Re: Fans

Stan or Patricia Griffiths <w7ni@...>

Michael Dunn wrote:

At 8:57 PM +1100 2000/9/28, Michael Dunn wrote:
Ah, I remember repacking many, many fan motors.... <g>
Tell us more please! The fans on our 549s don't work too well -
they start out seized, and as the scope warms up, they start to spin,
eventually reaching a decent speed...
Here is an excerpt from the "new" book I am writing. I have been working on
it for several years.



Not all fan motors found in Tek instruments can be easily rebuilt nor do
they need it. The fan motors typically found in the 530/540/550 scopes with
1/4 inch shafts are the ones that this procedure works for. The same basic
motor is also found in the 502 but it looks different because it has an
extra metal shield around it. Inside the shield, it looks the same as the

1. Inspect the motor wires for breaks in insulation. The insulation tends
to harden over time and flake off of the wires. You may want to replace the
wires at this point by splicing them where they enter the motor windings.
At least you are warned to be very careful with them if the insulation is

2. Remove the fan motor from the instrument and mark the ceramic strip
notches so you can return motor wires to the correct place. It has also
been suggested that cutting the motor wires just below the motor and
installing quick disconnect connectors will make the job of cleaning the
motor easier when it has to be done again in the future.

3. Disassemble the motor noting that the oil holes in the bearing pieces
point up and to the right side when you are facing the back of the
instrument. Also note that the wires exit the motor from the bottom and
toward the interior of the instrument. Make sketches so you can get it back
together correctly. There are several wrong ways to reassemble it.

4. Disassemble the motor by removing the bolts holding it together and
remove all of the old bearing washers including the fiber washers. Very
carefully remove the tight-fitting washer on the shaft near the fan blade
since you will need to reinstall this one later and I have not seen a source
for replacements.

5. Clean all of the old and solidified oil from around the shaft and the
area of the oil reservoirs in both bearing pieces. Mineral spirits work
well to dissolve the old oil and a rag soaked in mineral spirits works well
to wipe the windings and front and rear housings clean of dirt and old oil.
For especially bad cases, you might actually soak the end housings in
mineral spirits for a day, rinse well, blow out all the excess solvent, and
let them dry for a day before reassembly and re-oiling.

6. Put new oil in the reservoirs and place a small amount directly on the
shaft where it enters the bearing pieces. Do not overfill the reservoirs
but do saturate the fiber reservoir filler with oil. Use any medium weight
quality motor oil. Light mineral oil used to lubricate air conditioning and
heating system blowers works well since it does not change viscosity much as
the temperature changes.

7. Put nylon washers on both ends of the shaft such that the shaft has about
1/32 to 1/64 inch of end play when the motor is fully reassembled (and the
bolts are tight) and the rotating part of the motor is centered in the
field. You may need different washers on the front and back to achieve this
and you may not need all of the washers provided. (Tek used to sell a kit
to do this. Sorry, not available now.) This is a "trial and error" process
which may require assembly and disassembly of the motor several times to get
the correct end play. You can find a good assortment of 1/4" nylon washers
in various thicknesses at a well stocked hardware store.

8. Install the fiber washer that fits tightly on the shaft between the motor
and the fan blade to prevent oil from traveling down the shaft to the fan
blade where it will be thrown around the interior of the instrument.

Motors that tend to run slow almost always do not turn freely and this is
almost always due to the bearings being gummed up. Sometimes motors will
start slow and speed up after a few minutes. This indicates that a rebuild
is needed.

* * * * *
I have used this procedure hundreds of times and successfully fixed many
"slow" motors.


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