ac.10. Using an External VFO for Split Frequency Operation
With the SB-series split frequency operation is very easy, because there is a front panel
switch that selects either the internal VFO or an internal crystal oscillator to control the
transmit frequency. The following modification essentially duplicates the changes made
to accommodate the SB-640 External LMO. However, one is not limited to just use of
the SB-640, but rather could use any oscillator providing the necessary signal level over
the 5.0 MHz to 5.5 MHz range.
To perform this modification:
1. Remove the miniature RG-174 coax cable from the crystal oscillator as shown in the figure
and replace it with an RG-174 cable having enough length to reach an unused back panel
RCA phono jack and mark it ‘EXT VFO IN’.
2. Remove R220 (1K) to remove plate voltage from V5B
3. The Heathkit modification also calls for removal of L202 (36uH) and C222 (100pF) although
it is unclear why this would be necessary.
Figure 18. Using an External Transmit VFO
Mode SB-100 Setting SB-101/SB-102 Setting
Transceive using internal LMO LMO LOCKED NORMAL
Transceive using external LMO (or internal crystal oscillator) XTAL LOCKED AUX.
Receive uses internal LMO
Transmit uses external VFO (or internal crystal oscillator)
AUX. 1 UNLOCKED AUX.
Modifications Page 100
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