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AC Synchronous Motor Failure in old Turntable

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mike11298

New Member
Hello,
I have a Collaro 3RC531 radiogram from the 1950s. This unit uses, I believe an AC synchronous motor (in conjunction with an idler wheel) to drive the turntable. It was working well, until I lent the unit (foolishly) to a friend. Apparently, he spun the table in the opposite direction to normal, and the player was accidently switched on whilst the deck was still rotating. As you can image, there would have been a sudden jerk as the motor tried to reverse its direction.
Trouble is, now it doesn't work very well at all. The motor sort of, vibrates (like if you connect a DC motor to AC). sometimes, when i start it spinning with my hand, it will spin for a second or so, but not much longer. Occasionally it starts itself, but not at the correct speed (the turntable runs too slowly).

In an attempt to fix the problem, I have dissembled and cleaned the motor, checked the continuity of the two coils (200 ohms per coil, 2 coils in series @ 240VAC) and I also found that neither the rotor or the coils or the housing or anything is magnetized. (By the way, supply voltage was checked- ok)

I'm not at all familiar with this type of motor (or problem) and researching on the net didn't seem to yield any useful results. Therefore, any suggestions or comments would be hugely appreciated.
Thanks!

 
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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Mmm! Synchonous motors usually have a capacitor, but I can't remember any of the older turntables having one. When the cap is bad, the motor will tend to hunt for a direction and have poor torque. Many reverseable synchonous motors have 3 leads. A common and a capacitor between two of the leads. Power is applied to common and one of the ends to make the motor move.

Check the motion of the capstan with the platter removed.

What would come to mind is that the rubber idler got deformed and you would end up with an erratic turntable and possibly one that stops.

I did find this: Antique radio / phonograph repair center and this: AMI 200 turntable too slow • PhonoLand.com
 

mike11298

New Member
Thanks for your reply, but remember that this turntable worked perfectly until just a few days ago, until my friend spun the table in the opposite direction and accidentally switched it on -- somehow, I believe this is what caused the issue.

There is no cap inside or connected to the motor, and no evidence that there ever was -- the motor has two wires that come out of it and go directly to the mains supply (no switches or controls in between). I have been examining the motor with the platter removed already, it is the motor itself that is not spinning properly, and the idler wheel appears to be in good condition.
The motor *tries* to turn in the right direction, and occasionally it does (but at an incorrect speed. The motor spins effortlessly by hand.

Like I explained in the OP, the best way I can describe it is the motor "vibrates" like a DC motor would if connected to AC supply.

Thanks for the suggestions, though.
Any other thoughts?
 

Pommie

Well-Known Member
Most Helpful Member
I would suspect the bearings (bushings). Can the rotor touch the housing when a sideways force is applied by the magnetic field? Try pushing the rotor in various directions and see if you perceive any rubbing/binding.

Mike.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I'm trying to picture what this particular motor looks like. Most just spin without any gears and the capstan moves up and down.

Any possibility that the switch has high resistance in it such that the motor is not getting full power?

Does the shaft spin freely with no power?

The only real issues that I have ever seen is dirt.

Can you post a pic of the motor so I know what type we're dealing with?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks for your reply, but remember that this turntable worked perfectly until just a few days ago, until my friend spun the table in the opposite direction and accidentally switched it on -- somehow, I believe this is what caused the issue.
That wouldn't damage the motor, and as you say there are no capacitors, and no magnetised parts to the motors - they either work fine (which they almost always do), or they don't. Most common problems are worn out bearings, but apart from that and the occasional O/C windings nothing else goes wrong.

I've no idea what your problem might be, I'd be inclined to try and find another deck to get a motor from.

Back in the day I repaired hundreds of record decks, and it was VERY, VERY rare to get motor problems.
 

mike11298

New Member
Thanks for everyone's replies, I decided to take apart the motor again and have a really close look. After Pommie's suggestion, I checked the bearings and cleaned them so they were free of absolutely everything.
The motor required adjustment of a number of screws on the bearings so they were 'just so' and I also added a fabric washer to bulk up an old, degraded fabric spacer. The motor would now spin after giving it a light 'twist' to get it started, and after a few hours of screw fiddling, I managed to get it just right, so that it would start itself.
It now works well.

Thanks everyone for the suggestions! The turntable sounds great again!

EDIT: And of course a little machine oil in there for good measure (as recommended by another forum)
 
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OlPhart

Member
As postmortem, maybe the motor start with the platter spinning reversed jerked the armature out of alignment? :) <<<)))
 

BrownOut

Banned
I'm amazed to hear a simple motor has so many repairable items. Guess they don't make 'em like they used to :)
 

second286

New Member
I agree with Nigel; very rare to have motor problems. The nice thing about these motors is the simplicity in their design. The majority of issues is most commonly bearing wear/alignment. You say the turntable was turned the opposite direction-I would first concider the mechanical section between the motor shaft to the turntable (perhaps somthing here has been slightly moved or bent?).
 

unclejed613

Well-Known Member
Most Helpful Member
if the shaft were bent it might vibrate some, or if one of the bushings were worn out of round the shaft might wobble in it. i used to keep a bunch of these motors (i'm guessing we're talking about the "shaded pole" type motor here...) and use parts from one to fix another. the sintered bushings had oil soaked into them and it was common for the old oil to dry out and harden into a glaze that would jam up the bushings. a common method of removing the glaze from the shaft and bushings is to soak the items in light penetrating oil, and burnish the bushings with a q-tip wrapped in steel wool, and clean the shaft with steel wool. the capstan is often heat fitted on to the shaft. if the capstan is heated the brass expands faster than the steel shaft (best if the shaft is kept cool) and slips off. the same process of heating the capstan allows you to slip it back on to the shaft and cooling tightens it on the shaft. this will be useful if you want to completely disassemble the motor. after cleaning and burnishing i find it best to allow the sintered bushings to soak up some light (5W15) teflon based motor oil before reassembling it.
 
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