Technical information
The Model 810 external VFO is relatively easy to add the X-LOCK. You could also do the internal VFO
with more mechanical complexity. Only the external VFO is presented here.
I experimented with using the standard X-LOCK varactor control network connected to the VFO tuning
capacitor. I had to abandon this because of instability after resetting the dial end points – variable L too
far away from original design.
It turns out the VFO already has a varactor control network used for RIT with a 6VDC supply. I tried
using this instead with excellent results. The X-LOCK control voltage range is 0-6VDC. Simply coupling
the X-LOCK DC control voltage though a 10k resistor to VFO connector Pin#1 (now in parallel with the
RIT DC VFO input) seemed to play well at not affect the dial end point settings. From cold to warm (2
hour bench test) the X-LOCK control voltage ranged from 3.795 VDC to 4.175 VDC to maintain zero
VFO drift – EUREKA.
1. Build the X-LOCK kit. It is easy to test on the bench if you have a signal generator and 12 volt
supply.
2. Remove the external VFO top and bottom covers.
3. First, fix the T/R transition VFO voltage dropout problem by adding a 470uf, 25 volt electrolytic
between the VFO DC input and ground. This problem blocked a previous X-LOCK attempt since
it starts over every T/R transition due to goofy the 2020 switching. I apparently did not try
enough capacitance on my first X-LOCK attempt. I connected the cap to the bottom left
VFO/SPLIT SW terminal (looking from the back) and the ground lug on the VFO cover.
4. Remove the VFO can covers, easy, 4 screws.
5. Thread the DC control and RF sample wires through the adjustment holes. I grounded the coax
cable only at the X-LOCK connector to avoid a weird ground loop problem that had me
scratching my head why it wouldn’t lock with a perfect RF waveform.
6. Connect the RF sample coax to the PC board bottom where the short little coax connects to the
VFO jack on the back (see gallery pictures below). According to Cumbria, the RF IN signal can
range for 500mv to 30v pk-pk. The Tempo 2020 buffer outputs 1.9 v pk-pk – well within the
range, no need for a pad to reduce this level.
7. Connect the X-LOCK 12VDC input to the front panel SW with the fat red wire. I connected
ground to a lug on the VFO cover.
8. Connect X-LOCK DC control voltage through a 1N4007 diode (bypass X-LOCK side with a
10pf) then a 3.3K resistor to the pad where the current 10k RIT voltage resistor connects - a
summing junction of sorts. Use an ohm meter to find it from PIN 1 <-> 220uh choke <-> 10k <->
X-LOCK 3.3K. I used a twisted pair and grounded nearby the resistor install. The diode is
required to block the RIT voltage from getting back into the X-LOCK and getting its panties in a
bunch. 10K did not provided enough control voltage so 3.3K was arrived at by trial and error.
9. Program the post tuning delay to 4 seconds to give you time to tune on the beak before if locks
down. The default 2 seconds gets annoying when tuning with the DD-103, 10 Hz resolution
display.
10. I mounted the X-LOCK on a plastic box to avoid a weird problem I had if the X-LOCK pc board
got too close to the metal case area.
11. You will have to adjust the 3 RIT pots to center the knobs due to the now shared use of the RIT
varactor network. This is kind of tricky. R215 is the CENTER pot. R214 is the WIDE center POT.
R213 is the NARROW center pot. If you can’t set R214/R213 then move R215 and try again
until you can turn RIT on and the WIDE and CENTER knobs are close when the dial shows
centered.
12. The X-LOCK can manage 2 set points so this enables receiver RIT operations very well and
does not walk down the band on transmit – a brilliant design.
13. Since the radio frequency = VFO + LO OSC, there will still be a small drift until the LO OSC
stabilizes but after that, stability rules.