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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.

-Dave Drumheller, K3WQ

LMOs (or PTOs) are never perfect and a +/- 1kHz error, or even a little more, is
normal.

+/- 2kHz, as in your case, is probably on the high end.

But hard to tell how you can improve linearity any further.

Obviously it would be better to tune the oscillator so that it is at -2kHz on
3.5 and 4 MHz, and at +2KHz on 3.6, 3.7 and 3.8 MHz. Or even better at -3kHz on
3.5 and 4 MHz, and at +1KHz on 3.6, 3.7 and 3.8 MHz, so that you are closer to
true frequency on the major part of the scale without recalibration.

73

Tony I0JX