The Johnson Viking Ranger
by Greg Latta, AA8V

Oscillator Schematic and Circuit Description

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General Information:
The heart any transmitter is, of course, the VFO itself. It is here that the ultimate stability of the transmitter is determined. The series tuned Colpitts or Clapp oscillator used in the Viking Ranger is by and far the circuit of choice for this application. The circuit features a large inductance to capacitance (L/C) ratio which limits the current flowing in the main oscillator coil. Limiting the current in the coil minimizes temperature changes in the coil which lead to drift. The circuit also uses very small coupling between the tuned circuit and the oscillator tube, which minimizes the effects that changes in the tube (such as those that occur when the oscillator is keyed) have on the frequency of the oscillator. This helps to minimize the chirp (frequency change) that occurs when the oscillator is keyed, resulting in a much better sounding CW signal.

The Ranger VFO operates over three frequency ranges. Two electrically independent bandspread/bandset/coil combinations are selected by an internally mounted VFO band switch that is linked to the front panel band switch. A third VFO switch position switches in extra capacitance on the 11m band. The two basic ranges covered are 1750kHz - 2000kHz and 7000kHz - 7425kHz. For clarity, the VFO bandswitch connections and Operate switch connections are NOT included in the schematic below. The schematic below assumes that the Band Switch is in the 160m position and the Operate switch is in the VFO position. Locations in the circuit that connect to the band switch or operate switch are notated and marked with an "X".

VFO Circuit - Series Tuned Colpitts (Clapp) Oscillator
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Series Tuned Colpitts (Clapp) Oscillator:

Main Coil: The oscillator is the heart of the transmitter, but the oscillator coil is the heart of the heart. The coil must be constructed to minimize any changes in inductance due to temperature changes. Even the normal oscillator current in the coil can cause sufficient heating to alter the inductance of the coil. It is best to use an air core coil, since changes in the permeability of ferromagnetic materials (powdered iron, etc.) with temperature will cause changes in inductance. The wire should mounted so that no movement is possible. In a Clapp oscillator, the tuning capacitance should be as small as possible, which requires that the inductance to be relatively large. The Viking Ranger use two electrically separate coils wound on the same ceramic form. L1B is used used on the 160m and 80m bands, and L1A is used on the other bands.

Tuning, Bandspread, Bandset, and Temperature Compensation Capacitors: This tangle of capacitors is likely to scare anyone off, but in reality it isn't as complicated as it looks. The entire arrangement is nothing more than the equivalent of a temperature compensated variable capacitor with adjustable maximum and minimum values. This arrangement was very common in the 1940s and 1950s, and allows the range of frequencies covered by the oscillator to be easily adjusted. The values shown in the schematic are the maximum values for the variable capacitors C5 and C6. The actual values are determined when the VFO is calibrated. Variable capacitor C6 and fixed capacitor C17 are in parallel and form a variable capacitor with a maximum value of 180pf. They are in series with main tuning capacitor C1B, and the combination forms a variable capacitor that changes value by an adjustable amount as C1B is varied from its minimum to maximum position. The smallest change in value occurs when C6 is set to its minimum value (plates fully unmeshed). The maximum change in value occurs when C6 is set to its maximum value (plates fully meshed). Because the change or spread in capacitance is controlled by C6, it is called the bandspread capacitor. Variable capacitor C5 and temperature compensating capacitor C16 are in parallel and form a variable capacitor with a maximum value of 106pf. C16 is an NP080 capacitor which means that its value decreases by 0.008% for each 1°C rise in temperature. This is to eliminate frequency drift by offsetting other changes that are occurring throughout the oscillator as the temperature changes. The value of C16 and its temperature coefficient were determined largely by experiment when Johnson designed the Ranger transmitter. The capacitance of C5 and C16, the bandset combination, is added to the effective capacitance of the bandspread and main tuning capacitors and allows the maximum capacitance of the entire combination to be adjusted. Since C5 sets the maximum capacitance of the entire combination, it is called the bandset capacitor. Typically, the main tuning capacitor is fully opened and the bandset capacitor adjusted to the highest frequency desired. The main tuning capacitor is then fully meshed and the bandspread capacitor is adjusted to the lowest frequency desired. This process is then repeated over and over until the desired results are obtained. This process is called "aligning" or "calibrating" the VFO.

