Originally posted in January, 2017

I was recently fortunate enough to acquire a Tektronix 556 Dual Beam Oscilloscope with many accessories and in good condition.  Unfortunately, the cooling fan on the scope was very loud and more concerning, it emitted an alarming metal on metal contact sound.  After verifying basic operation of the oscilloscope, I decided to repair the cooling fan.  This process is documented in the images and commentary that follow.

Removing the Noisy Fan Motor

556 Cooling Module with filter and mesh screen removed.  The fan is connected to the chassis wiring at the ceramic terminal which is circled:

Inside of cooling module and bottom side of motor (fan blade removed):

The back side of motor (fan blade removed).  This is a 4 pole 1725 rpm (loaded) motor.  It moves a very large volume of air.

An  inspection of the cooling module showed that:

• The fan blades were mounted “backwards” reducing their efficiency
• The fan blades were not hitting any of the heatsinks or wiring
• The “metal on metal” noise was coming from the motor itself
• There were indications on the motor itself that it had been taken apart before

That covered the overview and mounting of the fan motor. After desoldering the wires connecting the motor to the chassis wiring, I removed the fan and mount (blue frame attaching the motor to the rest of the cooling module) as a unit.

The next few images show the disassembled parts of the motor:

Wear on rotor shaft

At this point I found several issues by visual inspection:

• The original rear bushing had been replaced, probably due to excessive internal wear.
• The replacement bushing itself had excessive internal wear and it did not fit in the retainer properly due to its shape.
• The replacement bushing had been drilled off-center
• The bushing felts were stiff (almost solid) and dirty
• There was a large amount of solidified grease and dirt on the shaft and bushings
• The front bushing was in good condition

Original bushing is on the left, the replacement bushing is on the right.

Measurement of the rotor shaft showed that the wear was about 0.001″ undersized. This was not excessive, so I reused the shaft.

I cleaned all the parts (felts, end bells, shaft, rotor, spacers)  and used mixture of Teflon lubricant and non-detergent oil to lubricate the felts and bushings.  I obtained  another fan motor with one bad bushing and one good one. Since the bushings are interchangeable, I simply replaced the defective bushing with the good one.

I also statically balanced the fan blade using jig originally intended to balance drone motor blades.

I re-assembled the motor and reinstalled it into the scope.  The result was that the fan motor ran quieter than before, with little vibration and no rattling.  This fan will never be whisper quiet as it moves a very large volume of air through the scope.