the same 24XO-TD reel motors, for 3uF or 4uF.

Careful the motor portion can be hazardous live if the mains cord is flipped.

[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]

There is no belt for changing speed. This is a 3 motor deck with direct drive.

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]

Ouch that sounds terrible 
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.

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]

There is no belt for changing speed. This is a 3 motor deck with direct drive.

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]

The capacitor's value should be chosen to cause an ideal 90 degree phase shift in the magnetic field.

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

[floobydust]

For tension, it looks like the reel motors are run in series with reduced voltage 100VAC & R2. The lower torque reel gets reverse and less current by resistor R4 shunting that motor.
So there needs to be somewhat of a balance between the reel motor circuits (run caps) for it to work.

The servo controls capstan motor speed by varying the AC voltage to the motor, not as a synchronous motor like in turntables.

[bdunham7]

The servo controls capstan motor speed by varying the AC voltage to the motor, not as a synchronous motor like in turntables.

OK, I see it now.  I haven't run into that setup before.  I'm guessing that at 60Hz, 3uF vs 3.5 still isn't going to matter too much.  And the capstan flywheel must be huge...