Installing the X-Lock into the Atlas 210/215 Transceivers

By Frits Geerligs, PA0FRI


The free running VFO's in old or vintage rigs are subject to frequency drift due to temperature variations. To cure this I have successfully used PAØKSB's VFO stabiliser in an FT-7 and also in a Corsair II with PA0CMU's PCB design but unfortunately the size of the PA0CMU PCB was too large for my Atlas-215x. The X-Lock VFO Stabiliser kit produced by Cumbria Designs is an improved version of (SK) PAØKSB's Huff and Puff stabiliser which instead of discrete logic, employs a microprocessor to detect and correct drift. This compact 35 × 60 mm module is complete with all components, sockets, and connectors, pin header connectors, PIC processor and a double-sided PCB. I ordered this relatively cheap but professional design for my Atlas-215x. The specifications of the manufacturer were confirmed during the testing. 


The diagram in Fig.1 below shows the interconnections between the x-Lock and the Atlas. The assembled X-lock board is mounted near the Ext. Osc. socket. More space could be made available by replacing the standard speaker with a shallower type. The RF input is taken from pins 2 and 3 of the Ext Osc socket. If you find that the VFO output is too low, you could increase the level with a simple BC547 pre-amplifier. I used this extra amplifier in my Atlas because of slight loading changes on the VFO, when going between RX - TX - RX. This was enough to cause a slight jump of ± 10 Hz in frequency on a couple of bands. The BC547 buffer amplifier is shown in the general schematic of Fig.2.


Fig.1 Interconnections

Note the use of a BB221 varicap diode instead of the LED supplied.

Fig.2 General Schematic showing BC547 buffer amplifier

Installing the X-Lock

Because space is at a premium in this compact transceiver, the options for mounting the X-Lock PCB are limited. I decided that the best approach would be to make a small mounting board out of PCB laminate (Fig.3) which would be fastened to the back of the loudspeaker with adhesive. The mounting plate is drilled to take the X-Lock which occupies the free space behind the loudspeaker.


Fig.3 Mounting Plate

Fig.4 The X-Lock in place


Open up the VFO enclosure and locate the junction of emitter the resistor R404 and capacitor C403 on the tracked side of the VFO board (PC-400/PC700) . Very carefully solder in place the four components that form the compensation circuit (shown in Fig.5 and Fig.6). One leg of the 10 nF capacitor is soldered to the R404/C403 junction, use C426's soldering tag for the other three parts. Install a short wire from the VAR connector to the components to connect the compensation voltage.

If required the three-colour LED may be remotely fitted to the front panel. Find a suitable place for the LED and connect 3 wires to the LED connector. I mounted the LED in the (empty) hole for the noise blanker.

Connect the PWR socket to the 13 V DC rail inside the Atlas.

That's all. If you assembled the board correctly the device should worked straight off on power up!


The device is an easily applied "add-on" module and the ability to detect rapid frequency changes during transmit and receive is a valuable innovation. It does exactly what is published in the advertisement and the Atlas-215x is astonishingly rock steady like a crystal controlled oscillator. I am very enthusiastic about this X-Lock kit and can recommend the Cumbria Designs.

OZ2QL has installed the X-lock in an Atlas-210x. It works just fine. This model has a different VFO circuit using two FET transistors. He used the supplied LED "varactor" and a 2.5 pF capacitor connected directly to the main tuning capacitor. He has also successfully applied the same arrangement in his Atlas-350-XL.

Fig.5 Compensation circuit

Fig.6 Compensation circuit components