Hi Chuck!!
I've fixed several of these over the years, and what I've found
is that the ground wiper on the capacitor gets dirty (see Bill Turners
response). I'll expand on what Bill said. These things tend to lose
their ground and if it's real bad, you'll get a "warble" when you tune
the radio across the band. If you partially disassemble the LMO, you
can get to the cap and clean and regrease it, and then you can get to
the adjustments inside to be able to re-linearize the thing. I've
gotten many of them to track almost as close as a Collins PTO, easily
less than 1/2 kHz off per 100 kHz. You'll find that TRW used low value
caps to pad the things and usually very little actual repair of them is
needed. FYI, you WON'T find any good info on the thing because Heath
didn't want you inside of it in the first place. Draw a schematic so
you have something to work from.
"Chuck Harris" wrote in message
...
wrote:
I just finished repairing an SB-301 receiver. It works well,
however,
I can’t reliably calibrate the dial. For example, if I zero the dial
at, say, 3.5 Mhz using the internal 100 Khz crystal oscillator, the
needle is about 4 Khz too high at 3.6, 3.7, 3.8 and 3.8 Mhz, but is
again zeroed at 4.0 Mhz. Is this normal behavior for linear master
oscillators (LMOs)?

NOTE: This LMO needed to be repaired. Two of its tabbed slip rings
on the tuning shaft were “glued” together (hardened grease) and its
capacitor plates moved beyond the fully meshed position on the low
end
of the tuning range. Anyhow, I removed and cleaned the slip rings
and
adjusted the rotation offset of the tuning capacitor. I’m now left
wondering if, out of necessity, I fouled up the LMO’s calibration.

This is a manageable shortcoming, as I can calibrate to the nearest
100 Khz mark. Nevertheless, I’m curious if such variability is
normal.

The LMO was spec'd to be +/- 2KHz from end to end. So, set it to be
dead on at the middle of the band, and it should be fine.

Try googling on Heath LMO recalibration... You should find some
instructions
on how to clean your LMO up, and recalibrate it for best linear
operation.

-Chuck Harris