Grid Coupling Capacitor: The effective tuning capacitor discussed above (the series/parallel combination of the bandspread, main tuning, and bandset capacitors) is connected in series with the grid coupling capacitor C14 and cathode feedback capacitor C15. The effective capacitance of these three series connected capacitors is then connected in parallel across the main tuning coil, forming a parallel resonant circuit. The fact that the junction between the effective tuning capacitor and the cathode feedback capacitor is grounded does not affect this result. The voltage developed across the main tuning coil is unequally divided across these three capacitors in inverse proportion to their capacitance. The HIGHER the capacitance, the SMALLER the voltage developed across it. Most of the voltage appears across the effective tuning capacitor. The remainder is split evenly between the grid coupling capacitor and the cathode feedback capacitor. The portion that appears across the grid coupling capacitor C14 is applied through C18 and R2 across the grid and cathode of the 6AU6 tube, where it is amplified. Because of the small value of C18 and the lower percentage of the total tank voltage that appears across C14, changes in the tube caused by, for example, keying the oscillator or changing the tube, have little effect on the oscillator. It is this weak coupling between the resonant circuit and the input of the tube that is an important characteristic of the Clapp oscillator.

Cathode Feedback Capacitor: The signal applied to the grid of the tube is amplified and part of the amplified signal is taken from the screen grid (not the plate) of the tube and is shunted to ground through the screen bypass capacitor. This signal must get back to the cathode of the tube, but it can't pass through the cathode RF choke, which blocks any RF. Instead, it travels from ground back through the cathode feedback capacitor C15 to the cathode of the tube. As it travels through the feedback capacitor, it causes a small voltage to appear across the capacitor. This voltage is in phase with the oscillation in the resonant circuit and tends to assist the oscillation, restoring any energy that was lost. The voltage across a capacitor is in inverse proportion to the charge on the capacitor. Because of the large size of the feedback capacitor, only a small voltage is developed across the capacitor. This minimizes the effect of the feedback on the oscillator, keeping the feedback to a minimum. This keeps the size of the oscillation in the resonant circuit as small as possible, minimizing any heating of the main tuning coil caused by the oscillating current. It is this weak coupling between the resonant circuit and the output of the tube (feedback) that is another important characteristic of the Clapp oscillator.

Grid Leak Resistor: When the circuit is oscillating, some of the RF is rectified by the diode action of the grid and cathode. This causes a voltage to develope across the grid leak resistor (R1 and R36 in series), providing operating bias for the tube. Resistor R1 also allows the grid-block keying voltage to reach the grid of the tube while keeping the RF on the grid from flowing back to the keying circuit.

Grid Leak Capacitor: The 500pf grid leak capacitor smooths out any voltage variations across the grid leak resistor keeping the tube bias steady. The 500pf capacitor also bypasses any RF leaking through the resistor to ground, preventing it from getting to the keying circuit.

Grid Resistor And Capacitor: Grid resistor R2 and capacitor C18 provide additional isolation between the grid of the VFO tube and the tuned circuit. The extra isolation helps to prevent changes in the tube operating conditions from affecting the frequency of oscillation.

Keying Input: Grid block keying is used in the Johnson Viking Ranger. While the key is up the keying circuit applies a negative voltage through the grid leak resistor to the grid of the tube, shutting it off. When the key is pressed, the blocking voltage is removed and the circuit begins oscillating.

6AU6 Tube: The 6AU6 is a 7-pin sharp cutoff pentode with high gain, and is perfect for use as an oscillator. The small size and lower heater power allow it to be confined inside of a cabinet without overheating. You can click here for a 6AU6 Tube Data Sheet.

Cathode RF Choke: Direct current must be allowed to flow to the tube cathode, but RF must be blocked so that it flows through the cathode feedback capacitor instead. This is done by connecting the cathode to ground through an RF choke, which blocks RF but passes DC.

Screen Bypass Capacitor: The screen grid of the tube serves as the plate of the oscillator in an electron coupled oscillator. Some of the RF amplified by the tube must get back to the resonant circuit to support oscillation. The screen bypass capacitor allows the RF on the screen grid to pass through to ground while preventing the screen DC supply from being short circuited. The RF eventually flows through the cathode feedback capacitor back to the cathode of the tube. When it flows through the cathode feedback capacitor, it provides the necessary feedback to keep the oscillator going.

Plate RF Choke: Direct current must reach the plate of the tube for proper operation, but the RF appearing on the plate must not be permitted to reach the plate power supply. The plate RF choke allows the DC to pass through, while preventing any RF from getting through.

Output Control Resistor: The output of most oscillators normally varies with frequency, and this is more so in the Clapp oscillator circuit. When the VFO band switch is changed from the 160m/80m position (1750kHz - 2000kHz) to the position for all of the other bands (7000kHz - 7425kHz) the quadrupling in frequency causes the oscillator output to drop considerably. To keep the output relatively constant, resistor R4 drops the oscillator plate voltage in the 160/80m position. When the VFO bandswitch is switched out of the 160m/80m position, contacts on the VFO band switch short out resistor R4, raising the plate voltage on the oscillator, increasing the output, and compensating for the change in frequency. It is this attention to detail that makes the Ranger one of the best transmitters of the 1950s.

Plate Bypass Capacitor: As extra insurance that any RF from the plate of the oscillator does not get into the plate power supply, a capacitor is used to short circuit any RF that might have leaked through the plate RF choke to ground.

Output RF Load Resistor: R6 is connected to the output of the oscillator and loads the oscillator for RF, since the bottom end is grounded through the plate bypass capacitor. Loading the oscillator lowers the overall Q of the circuit and tends to equalize the output from one end of the tuning frequency range to the other, keeping the oscillator output relatively constant.

Plate Coupling Capacitor: RF appearing at the plate must be fed to the next stage for amplification. However, the DC plate voltage must not be allowed to pass through to the next stage. The plate coupling capacitor allows the RF to pass through while blocking the DC.

Filament and Cathode L-Networks: There are many L-networks scattered throughout the Ranger transmitter. These block RF more effectively than a bypass capacitor or RF choke alone, and are used to prevent RF from going from one place to another. The L-network is very effective because the capacitor shorts any undesired RF to ground, while the RF choke blocks any RF that remains, and vice-versa. The cathode network prevents any RF that may have escaped through the cathode RF choke from getting through to the Crystal/VFO switch, and the filament network prevents any RF on the filament from getting away into the rest of the transmitter. The networks work either way, regardless of which direction the RF is coming from, so they also prevent stray RF from getting back to the cathode or filament from elsewhere in the transmitter.

Cathode L-Network Filament L-Network

VR Tube and Dropping Resistor: The frequency of the VFO is very sensitive to changes in the screen voltage, which functions as the plate of the electron coupled oscillator. A gaseous voltage regulator tube is used to keep the screen voltage constant, regardless of voltage variations in the low B+ supply. Resistor R3 is chosen so that the VR tube remains lit at all times. This guarantees that the voltage across the VR tube (and thus the oscillator) is constant. Resistor R3 is one of the weaknesses of the Ranger circuit. It is inside the VFO cabinet and is not well ventilated, so it often burns out. If you have never checked R3, remove the left side of the VFO cabinet to access it and replace it if it has burned out. You may have to remove the entire VFO cabinet to get to it. Use a 3W film resistor if you replace it. Do not use a 2W carbon resistor as was used originally. The carbon resistors are unstable and subject to thermal runaway.

Metering Resistor: When the "Crystal/VFO" switch is set to VFO or ZERO both the oscillator (V2) and buffer (V3) cathode currents flow through the metering resistor, creating a voltage drop across the resistor. (The connection to V3 is not shown in the schematic diagram above.) When the front panel meter is set to the "Osc" position this voltage drop is read and is used by the meter to indicate oscillator/buffer cathode current. When using crystal control, the cathode of V2, the VFO oscillator, is disconnected, and the meter indicates only the crystal oscillator (V3) current.

B+ Dropping Resistor: Resistor R5 drops the low B+ voltage down to the value needed by the plate of the VFO tube. In conjunction with the output control resistor it determines how much the VFO output changes when the VFO band is changed.

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