Frequency Stabilizer
Construction

The new Rev 2 boards currently being sent with the kits have the parts for integrating the stabilizer with the VFO on Board 1.

Some of the following instructions use the Rev 1 picture. Both versions are close enough in layout so there should be no confusion. In some of the Rev 1 pictures there is a cable connected to the "Freq Cntr" box, this is no longer used - just ignore. The "Freq Cntr" has been changed to a "Test" box.

Two cables connect to the receiver. First, a cable between the stabilizer and the stabilizer parts in the VFO. Second, a cable between the "Stabilizer" box between the two VFO amplifiers and the "VFO IN" at the upper left of the stabilizer board.

The reason for the output being labeled "Relay" was that the original stabilizer setup used a relay mounted on the VFO toroid to control frequency. The current draw of the relay caused problems with overheating the output of the CA3140.

A fix was found for the problem, but using varicaps and integrating the parts on the VFO (Board 1) made installation a snap and performance became consistently excellent. The magnetic coupling does work and for those that would like to experiment with magnetic coupling check out Frequency Stabilization using Magnetic Coupling.

The VFO coil is wound with 19 turns when using the stabilizer. If you wind it with 18 turns, just pull the windings together to get the 14 MHz VFO within range of the 8pf ceramic trimmer.

The varicap on the 8pf trimmer raises the 14 MHz frequency 300-400 kHz, so the VFO coil is wound with 19 turns, instead of 18 turns without the stabilizer.

Readjustment of the toroid windings will be necessary to get the 8pf trimmer in the 14 MHz range of the VFO. The addition of the varicap will raise the 14 MHz frequency and bringing the windings closer together will be necessary.

____Start with the resistors and capacitors. Picture Be sure to get the 4.7K in the right footprint. If it gets in a 4.7 meg spot, the integrator will not work. The 2.2K resistor above the T1-1T transformer is not installed. See below:

Be careful mounting the 7805. The front of the 7805 faces to the outside of the board. The bold line of the footprint indicates the location of the heat sink, the back of the 7805. Be sure to get good solid solder joints at the pins of the regulator to the PCB.

There are four jumpers (Picture) on the board:

There are two connections to Board 1 in the VFO section, a third one to the reset button and a fourth one to 12 volts and ground.

A box labeled "Stabilizer" between the VFO Amplifiers on the edge of the PCB goes to the "VFO IN" connection on the Stabilizer board.

At the VFO, above the main tuning capacitor, there is a box labeled "Stabilizer In" that goes to the box labeled "Relay" on the Stabilizer board.

A reset button, mounted on the front panel, is connected to the "Reset" box on the stabilizer board.

Power (12 volts) is connected to the input power connection (or wherever is convenient), and ground to a ground plane connection.

Inside the "VFO IN" box are the connections to the VFO "Stabilizer" connection on the receiver board.

The lower one is the VFO connection (connected to the .01 input capacitor) and the upper one is the "Gnd" (ground) connection.

It is easier to make this connection first, mount the stabilizer board, then make the connection to the VFO "Stabilizer" box on Board 1.

____Solder the shield of a 6" length of miniature coax cable to the ground plane underneath the board.

____Solder the center conductor to the trace connecting to the .01 cable underneath the board. You might be able to get the center conductor through the hole on the top side but it is much easier to solder to the trace underneath.

The yellow arrow points to the connection between the VFO amplifiers that connects to the cable from the "VFO IN" connection on the Stabilizer board. Miniature coax is used for the connection.

If desired, wire loops can be installed at both the "Stabilizer" box and in the "Ground" box for easy connection points.

The output of the stabilizer is marked "Relay" and "Gnd". Both connection holes are in the box labeled "Relay".

____Connect the cable to the stabilizer first. Mount the stabilizer board, then connect the cable to the receiver board as shown below. Use about a 5" length of the miniature 75 ohm coax.

Note the ground and center wire connections. There is a "Gnd" label next to the Ground hole. Or solder the shield underneath the board on the ground plane.

____Mount and connect the "Reset" button to the stabilizer. The Reset button is a small push-button switch, closes on push.

Be sure to use enough wire length to reach where the Reset button will be mounted on the front of the receiver. Twist the wires together to keep them neat.

The picture above shows the connection of the output of the stabilizer to Board 1. The connection is made underneath the board. The shield is soldered to the ground plane and the center conductor is soldered to the pad underneath the board where the "Stabilizer In" hole is located. See the picture below.

This picture shows the connection on the top side of the board. Note the yellow arrow pointing to the connection hole. This hole and both resistors connect to the same pad underneath the board.

