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Notes on installing the Cumbria Designs X-Lock 3 into the Kenwood TS120S Amateur Bands Transceiver.

By Roger Wincer, ZL2RX

Varactor Coupling Capacitance

Interfacing the varactor circuit to the VFO was fairly simple once I had established the coupling capacitance values. The coupling capacitor to the correction varactor circuit consists of two 5.6pf capacitors in series and connects to the hot side of C17 and C18. This arrangement required only a relatively small adjustment to L9 to bring the dial setting back to original. Using more coupling capacitance than the 2pf or so that I used may result in some warbling of the received and transmitted signals. As I am using my TS120S with WSPR slow data mode and as WSPR is very sensitive to any type of frequency shifting, I set the X-Lock's "Correction Rate" to minimise the "warbling" effect of the correction process. I had previously removed C18 (15pf) and C18 (12pf)  from the VFO in an unsuccessful attempt to stop temperature drift, these were replaced these with 18pf and 5.6pf capacitors to restore the VFO circuit to it's original state. These capacitors are in parallel across the main tuning circuit which includes TC1, L9 and the main tuning capacitor. (Note: In an unmodified rig C17 and C18 should not need to be changed).

VFO Connections

The varactor diode circuit was mounted to the VFO by using an old recycled 3 terminal tag-strip soldered to the chassis. The RF output was taken from the back of the VFO RF output socket using a screened wire. There was a handy hole in the rear of the VFO outer screen for the two wires for the X-Lock board.

 X-Lock PCB Installation

Mounting the X-lock board is tricky as there a no spare space in the rig, so the loudspeaker had to be sacrificed! The board was hot glued into place using thin cardboard spacers as can be seen in the photograph. The power was taken from the cold end of R14 the large 2 watt resistor on the AF Gen board, as this point provided about +11.5 volts. I had to solder connections directly onto the plug socket inputs because there is no headroom for the plugs! Any movable wires and solder joints were stabilised with hot glue to prevent shorts etc. One more advantage of removing the speaker is that the indicator LED can be monitored from outside the rig, through the gauze cloth.


I have tested the rig on transmit and receive using WSPR and it appears to work wonderfully well and is stable day after day after day. I have also used it on CW mode where the Tx frequency is shifted 800 Hz from the Rx frequency and it works flawlessly. I have set the "post tuning" to about 8 seconds, which seems a reasonable time.