Installing the X-Lock into the Drake
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,
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
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
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
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
I determined that I could keep the plugs provided that I installed the pin headers at 90
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,
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
Fig.2 (Right) Pin Header installation.
The headers are mounted at 90 degrees to provide clearance for the
plugs and wiring.
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
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.
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
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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