Vanguard Labs

tions in Michigan, Tennessee, Pennsylvania, and West Virginia - as well as the expected Kentucky and Indiana. Although I don't have the equipment necessary to make a good evaluation, receiver noise is well below thermal and atmospheric noise. I hear everything anyone else in the area hears, and more than some. Birdies are not evident, another advantage of an internal arrangement over a converter set-up. The conversion was completed by making the appropriate changes in MHz dial calibration and the band switch index. A conversion of this type is easy and suitable for any receiver having a crystal controlled front end. Even if you don't have an extra band (like 11 meters) to play with, you can add a position on your bandswitch, if you're the ambitious type*

The whole thing goes together easily, is uncritical, and stable. It's nice to be able to flick up to 6 without having to wade through the usuai rat s nest of wires, and there's absolutely no compromise between convenience and performance both are improved! The only thing I don't like is that empty socket that used to hold the 6BE6 mixer. It just sits there begging to be used. Let's see now,,, what could we put in there..,?

Alton E. Glazier, K6ZFV 3154 Jordan Road Oakland, California 94602

The need for Monitor 2 arose from the many hams who want to monitor two meter repeaters while still leaving their main equipment free, and possibly is an answer to 24-hour-a-day emergency listening. It was decided to make the project as easy as possible, with high reliability and low cost.

The basic receiver is the old standby, the five-tube ac/dc broadcast radio (12BE6, 12BA6, 12AV6, 50C5 and 35W4), purchased from most catalog houses or drug stores for about $10, Most of these imports are hand wired, and this is recommended for ease of conversion.

The major change is at the converter (1 2BE6), The if, second detector, and audio stages are left as is. Remove the variable capacitor, antenna coil and oscillator coil. This will give you the space needed for the crystal oscillator tube and components. The oscillator is the one described by Frank Jones, W6AJF ("The Overtone Harmonic Crystal Oscillator/' CQ Magazine, February, 1963), This is an excellent circuit, and makes possible this simple conversion.

Remove everything from pins 1, 2, 6 and 7 from the 12BE6. Bypass filament pins 3 and 4 to ground with a -002 capacitor. Mount L-2, C-2 as close to socket as possible. One side of L-2? C-2 connects to pin 7, The other ends of L-2, C-2 connect to ground. Connect a 27 K V4 watt resistor and a 4,7 pf capacitor from pin 1 to ground. Dress this resistor close to the chassis. I n is completes the mixer conversion.

The oscillator uses a 6CW4. Although the filament current is slightly different from the rest of the tubes, no harmful effects have been noted. I race the filament series string and find that filament lead which goes to ground. This will vary according to the manufacturer. Remove from ground, and install a choke made up of ten turns of hookup wire 1/16" in diameter. From the previous filament, connect to pin #12 of Nu-vistor socket. Also bypass to ground with a .002 capacitor. Ground pin #10. This completes the filaments. From pin #8, connect a 4uh choke and a 10 pf capacitor. Ground the opposite ends. From pin #4, install a

The Monitor 2.

100K i^-watt resistor, also a lead to crystal socket. The opposite end of resistor and crystal socket go to ground. From pin fl connect a lead to top of L3 and C3. Also connect lead of 4.7 pf capacitor. The opposite end goes to pin #1 of 6BE6. At the bottom oi coil L3 and C3, bypass to ground with a .002. Also connect a 4700 ohm resistor. The opposite end of resistor goes to nearest B-plus lead. This completes the oscillator circuit.

For those who live within line of sight of the repeater, this simple mixer-oscillator should provide enough sensitivity. For an antenna, connect a piece of insulated wire from the top of L-2, C-2 to back of receiver, then to a wire rod. Because the receiver is hot to ground, be sure to use insulated sleeving over antenna. The connecting wire and rod should measure 38" overall. It is interesting to note that this directly coupled antenna outperformed any attempt to use an outside ground plane.

For those not in line of sight of the repeater, the following rf amplifier is very worth while. The rf amplifier is quite straightforward. Just be sure antenna coil is shielded from the mixer coil or at feast at right angles. The transistor was taken from the rf section of a junked RCA FM receiver. Most PNP vhf transistors should work quite

all resistors are im*

Fig. 1* Complete diagram for Monitor 2.


all resistors are im*

36 " overall see text



Fig. 1* Complete diagram for Monitor 2.

L1— 5T, Va" diameter, 3/4" long, airwound center tap.

L2-3T, 14" diameter, 3/4" long, airwound center tap.

L3-4T, Ya" diameter, long, Milier #4300. L4—3.9 uh choke.

well, and perhaps a FET would be better. The power for the rf amplifier is taken from pin (cathode) of the 50C5. Be sure to use a cathode bypass capacitor if it is not already present. Ten to twenty-five mf, ten volt.

The crystal is a third overtone, and is lower in frequency than the receive signal. The reason for this is that the receive signal that I wanted to monitor was at 145. 00 mhz, Due to the fact the if frequency is 455 hkz, this puts the image on the lower end of the two-meter band. In one month of monitoring, no image signal has been heard; however, if the receiver is to be used in the upper end of the band, perhaps the crystal frequency would be better on the high side. This will depend on the activity in your area.

As to the crystal frequency, after it has been multiplied three times, it must be 455 khz different than the receive signal or thereabouts. If a surplus crystal is found near enough in frequency, the if transformers can be shifted to allow for some difference, for 455 khz is the design center of the // transformer, and it can be moved up or down in frequency. i?or example, in the author's receiver, the crystal used (surplus)


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