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Nagra repairs

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I'm about to tear a Nagra III apart to replace a failed motor (broken commutator).  I wonder if anyone here has attempted such a replacement, and would be interested to know how that turned out. I know that the Nagra III Service manual has dire warnings about disconnecting the motor shaft from the encoder wheel.  No doubt Kudelski's ghost will haunt me, but I cannot see that this would be impossible.  I will of course be replacing belts and clutch felts as well, but the motor encoder system is my current main concern.

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I've had 4.2 motors apart but never a 111

 

I wonder if the warning is related to putting a magnetic shunt over the rotor??

 

Contact Kudelski if no local responses occur

 

mike

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No doubt there was concern about the "shorting bar" being correctly applied, but I see no way to do that without detaching the motor from the mounting plate and encoding wheel, which I suspect you would have had to do on the 4.2 as well.  I assume you were successful in removing and replacing the 4.2 motor, and checking out the motor speed afterward.  Did you have to r/r the encoder wheel?

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The one person that has ever worked on the III recorders I own besides myself is unfortunately no longer with us. He did do a motor repair once for me (bearings), but I sadly don't know the details. I do know you need the magnetic shunt. The only other issue I can think of is that if you remove the encoder wheel, it will probably need to be re-balanced. Have no idea how that would be done. My guess is the factory had a special test bed to do dynamic balancing.

Might want to let an expert attempt this one.


Sent from my iPhone using Tapatalk

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I've heard tales of in the field repairs from Blake Wilcox at Wilcox Sound. I don't know of his level of expertise or if he even remembers what exactly he did, but it may be worth a call over there to see if you can pick his brain. 

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No doubt there was concern about the "shorting bar" being correctly applied, but I see no way to do that without detaching the motor from the mounting plate and encoding wheel, which I suspect you would have had to do on the 4.2 as well.  I assume you were successful in removing and replacing the 4.2 motor, and checking out the motor speed afterward.  Did you have to r/r the encoder wheel?

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Thanks Jon, Clay and Scott.  Based on a re-reading of the 1967 version of the Instruction Booklet for my 1966 Nagra III, my fears were overblown.  It seems that the heart of  the rotor portion is a large permanent magnet "fixed in the assembly by a left hand threaded set screw. The winding surrounds it.  To remove the rotor, after first having unscrewed the center screw (normal right hand thread), it is necessary to introduce a rod 3mm in diameter into the hole in the rotor between two wires."  The manual goes on to describe how the magnet can be unscrewed and the entire rotor removed from the machine into a "keeper" tube of 40 mm in ID and a minimum of 60 mm in OD. 

So the rotor assembly detaches from the shaft protruding from the motor side of the encoding wheel (which is described as a "phonic" wheel).  So the phonic wheel and its shaft stay connected to the capstan and the clearance to the related read head are maintained.  What one needs to do the repair I need is a NOS motor, which I may be able to get, and a keeper tube which I can have made locally.  While the motor outer shell and brush assembly are disconnected, I can pull the mounting plate to replace the belts and clutch felts (assuming I can find some felts).  So I appreciate your and Clay's comments and will be touching base with the various sources that have been suggested.  A mixer friend has an alignment tape and a couple of scopes and signal generators as well as a known good 1,000 Hz tape and a cycle counter if we need to dig further into the innards to correct timing beyond what the flutter wheels indicate. 

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Yep from my memory the the 4.2 phonic wheel was on the same shaft so no balancing issues.

 

Sound like to are well prepared to take a big breath and do it, and you'll feel good too!

 

Cheers

 

mike

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