1. Capacitors marked * have a direct effect on the frequency of oscillation and should be qood quality components. Fixed values should be silver mica. Avoid sub miniature leaded ceramic types. The variable capacitor should be a dual bearing type. I have used one from a Heath LMO and one from a WWII RF27 unit.
2. D1 = BA244 or similar VHF switching diode
3. The offset switching components shown provide the same functionality as in the SB101 for offsetting the VFO between USB and LSB. An additional set of offset components will be required if you wish to have a different offset on CW.
4. As shown, the offset switch requires a positive voltage to reverse bias it and a negative voltage to activate it - I use plus and minus 9v as this is conveniently available. To reverse the switching polarity simply reverse the diode D1.
4. The VFO components are contained within the screened enclosure. The VFO inductor L1 and 75pF tuning capacitor are mounted directly onto the inside of the metal enclosure, the rest on a fibreglass PC board. The buffer and regulator module are mounted on the outside wall of the enclosure to provide additional isolation. A simple metal screen may be used if required over the buffer and regulator module.
5. The 9v supply is obtained from another Low Drop Out (LDO) regulator fed from the common 12v supply.
6. Ensure that all stages after the VFO oscillator do not saturate otherwise some isolation will be lost.
7. Inductor L2 is very small compared to L1 and is to allow incremental changes to be made for calibration.
8. If you do not require the RIT facility then leave it out. The two varicaps and the adjacent 47pF coupling capacitor should be selected for the desired pulling range. As shown, the RIT control voltage should be positive with respect to the chassis, well regulated and hum free.
9. During development, two oscillator devices failed when running on a 9v supply so this has now been lowered to 6v with no further failures.
10. No attempt has been made to obtain a linear dial calibration as a frequency counter is used instead.
1. The horizontal width of the enclosure is 2.9 inches.
2. The PC board nearest the camera holds the buffer and regulator. The PC board on the right holds the transmit receive RIT and offset logic and 9v regulator.
3. The small inductor inside the enclosure is the fine adjust with the main inductor below it. The tuning capacitor is from an old WWII RF27 unit, given an initial wash, immersion in a silver dip liquid followed by another wash, dry and lubrication of the bearings and sliding contacts.
4. The enclosure is made from 16swg NS4 half hard aluminium sheet fixed to 0.25inch square aluminium bar using 6BA tapped holes. The complete enclosure is extremely strong and does not move!