Heathkit SB-10 Series Troubleshooting Manual page 93

On my particular HW-101, the actual measured carrier oscillator frequencies were
3393.8 kHz LSB, 3395.9 kHz USB, and 3395.17 kHz for CW. This resulted in a "tinny"
sounding audio response in LSB compared to USB, and a very "bassy" sounding USB.
The CW power output while using the SSB filter was 110 watts, but since the CW carrier
oscillator injection was so far from the CW filter center-frequency of 3395.4 kHz, the CW
power output was 50 watts while using the CW filter!
Heath's intended frequencies for the carrier oscillator were 3393.6 kHz LSB, 3396.6 kHz
USB, and 3395.4 kHz for CW. With the specified filter center-frequency of 3395.0 kHz
the USB & LSB carrier positions would be 1.6 kHz each side of the filter center
frequency.
Unfortunately, the filter center-frequency may not be exactly 3395.0 kHz, so simply
placing the carrier injection points equally-spaced from 3395 kHz may not have the
intended result. In order to determine the filter center-frequency one must balance the
audio response between both sidebands, measure the USB and LSB carrier
frequencies, and finally subtract ½ the difference between the USB & LSB frequencies
from the USB carrier frequency. The result will be the I.F. filter center-frequency as it
exists within your particular rig. For example, if the audio response is exactly the same
between USB/LSB, and the measured USB carrier frequency is 3396.31 kHz while the
LSB carrier frequency measures 3393.51 kHz, then the difference is 3396.31 - 3393.51
= 2.8 kHz. One-half the difference is 2.8 ÷ 2, or 1.4 kHz. Then the USB frequency of
3396.31 kHz - 1.4 kHz = 3394.91 kHz, which in this example is the actual SSB filter
center frequency.
I like audio with a tad bit more bottom end response, so I placed my carrier points just a
little closer to the filter center frequency than the Heath spec., i.e., rather than 1.6 kHz
off center I went with 1.4 kHz.
The frequency of the oscillator is lowered by placing a small amount of capacitance in
parallel with the crystal, and the frequency is raised by putting capacitance in series with
the crystal. To put a capacitor in series with the crystal simply cut one circuit board trace
just before the crystal pin as indicated, and solder the capacitor across the opened trace
(see Figure 15. Modulator Circuit Board Modifications on page 86).
A 100 pf capacitor in series will move the crystal frequency up about 100 Hz, but the
same frequency change in the downward direction would only require about 10 pf
connected in parallel with the crystal.
Note: Series capacitor values versus frequency shift: 100pf up about 100 Hz, 47pf up
about 330Hz, 33pf up about 450 Hz, 22pf up about 600 Hz. If the series capacitor is too
small, the oscillator may fail to start.
Modifications Page 93