More on the LMO Wobblies

[Roger A. McCarty]

I have discovered a heretofore unmentioned source of the
wobblies.

On my SB300, I found the problem to be different than those
items mentioned in the archived posts. On this unit, the
problem source is the backlash gears.

The backlash gears are two sets of concentric gears meshed
together that are able to rotate a very few degrees . They
are positioned to enable the gears to rotate in opposite
directions, with pressure applied by a spring so as to
enable an opposing pressure of these two teeth, against the
third or drive gear. This arrangement is necessary for
without it, when changing tuning direction, you would feel
and hear a delay before the vfo actually began to track the
change i.e., you might turn the knob 1/32 to 1/16" before a
change was heard in the receive frequency.

On many older units I have refurbished, I have found these
gears to be "sluggish", due to gunk (tech term) accumulating
between these sandwiched gears, disallowing the full force
of the spring to track the change of tuning direction. In
this particular SB-300 LMO, the gears would jump or step
jitter as the spring attempted to close the gap. Sometimes
only when changing frequency but more often than not, while
stationary as the gears would suddenly close against the
drive gear.

The cure can be easy or a royal pain, depending on the
extent of the problem.

If the gears are slightly sluggish, a dose of penetrating
oil on the gear assembly, and then rotating the LMO back and
forth a few dozen times MAY solve the problem. I used a
battery powered screw driver with the chuck tightened on the
LMO shaft to accomplish this.

If the two gears are virtually stuck together, which I have
seen more than once on other Mfg. VFO assemblies, you will
need to disassemble the gear assembly, pry the two gears
apart (careful! Don't lose the spring), clean off the gunk,
and re-assemble.

I don't know about using a lubricant between the gears,
perhaps others will want to comment. It is my belief that
you should NOT use a lubricant, as it will attract dust and
grime (=gunk) again. I do not use a lubricant.

When it re-assembled it is imperative that you rotate the
gears against the spring two or three teeth before meshing
the assembly with the third drive gear. Otherwise you may
solve the wobbly problem, but you will have a terrible
back-lash problem.

FWIW

73

Roger KD6CC

Roger A. McCarty ARS KD6CC So. Calif.
http://www.qsl.net/kd6cc
http://www.qsl.net/kr6lp
Qrp-L #1555 KR6LP #1

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