Treetop Circuits
Owner’s Manual for SB-600 Adapter
Version 2
6
Change to the 40 or 80 meter band, and repeat the procedure to determine the best settings for
LSB. Ideally, the best settings will correspond to the ones you already found, but will be on the
MINUS side. However, some asymmetry is to be expected because of crystal aging and similar
effects.
The SELECTIVITY settings discussed here are typically the most useful for SSB. You can, of
course, use other selectivity settings. In general, the narrower settings are best in a crowded
band, but wider settings produce more natural audio.
The blue trimming potentiometer (R11) on the board can be adjusted if desired to get the AM
and ssb audio levels to match. If you adjust it, make sure to tune in a strong ssb signal to make
sure that the audio section of the receiver does not overload.
Circuit Description
The SP-600 SSB adapter is installed under the chassis of the receiver, and wired into the
receiver circuitry. When the receiver BFO is turned off, the adapter is inactive, and AM
reception is unaffected. If the BFO is turned on, the adapter detects the presence of the BFO
signal and takes over the detection and AGC functions. Referring to Fig,3a , operation is as
follows:.
If the BFO signal is not present, as in AM operation, relay K1 is not operated and the original
receiver circuitry is restored via the normally-closed contacts of K1; R11 in the receiver is
replaced by a similar resistor R12 in the adapter, and C143 in the receiver is replaced by C4 in
the adapter.
If the BFO signal is turned on, the BFO signal is amplified and amplitude-limited by Q1, Q3, Q4
and associated components. The BFO signal appearing at the collector of Q3 is rectified by D12,
and the resulting DC voltage drives part of U2, which operates a Schmitt Trigger comparator
and operates relay K1, which connects the audio and AGC signals from the adapter to the
appropriate points in the receiver circuits.
The BFO signal appearing at the collector of Q4 is coupled by C1 to a resonant circuit consisting
of L1, C5, and C6. This circuit is configured so that the BFO signal on C6 is 180 degrees out of
phase with the signal on C5; the result is that D1-D4 conduct on one half-cycle and are cut off
on the next half-cycle. This drives the junction of R7 and C9 to ground on alternate half-cycles
