af.2.2. Heath Tube-Type LMO Repair
by Glen E. Zook, K9STH Copyright 2012 by author
The tube-type Linear Master Oscillators (“LMO”) used in the Heathkit SB-Line
equipment is a very stable and accurate means of controlling the frequency in the
equipment. Unfortunately, as the units age, many LMOs develop a “warble” when
tuning. This “warble” usually stops when the frequency control knob is not rotated.
However, accurately “zero-beating” another station is made harder by this “warbling”
and it is generally a major distraction from the otherwise excellent performance of the
SB-Line equipment.
The primary cause of this “warble” is a break-down of the original lubricant used on the
pivot points of the shaft of the main tuning variable capacitor. Removing this unwanted
sound is not hard.
Next, one of the “L” shaped side panels has to be removed. It is best to remove the
panel on the same side as the oscillator tube. There are no internal parts connected to
this side whereas there are components attached to the other side. To remove the
panel, there are a total of 10 hex-head sheet metal screws that have to removed. The
bottom 2 sheet metal screws also hold the lugs which attach the LMO to the chassis. A
1/4 inch “nut driver” is the tool to use to remove the sheet metal screws.
It will be noted that 2 or 3 of these sheet metal screws are covered with a red lacquer.
The purpose of this lacquer was to act as a seal which would be broken if the covers to
the LMO were removed and thus the factory warranty on the LMO would be violated. Of
course, the warranty has long run out, considering that the LMOs are 40, or more, years
old!
After removing the panel, make sure and do not disturb the small variable capacitor
inside the unit. Look closely at the rotor of the main variable capacitor. Depending on
the actual individual who assembled the LMO, there will be varying amounts of dried
lubricant (“grease”) showing. This has to be removed. I generally use WD-40 and a
toothbrush to eliminate this old lubricant.
Then, check the “stops” on the main tuning shaft. These rotate as the dial is tuned with
each stopping at one full turn of the shaft after the last one stopped. Over the years,
these “stops” start sticking to each other and the shaft may not be able to be turned
through the slightly over 5 revolutions to tune the 500 kHz frequency range (with a few
kHz over at each end). Spray WD-40 on these and make sure that the shaft easily turns
the 5+ revolutions. Watch the main tuning capacitor to make sure that it mostly opens
Heathkit Linear Master Oscillators (LMOs) Page 124
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