Here is how I put several Johnson Messenger CB radios on 75 meters. This covers all of the tube type Messengers, One Two, and 223. This is lengthy, but it covers all mods necessary to convert the receiver and transmitter into a 75 meter rig that really works.
For schematics see:
http://www.cbtricks.com/radios/ef_johns ... /index.htm
73,
Dan K9WEK
K9WEK Johnson Messenger conversion to 75 meters AM:
The following makes more sense if you are looking at a schematic which can be found in Sams CB-1 for the Messenger 1 (White face) or CB-5 for the Messenger Two (Black face). The Messenger Two has 4 double tuned IF transformers vs. only 2 for the Messenger 1, so receive selectivity is much better. The Messenger 223 found in Sams CB-23 is the very same chassis turned sideways for a more modern look. The transistorized synthesizer is not used on 75 meters, of course.
Receiver Conversion:
The original oscillator coil (actually a so called "impedance inverting" coil) was about the right inductance so I tapped it at 7 turns from the bottom for the cathode of the 6BE6 and put the 330 pF mica across it. I used a 25 pF variable for limited tuning of the 75M AM frequencies. The tuning capacitor was mounted in place of the original crystal switch. Since the tuning capacitor is located across the chassis from the coil I connected it to the coil with a piece of RG-59 coax. You could probably use RG-174 instead. This parallel tuned circuit is coupled to the oscillator grid of the 12BE6 with a 47 pF mica or NPO ceramic capacitor. A 22 K grid resistor was coupled to the transmitter final grid similar to the original circuit since this is part of the TR switching scheme. I did away with the 820 ohm cathode resistor and coupled the cathode directly to the tap on the coil.
For the coupler from RF to Mixer I used a 4.5 MHz sound IF can from an old TV. In another conversion, I rewound an old IF can to resonate at 3.85 MHz - not easy! Just one tuned circuit (like the antenna coil) could be used here, and image rejection would be adequate since we will very likely be using a tuned antenna. In this case replace the second tuned circuit with a 220 K resistor and couple it to the plate of the RF amplifier with a 100 pF cap.
For the antenna coil I rewound the original ceramic form with 48 turns of No. 28 enameled wire and put a 100 pF mica capacitor across it. The antenna link was 2 turns of hookup wire.
To defeat the squelch circuit, remove the 12 K, 2 watt resistor (R35 in Messenger or R30 in Messenger 2) and ground the circuit node where it came from -- not the B+ end! The Messenger 223 uses a different squelch circuit, so this doesn't apply to it. Because I haven't converted a 223 I don't have any recommendation for it.
Transmitter Conversion:
The 7054 is basically a 12BY7 and the 7061 is practically the same as a 12AB5. For a crystal oscillator see any Handbook circuit. For an external VFO couple the 330 pF cap from the grid of the 7054 to a BNC on the rear chassis apron. (I use a NorCal FCC-2 DDS VFO.)
Replace the cathode choke L9 with a 150 ohm resistor for self bias, and bypass the cathode to ground with a .01 ceramic capacitor. Use a 150 K resistor for the 7054 screen resistor
The 7054 plate coil can be the same as the receiver antenna coil, again with a 100 pF capacitor across it.
The final grid circuit is modified somewhat. Grid choke L-10 is removed and replaced with a 15 K resistor. A 10 K resistor goes in series with this resistor to ground. The junction of these 2 resistors is bypassed to ground and goes through a 22 K to the grid of the 12BE6 receiver converter tube similar to the original hookup.
For the final plate circuit a pi network is used. Use a 2.5 mH RF choke to feed B+ to the plate of the 7061.
The final plate coil was wound on a piece of 3/4 inch PVC pipe (1 - 1/16 inch OD) 1 - 3/4 inch long. Drill two small holes at each end to terminate the windings. These holes are spaced to allow the winding to be 1 - 3/16 long. They should be close to the top end to space the coil up from the chassis when mounted vertically. The winding is 31 turns No. 24 enameled wire space wound using No. 28 wire as a temporary spacer. This gives 17.9 uH with an unloaded Q of about 225. The plate capacitance can be the original plate tuning capacitor if it is 4.5 to 100 pF. Later Messengers only had about 65 pF. These can be used if some fixed capacitance is added. About 600 pF is needed for the output capacitance when feeding a 50 ohm load. I used a 1000 pF mica compression trimmer.
Of course, short out the series 3900 ohm 10 watt resistor for more power to the final.
The VFO output level should give of about 1.5 – 2.0 mA final grid current that can be measured by the voltage drop across the 10K resistor in the grid circuit
When converted as described the OUTPUT power will be about 8 watts.
73,
Dan K9WEK
