Installing the X-Lock into the Drake TR-7

By Ron Wagner WD8SBB


The free running VFO's in old or vintage rigs are subject to frequency drift due to temperature variations. Whilst the Drake TR-7 Permeability Tuned Oscillator (PTO) is generally very stable for an LC circuit, it still drifts several hundred hertz during a two hour net. To cure this drift, many have successfully used (SK) PAØKSB's Huff and Puff VFO stabilizer in the TR-7. Carel Mulder PA0CMU, has produced a very documented section on his website describing his experiences in doing this.

The Cumbria Designs X-Lock VFO Stabilizer kit is an improved version of the Huff and Puff stabilizer which instead of using discrete logic, employs a microprocessor to detect and correct drift. This compact 35 × 60 mm kit is supplied complete with all components, sockets, pin header connectors, PIC processor and a double-sided PCB. This relatively inexpensive and professional design dramatically improves stability of the TR-7 and also detects and "understands" RIT changes, something that other Huff and Puff circuits do not.

Below is my description of of how I installed the X-Lock into my Drake TR-7.


The X-Lock generates a control voltage for tuning the VFO by means of a varicap diode. A power diode (1N4004) and several other parts are provided with the kit to build a varicap "correction" tuning circuit but I decided instead to use the existing RIT line in the TR-7. To do this I had to combine the X-Lock control voltage and the RIT voltage, I used a circuit designed by Joe KC9LAD which with only three resistors and one signal diode, does the job nicely!

The diagram in Fig.1 below shows the additional diode and resistor network together with the interconnections for the X-Lock output and the voltage from the RIT control point on the main board.

Joe also suggested that it would help with physical installation if I were to exchange the 6 electrolytic capacitors for titaniums which are much smaller for the same electrical specification. Although I did not do this, it would be a desirable modification for installations where space is at a premium.

The clearance above my chosen mounting point for the X-Lock would have prevented me from using the straight pin headers supplied with the kit and their associated plugs. After quite a bit of analysis, I determined that I could keep the plugs provided that I installed the pin headers at 90 degrees, as shown in Fig.2. By carefully bending the solder side of the pin headers down 90 degrees, the pins would still be inside the PCB mounting holes a little over half way. As the PCB has high quality plated through holes, I could solder the top side and then flow the remainder of the hole with solder from the bottom. Since there is not a lot of ongoing pressure on the pin headers, this modification should last the life of the devices. NOTE: Be sure to bend the pin header pins so that the tab for the plug is downwards (against the PCB).

The assembly sequence of the board components should be altered slightly due to the modification for the pin headers. To avoid having difficulty with other components being in the way, install and solder the modified pin headers first. Remember to solder from the top first, and then flow solder the bottom. After this, follow the instructions as supplied.

The assembled X-Lock board is mounted on the bottom of the TR-7 main board. As mentioned, more space could be made available by replacing the 6 electrolytic capacitors with tantalums, but it is not an absolute necessity. All interconnects are close and easily accessible on the bottom of the TR-7 main board.

Fig.1 Diode/resistor network

The resistors carry virtually no current and can be miniature 1/8 watt.

Fig.2 (Right) Pin Header installation. The headers are mounted at 90 degrees to provide clearance for the plugs and wiring.

Installing the X-Lock

Space is somewhat tight inside the Drake TR-7 transceiver. I decided that the best approach would be to install x-lock on the bottom of the main board as shown in Fig.3. I used double sided foam tape, and used black electrical tape over the main board's protruding pins for added safety. A picture of the service manual with various points of interest circled in red is show in Fig.4. (Click on the images to enlarge).

Take a close look at the pictures, schematics and comments. These should provide you with enough information to install the X-Lock into your Drake TR-7 with minimal effort.


Fig.3 Installed X-Lock. In this picture, the red jumper wire to the resistor-diode network is RIT from main board, brown/white wire is RIT to PTO. 

Fig.4 Service manual. Click for larger image.

General Comments

Status LED If you wish, the tri-color LED can be mounted so that it shows through the fixed "window" on the front of the transceiver, to the right of the meter.

Power I opted to get power from the power supply board. The board also has the bandpass controls on its top side. It is to the left of the main board in the pictures. Pin 3 from the right is ground, pin 9 is the +13.6VDC rail.

Assembly Time If you assemble the board with care, the X-Lock should work at power up. Mine took about 10 hours to assemble and install, and worked first try.


The X-Lock is an easily applied "add-on" module. It's ability to detect and respond to the rapid frequency changes during transmit and receive transitions to support RIT operation, is an invaluable feature. It does exactly what is advertised, and my Drake TR-7 now behaves like it has the optional RV75 synthesized external VFO installed. I am very enthusiastic about the X-Lock kit and recommend Cumbria Designs.


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