af.2.4. Repairing and Recalibrating a Stuck LMO
By KI4IUA
The LMO on my SB-301 would only make about 2 ½ of the five turns before hitting a
stop.
Here’s how it's supposed to work: The TRW LMO contains a variable cap that will turn
continuously through 360 degrees without some external mechanism to stop it. It is
driven by a gearing system that allows one revolution of the variable cap for around 5
revolutions of the drive shaft which extends out the front of the LMO housing. On the
drive shaft there are six control washers whose purpose is to provide a physical stop at
either end of those 5 revolutions. Each washer, in turn, will allow part of one revolution
before a tab on that washer contacts a tab on the adjacent washer. The adjacent
washer then joins in the turning until its tab contacts the next washer and so on until
approximately 5 turns have occurred. The last washer's tab will contact a protruding pin
on the frame of the LMO and will stop the turning of the entire assembly at the extreme
end of the tuning. This same situation occurs when turning in the opposite direction.
The brass collar is adjusted to the proper stopping place in the capacitor travel since it
has a pin locking it to the first washer. This whole mechanism is preset at the factory so
normally you would not have to worry about it. On mine, the grease had hardened and
at least a couple of the washers would not slip, reducing the number of turns possible.
I maybe could have sprayed some solvent into the washers and freed them, however, I
wanted to clean out the old grease and start with some fresh so I discarded the solvent
idea and proceeded with the disassembly. With the washers stuck as they were, I could
not determine the exact end of the tuning so marking the shaft for reassembly wasn't an
option. I would have to re-determine the stopping points electronically and I will detail
that later.
With all the front panel controls removed and the panel itself removed, I took the dial off
the drive shaft of the LMO. It was held with one setscrew. If you don’t have a long hex
wrench to use I would suggest that you obtain one as you will need it when you later
perform the adjustment of the dial. This can be quite tedious. I found a really handy one
at my local ACE hardware. It’s called a Precision Screwdriver and you will want the
5/64” one, ACE part no. 2167310.
With the dial removed, the shaft and washers will be clearly visible at the front lower left
of the LMO housing. The collar holding the washers takes a 1/16” hex wrench. With the
collar removed, you can see the hole in the first washer and the corresponding pin in
the back of the collar that fits the hole. Remove the washers and give them a good
cleaning.
Heathkit Linear Master Oscillators (LMOs) Page 130
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