The W8EXI Wingfoot VFO Exciter
High Technology In The 1950's
by Greg Latta, AA8V

Power Supply, Interior, and Rear Views

Wingfoot VFO Exciter Pages:
Introduction and Front and Side View Photos

Power Supply, Interior, and Rear View Photos

Schematic Diagrams and Circuit Descriptions

Special Features

Block Diagram

Chassis Layout

This page continues the tour of the Wingfoot VFO Exciter with pictures of the exciter power supply, interior photos that show the inner circuitry of the exciter, and rear views of the exciter and grid box..

This page is dedicated to Jim Trutko, W8EXI, who was a shining example of what every amateur radio operator should be. We miss you Jim!

Power Supply, Interior, and
Rear Views of the Wingfoot VFO Exciter:
Exciter Power Supply:
The exciter power supply is solid state and very conservatively rated. The supply provides filament, bias, low B+, and high B+ voltages. The bias and B+ supplies use choke input filtering, and the two chokes are visible on the top of the chassis, just in front of the B+ power transformer. Note the use of the Amphenol aircraft connectors on the front of the power supply, which are used throughout the exciter to connect the various assemblies together. The whole exciter is built like a battleship, with 1/4 inch aluminum bulkheads throughout.
exciter power supply
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Exciter Grid Box Interior View:
The interior of the grid box is quite crowded, even though it contains no active components. The grid box contains the oscillator coil, visible in the center of the photo, the main tuning capacitor, between the coil and the front panel, and twenty (yes, twenty!) air variable trimmer capacitors. The twenty trimmer capacitors control the bandset and bandspread for each of ten band segments selected by the band switch (not visible in this photo) on the front of the grid box. The sides of the grid box are 3/8" aluminum and the front and back feature double paneled construction: a pair of inner panels, where the trimmer capacitors are mounted, and a pair of exterior, cosmetic panels. The main tuning capacitor is operated via a National planetary reduction drive.
Grid Box Interior
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Exciter Grid Box Rear View:
This rear view of the grid box shows the two SO-239 connectors that connect the grid box, via a pair of coaxial cables, to the main exciter chassis. Note how the connectors are mounted on a sub-panel, below the exterior cosmetic panel. Ten of the trimmer capacitors are accessed through chrome covers on the back panel. (The other ten are similarly accessed from the front panel.) The frequency of each trimmer, and the function of each of the coaxial connectors, is embossed directly into the back panel.
Grid Box Back
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Exciter Oscillator/Cath. Follower/1st Buffer Stages:
Though the grid box contains the LC circuit for the oscillator, the 6AG7 oscillator tube and other oscillator components are on the main exciter chassis. The chassis compartment closest to the front contains the oscillator, cathode follower, and 1st buffer/multiplier circuits. The Clapp VFO oscillator is followed by a 6J5 cathode follower to isolate it from the following circuits, and the cathode follower is then followed by a 6AG7 buffer/multiplier. On the 80m, 40m, 20m, 15m, and 10m bands the oscillator runs on 3.500 MHz. The buffer/multiplier stage operates straight through on the 80m band, doubles to 7 MHz on the 40m and 15m bands, and quadruples to 14 MHz on the 20m and 10m bands. On the 30m band the oscillator runs at 3.367 MHz, which is then tripled to 10.100 MHz. Clockwise from top, the coils mounted on the bandswitch tune the multiplier output to 10.1 MHz, 14 MHz, and 7 MHz respectively.
Oscillator Stage
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Exciter Driver/2nd Multiplier Stage:
The output of the 1st buffer/multiplier stage is fed through a 1/4" aluminum bulkhead to the 6AG7 driver/2nd multiplier stage. On 80m, 40m, and 30m the stage operates straight through. On 15m the stage triples from 7 MHz to 21 MHz, and on 10m the stage doubles from 14 MHz to 28 MHz. The output is fed through another 1/4" bulkhead to the final amplifier. The coils around the bandswitch tune the plate circuit to the desired output frequency. At lower left are the sockets and associated circuitry for the 0A2 and 0B2 regulator tubes which regulate the oscillator plate voltage and the screen voltage for all stages except the final amplifier.
Driver Stage
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Exciter Final Amplifier and Keying Stages:
This crowded compartment contains the final amplifier, final amplifier screen control, and differential keying circuits. The final amplifier is a 2E26 beam power tube. Power output is a very conservative 27 watts. Power output is adjusted by electronically controlling the screen voltage on the 2E26 with 5814A (12AU7) tube. Output is controlled by adjusting the potentiometer at the upper left from the front panel. The exciter also features grid-block differential keying using another 5814A tube. With differential keying, the oscillator is keyed on before the rest of the exciter, and keyed off after the rest of the exciter. This eliminates any chirp in the output signal, and gives the exciter its exceptional sound. The string of resistors at right is used for metering the various stages in the exciter. The resistors are in series with all important tube elements in the exciter. By measuring the voltage across any resistor, the current through that tube element can be determined. This was an invaluable troubleshooting feature while restoring and redesigning the exciter!
Final Stage
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Exciter Back Panel:
All of the connections to the main exciter chassis are made on the back panel. The antenna connector is at the upper left, the the two connectors for the VFO grid box are at the lower right. The power supply connects to the male Amphenol connector at the lower left, and the meter panel connects to the female connector just below it. The key connects to the 1/4 jack in the middle, and the control just below the key jack is used to adjust the differential keying. Note how the function of each connector is embossed directly into the metal. This is also done for the tube sockets, for the buffer and driver coils, and for the trimmer capacitors on the grid box.
Exciter Back
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Amplifier ImageClick here for pictures and information on the matching Wingfoot 813 Amplifier

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