Please note that connecting the stabilizer to the VFO will change the frequency of the VFO. To set the 14.000 (or 14.068) MHz base frequency of the VFO, compress (too high frequency) or expand (too low frequency) the turns of the VFO coil.

If spreading (expanding) the windings on the VFO does not get the VFO to 14.000MHz, take one winding off the VFO coil.

When adding the stabilizer, you are adding some capacitance to the VFO tank, so the most likely adjustment will be raising the VFO frequency from 13.xxxMHz back up to 14.000MHz.

____Connect the 12 Volts and Ground to the stabilizer board.

Even though pictures of the receiver show the 75 miniature coax connecting to 12 Volts, regular hook-up can be used. Two pieces of wire twisted together work great.

There are enough holes here to install wire loops for connection points for the 12 volt and ground connections if desired.

The 12 volt power supply to the stabilizer should be filtered from the receiver. Make a connection to the receiver only at the BOLD 12 Volt connections, where the RF Chokes are located. Using an additional RF Choke at the 12 Volt connection to the Stabilizer PCB wouldn't hurt.

Have the stabilizer turn on at the same time as the receiver. The VFO will do some short term quick drifting, as the 2N5486 warms up from the operating current.

The stabilizer should lock very fast. The current integration of the stabilizer with the VFO has provided excellent stabilization. The stabilizer should be reset by pushing the reset button after about five minutes.

Depending on how fast the room temperature is changing, normal reset times are about two to four hours.

There will be a slight, short term drift when changing VFO frequencies as you change bands.

If the stabilizer does not lock up after a minute or so of operation, then some troubleshooting is in order.

Drifts in one direction

Using a DVM, check the voltage at the output (lead going to the varicaps). It will start at about 6 volts (after reset) and rise to about 10 volts and stop. (Rising voltage has been an indication of a bad oscillator or 74HC4020 failure.)

This type of drift indicates that one of the signals to the integrator is missing or the canned oscillator is bad.

The first thing to check is the canned oscillator. It should show 32 MHz at the upper right hand corner pin or at Pin 2 of the 74S74. (A 74F74 is also used.) (see pic below).

If the canned oscillator is working, the frequencies should be checked at the .01 caps coming from the ICs in the center of the board.

Arrows indicate the places to check for a frequency, the readings will vary depending on band, etc, but you are looking for a missing frequency. The reading should be varying slightly, a locked or no reading indicates the bad part of the integrator.

If a no signal is indicated at one of the .01 caps:
First, check the 22K resistors for correct value and correct polarity of the diodes.
Second, check the ICs (74HC4020 and 74F74) for unsoldered active pins. (Not all pins are used on the ICs). If Pin 8 or 7 (ground pin of the IC) was missed and soldering the connection does not fix the stabilizer, the IC will have to be replaced. Email for a replacement part.

Sudden drifting

Sudden drifting, with no jumping, is a bad solder connection at a ground or 12 Volt pin at an IC or a low value integrator resistor (the 4.7 Megohm resistors). Check the connections to the transistors also.

Jumping of the frequency, around the stabilized frequency, can be a bad ground at one end of the miniature coax. A lot of movement during setup of the boards can cause the ground lead to break right at the cable where the tinned ground shielding ends against the insulation. Pull gently at the cable to make sure there is a strong connection.

If the output of the stabilizer (relay outut) immediately goes to 10 volts or zero volts at turn on, check the 4.7 Megohm integrator resistors.

You can double check this by taking the input frequency (the VFO) off the stabilizer input and reset the stabilizer. The output voltage should be around 5 to 6 volts, then touch the VFO output to the input of the MOSFET amp, the output will jump to 0 or 10 volts immediately. One of the 4.7 Megohm resistors is not 4.7 Megohm, instead, a very low value.

All the readings below are with the VFO at 14 MHz.

It is very difficult to blow a CMOS chip, unless you put the 5 Volt regulator or IC in backwards. Otherwise, your problem has to be a bad solder connection on the board. An unsoldered pin at one of the ICs is the most likely problem to occur. Check carefully!

Resistor/Capacitor Mistakes

If both the 4.7 Megohm resistors are too low in value at the integrator, there will be modulation in the output of the VFO. This modulation will be heard in the speaker. Remove the wire to the varicaps at the main tuning capacitor, ground the wire to the PCB board, to test if it is definitely coming from the stabilizer.

If the varicaps are installed and not connected to ground or 12 Volts, the VFO will pick up 60 cycle hum and the VFO will be very unstable. When testing the VFO without the stabilizer, ground the varicap input voltage connection - between the two 100K resistors.

Check the integrator resistors - two 4.7 meg. There is one 4.7k resistor in the kit that could have been accidentlly placed here.