MOTOR MAINTENANCE

3M M79 DC SERVO MOTORS 1480rpm / 24 volt /.45 amps

Refurbishing and preserving your motors is a must because finding an exact replacement is nearly impossible. They are easy to do and real messy so don't try it on your wife's new couch.  HINT: take pictures/videos as you disassemble the motor for proper orientation when it's time to reassemble.

STEP BY STEP 

Turn off machine.

Raise deck and disconnect Molex power connector.

Lower deck and remove four top screws holding down the motor plate.

Carefully lift the motor out of the deck.

Take off hub

Take apart motor

Mark your motor with white grease pencil for proper orientation when you are reassembling. Polarity is important on DC motors.  

The rear bell will have carbon brushes on springs (brush springs). Remove the brushes by unscrewing the two black plastic brush covers which expose the brushes and pressure springs. Inspect the brushes for excessive wear. They should be at least 1/4" long (my guess, not a fact). Place the brushes in plastic bag for protection.  I am looking for a good source for replacement of these.

Remove the two regular slotted screws on the top of the motor.

Clean with steel wool...

The motor with come apart in three pieces. The two end pieces are called "Bells" and the long center section is the "Frame". Carefully take off the Rear Bell first and then the Top Bell.

You will be left with the frame and the main motor sticking to the Field Magnets. 

Remove Shaft from the Frame.

Inspect motor for any damage.

WARNING: If you shaft is bent even slightly you have wasted your time because the flutter will render it useless. If you suspect it is not true, bring it to a good machine shop who can test with a laser. They may even be able to fix it???

REMOVING BEARINGS

Remove bearing from the shaft using bearing puller which can be had on Ebay.

REALITY CHECK #1:  Removing the rear bearing can be very difficult. I have destroyed two (bad) bearings trying do do this and will detail the correct way to solve this problem when I have mastered it. Be careful not to damage motor coils when removing the bearings. I  put masking tape over the coils.

BEARINGS

There is nothing good about bad bearings.  They cause Amplitude and Frequency Modulation, physically noisy and wear out other parts of the machine faster. Preloading the bearing is critical to getting the bearings right. Preloading is where pressure is evenly applied to the inside and outside races of the bearings to control the internal clearance. This removes resistance to make inner balls roll smoothly and space evenly.

Preloading on the reversing idler is done with a rubber O-ring in the bottom. On the Capstan motor the is a "washer sandwich" with a wavy spring washer as the meat in the middle. Dale Manquen worked for 3M Mincom in the mid 1960's as a key developer and is the best resource on the perils and pro's of bearing preloading and it effects on A.M. and bearing life. Use the above links to find his site and read all of his material (twice minimum). I am trying to find out exactly how each motor, idler and roller bearing should be optimally preloaded for best results. will continue to update this info.

Dents in the shields cause brinneling. Brinelling is where dents in the bearing case or balls cause measurable mechanical vibration distortion friction, heat and reduce the life of the bearing. The worst part of bad bearings is the list of problems that are associated with performance to the tape machine.

REMOVING PRELOAD WASHERS

BEARING WASHERS used for "Preloading"

Preloaded washers setup #2

Preload Final setup (top to bottom)

Shaft/Armature/Bearings

Commutator is the 1" long by 1/8"wide copper strips around the the shaft.  The brushes are spring pressured lead blocks that pass over the commentator to make electrical contact to the input leads. The Commutator needs to be resurfaced when it is charred or out of round.Very carefully sand the commuter with fine,fine sandpaper evenly. Never take off more than needed. The gaps between the Commutator are Mica and need to be undercut or hand slotted after the commutator has been sanded. I use a  knife and clean out the in-between spaces but I've read to use a hacksaw blade. The spaces should be as deep as they are wide. Used contact lubricant (the blue stuff) on the commentator when done. Spray all of the shaft parts clean as well.

DIRTY MOTOR   

Clean all the parts with Motor spray cleaner.

Spray motor case with degreaser/cleaner. The frame will have field magnets fixed to the sides which can be spray cleaned.                                                                           

BEWARE this stuff melts plastic so DO NOT SPRAY ON ANYTHING BUT METAL!

Reverse order to reassemble.

REALITY CHECK #2: Make sure you realign all of the parts in their original order by matching the white grease pencil marks.

REALITY CHECK #3: The two long screws are very difficult to align and can take much patience. I have no special tricks here but encourage you to preserve.