The Johnson Viking Ranger
by Greg Latta, AA8V

Accessory Socket Schematic Diagram and Circuit Description

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Introduction:
The accessory socket on the Viking Ranger is a 9-pin (not octal) socket that provides connections to the transmitter power supply and the modulation transformer secondary. A 9-pin plug normally connects the power supply and modulation transformer secondary to the internal transmitter circuits via three jumpers. The transmitter will not operate normally without the plug installed. 9-pin plugs are very hard to find, so treat the plug with care! If the plug is missing, the jumpers can be installed internally, but it is best to use the plug if possible.

The accessory socket can serve three possible functions. The primary function is to supply power to station accessories. The Ranger power supply is very robust and can supply a substantial amount of power. It has plenty of reserve for small station accessories, but can be used to power larger equipment if the transmitter RF circuits aren't used at the same time.

Another function of the accessory socket is to connect the Ranger to an external high powered modulator. The connections to the modulation transformer secondary are available at pins 1, 2, and 3. These can be used to directly drive the grids of a push-pull modulator.

The last function of the accessory socket is to use the Ranger as a PA amplifier. If a speaker matching transformer is connected to the modulation transformer secondary, nearly 50 watts of audio power are available. Frequency response is limited and not suitable for music, but is perfect for speech. An audio output transformer with a primary plate-to-plate impedance of about 3800 ohms and a secondary impedance of 8 ohms is needed.

Accessory Socket
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Accessory Socket:

Modulation Transformer: The modulation transformer matches the output of the modulator to the final RF amplifier. The transformer turns ratio is selected so that when the modulator is delivering enough power for 100% modulation (32.5 watts) the peak positive voltage in the secondary is exactly equal to the output of the high B+ supply (about 500 volts). Under such conditions, the instantaneous voltage on the final RF amplifier will swing at an audio rate from 0 volts to 1000 volts (twice the high B+ voltage).

High Frequency Cut Capacitor: To help limit the bandwidth of the audio system and to help stabilize the system, capacitor C69 is connected across the modulation transformer secondary. This shorts out some of the higher frequencies, preventing them from modulating the final amplifier tube.

L Network: To prevent any RF from entering or exiting via the accessory socket, L networks are installed on all of the accessory socket leads. These consist of an RF choke which blocks any RF, and a capacitor that shorts any remaining RF to ground. Such L networks are used extensively throughout the Ranger to prevent any RF from getting where it shouldn't.

Modulated High B+ To Final Amplifier: Pin 5 carries modulated B+ from the modulation transformer to the plate of the final amplifier tube. This pin is normally connected through a jumper on the 9-pin accessory plug to pin 3 and the modulation transformer secondary. The other side of the modulation transformer secondary is normally connected to the high B+ supply through pin 2 and a jumper on the accessory plug to the high B+ on pin 6.

High B+ From Power Supply: High B+ of approximately 500V DC is available from pin 6 of the accessory socket. This pin is normally connected through a jumper on the 9-pin accessory plug to pin 2 and the modulation transformer secondary.

6.3V AC To V1, V3, V4, and V5 Filaments: Pin 8 supplies 6.3V AC to the filaments of V1, V3, V4, and V5. It is normally connected to pin 7 through a jumper on the 9-pin accessory plug. If the plug is not inserted, or if the jumper is absent, V1, V3, V4, and V5 will not operate and the RF circuits will be disabled. This feature can be used to disable the RF circuits when the Ranger is being used as a PA amplifier or power supply.

6.3V AC From Power Transformer: 6.3V AC is available from pin 7 of the accessory socket. Be very careful not to accidentally short this connection to ground! A large amount of current is available that can easily burn out the RF choke in the L network connected to this pin or even the pin itself.

Low B+ From Power Supply: Low B+ of approximately 300V DC is available from pin 4 of the accessory socket.

Accessory Plug: The accessory plug is a 9-pin plug that normally contains three jumpers between pins 7 and 8, 2 and 6, and 3 and 5. The jumper between pins 7 and 8 connects the filaments of RF tubes V1, V3, V4, and V5 to the 6.3V AC filament supply. If this jumper is absent the RF stages in the Ranger will not operate. The jumpers between pins 2 and 6 and 3 and 5 connect the modulator transformer secondary in series with the high B+ supply to the final amplifier. If these jumpers are absent the final amplifier will not operate in AM mode.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu