Using the LM-13 Frequency Meter As A VFO
by Greg Latta, AA8V
Grid Blocking the Oscillator During Receive

Introduction:
The VFO in a transmitter must be disabled during receive periods, otherwise it will be picked up in the receiver and interfere with reception. One way to disable the LM-13 during receive is to turn off the plate voltage with the "Plate" switch, but it was found that the LM-13 drifted too much and took too long to stabilize when the plate voltage was turned back on. By experiment it was determined that applying blocking bias to the oscillator grid during receive and removing the bias on transmit allowed the oscillator to stabilize much faster. It was also determined that attempting to key the oscillator by this method resulted in excessive chirp, so that keying the LM-13 is not desirable.

Bias Power Supply:
By experiment it was found that about -16 V of bias was needed to reliably cut off the oscillator. This was obtained by using a half wave voltage doubler connected to the 6.3 VAC filament supply. The bias supply and its associated switching circuitry is contained in the power supply that I built for the LM-13.

Grid Block System Schematic Diagram:
The schematic diagram of the grid block circuit is shown below. My bias supply supplies -15.9 V open circuit, but this drops below -12 V when loaded by the 500 ohm relay coil. However, the current is sufficient to reliably pull in the relay, so a separate power supply for the relay coil was not needed.

The relay, Mode switch, and Standby/Operate jack are contained in the power supply. The RF choke and bypass capacitor are in the LM-13 itself near the oscillator tube.

During Standby/Receive -15.0 V is applied to the grid of the oscillator tube through the normally closed relay contacts and the 2.4 mH radio frequency choke. When the Mode switch is switched to Operate or the Standby/Operate jack is shorted to ground the relay is activated, opening the contacts. The bias is then removed from the oscillator, turning it on. The 2.4 mH radio frequency choke and 0.001 uF bypass capacitor insure that no RF escapes from the oscillator into the rest of the grid block circuit.

In normal operation, the Mode switch is set to Standby, and the relay is activated via the Standby/Operate jack, which is connected to my automatic T/R system. Among other things, the automatic T/R circuit grounds the Standby/Operate jack during transmit, turning on the VFO.

RF Choke: The inductance and current rating of the RF choke are not critical. I happened to have a nice, encapsulated 2.4 mH RF choke on hand, so that is what I used. My guess is that just about any value from 1 mH to 10 mH will work fine. Only about 32 uA flows through the choke, so the current rating isn't critical either. There is absolutely no reason why a toroidal choke couldn't be used here, so feel free to wind your own. Just be sure that the inducatance is at least 1 mH.

Connecting to the Grid of the Oscillator Tube:
Making the connection to the oscillator grid is quite easy, since the cap of the 77 tube is the grid, and NOT the plate. A small two terminal tie strip is mounted on one of the already existing screws near the 77 oscillator tube. One terminal of this tie strip must be grounded. The 0.001 uF capacitor is mounted across the two terminals of the tie strip, and the RF choke is connected from the other tie point to the grid cap of the tube. Be sure to remove the grid cap from the tube when soldering the RF choke to it, otherwise you may damage the tube! The wire is simply laid on the grid cap connector and soldered to it. This method of soldering allows for easy removal should you wish to undo the modification, while still providing a secure connection.. A wire (white in the photo below) is then run from the ungrounded tie point to the power supply cable. See the picture below.

Click here for a larger view.
A 2.4 mH RF choke (black in the photo) is connected from the grid cap of the oscillator tube to a tie point near the tube.
The other end of the RF choke is bypassed to ground with a 0.001 uF capacitor and then run, via a wire (white in the photo) to the power supply connector.

The picture below shows the modification from further back. The white wire runs down next to the chassis behind the voltage regulator tube, and then across and under the chassis to the power supply connector. The white wire is barely visible at bottom right in the photo, just to the upper left of the audio tube socket. The nice thing about this modification is that is can be easily removed with no permanent changes made to the LM-13.

Click here for a larger view.
A 2.4 mH RF choke (black in the photo) is connected from the grid cap of the oscillator tube to a tie point near the tube.
The other end of the RF choke is bypassed to ground with a 0.001 uF capacitor and then run, via a wire (white in the photo) to the power supply connector.

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