sb-301, 303 LMO problem

[Murray]

Hello:

Sounds to me like a thermally intermittent passive component in the vfo to
me.If you were careful and stayed away from the HOT parts like the tube, you
could apply a technician's troubleshooting freeze mist spray to each
component and observe the frequency drift characteristics...a good part will
exhibit a smooth change in frequency as the chilled component warms back up.
A thermally intermittent part or cold solder joint  will shift more abruptly
or erratically as it goes thru the temperature change from cold back to warm
(or operation wil cease upon application of the spray.)

The tubular part, if adjustable, might be the sideband shift inductor...I
think that was to shift between USB and LSB.

The main thing about not opening the secret voodoo fetiche LMO boxes (I was
under their spell, too as a young ham), that the people who carefully
aligned them didn't want you screwing up the linearity. There is a group of
tabbed washers stacked on the vfo shaft that catch on the next adjacent
washer in the stack as each one is rotated. Rotating from one extreme of
rotation to the other results in the entire stack of tabbed washers hooking
onto it's respective neighbor and rotating it's respective fraction of a
turn.

Loosening the set screws intentionally or accidentally thru vibration and
wear causes problems because the vfo capacitor is a 'butterfly' type with no
stop...there is a point at which it reaches a capacitance max. and a min. in
it's rotation. If the set screw moves from it's proper position, the
linearity and calibration of the vfo changes. With a digital readout
accessory, you won't suffer any practical problems...you'll be closer to
having an HW-101-like dial calibration. If you are 'way off' with the
capacitor shaft position, the frequency change direction can reverse at some
point near the end of the rotation limits.

Similarly, but probably not as critical, changing a fixed component, say,
one you determine to be thermally intermittent, will probably affect the
calibration and linearity, but in my opinion not as badly.

I have owned both SB-301 and SB-303 receivers. I really loved the
SB-301...much quieter and lower audio distortion than the '303, whic hI
found fatiguing to listen to . I could listen to the '301 for hours on end.
I worked some of my best 80 & 40 m DX with it, with my humble antennas at my
parents house.

Murray

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