The TMC Model VOX VFO
By John Mackesy VK3XAO

24 March 2000

Part 1 - General Description

The VOX is an all-vacuum-tube (13 tubes) VFO covering 2 to 64 Mhz, with provision for 4 crystal-controlled HF frequencies and 2 low-frequency crystal-controlled BFO frequencies. Physically, it's a 19" rack mounting instrument, 10 1/2" H x16 1/2" D, and weighs 65 lb. Power is 110 or 220V 50/60 Hz.

Structurally, the VOX is divided along the centerline into 2 compartments. The LH compartment contains the Power Supply Chassis and the RF Chassis, the RH (subdivided horizontally) contains the temperature-controlled VFO and 100 Khz xtal calibrator assembly (rear compartment) The tuning drive, control extension shafts, thermostat relay and indicator lamps occupy the front compartment.

Front-panel controls

Tuning
Frequency readout (mechanical digital indication)
Calibration
Output
High Frequency Oscillator (HFO) tuning: 2-4 Mhz
Band selection: 2-4 Mhz, 8-16 Mhz, 16-32 Mhz and 32-64 Mhz
VMO (Variable Master Oscillator)/Xtal switch
Xtal tuning.

Indicator lights

Power, Inner Oven, Outer Oven, Beat

At the top LH side of the front panel is a small door which gives access to the Power on/off switch, with switches controlling HFO, Intermediate Frequency Oscillator (IFO), Beat Frequency Oscillator (BFO) plus a meter selector switch and a phone jack for aural beat monitoring. The output meter is at top RH of the panel.
By now you should have some appreciation of what the VOX looks like and its control facilities. In the next section I'll describe the various sections of the VOX, starting with the Variable Master Oscillator (VMO).

VMO

The VMO is a conventional Colpitts oscillator using (in my VOX) a 6AB4 triode, although the drawing shows a 6C4. Output is to one-half of a 12AU7 cathode follower; the other half of the 12AU7 is used as a 100 Khz xtal oscillator. The 100 Khz oscillator is used as a calibration reference (beat oscillator). Linear tuning in the range of 2 to 4 Mhz is by a mechanically linearized variable capacitor. The tuning capacitor and its associated mechanism is very precisely and ruggedly executed, something of a work of art, in fact. The associated inductor is adjustable over a limited range by the CALIBRATION control. A trimming capacitor is provided to adjust the 100 Khz xtal oscillator (accessible from the rear panel). The complete oscillator assembly is rigidly mounted in a cast aluminum box with external heaters for temperature stabilization.

There are 2 heaters; the Outer Heater (150W) and the Inner Heater (18W). The outer heater is controlled by a 50° C temp switch, the inner by a 70° C glass-encapsulated thermostat (normally open). This controls a (normally closed) relay which switches heater power on and off as required to maintain a stable temperature. There is also an 80° C temp switch inside the oven to protect against over-temperature due to thermostat or relay failure.

The mesh-type heaters (separated by a layer of thermal insulation) are wound around the oscillator box. Each heater is equally split; series for 220V operation, parallel for 110V. All the wiring (there's a fair bit of it!) is brought out to 2 rear-panel terminal strips. Heater activity is indicated by front-panel neon lamps. The whole assembly occupies the RH rear compartment and measures approx 7" x 7" x 7". The 6AB4 and 12AU7 tubes can be accessed by removing a small panel at the rear of the assembly.

The VMO tuning is linear over the range of 2 to 4 Mhz and is controlled by a 2 1/4" diameter knob. Frequency is indicated by a large 4-digit mechanical counter for Khz, Hz indication is by an analog scale attached to the tuning drive shaft; tuning rate is 1 Khz per turn. At 1 Khz/turn it takes quite while to make large frequency changes; according to my drawing a motorized tuning drive is available, although (sadly) this is not fitted to my VOX.

The RF Chassis

The RF Chassis occupies the bottom of the LH compartment and contains 3 RF amplifier stages (6C4 and 2 x 6AQ5) and 3 x 6AQ5 multiplier stages. These stages receive the 2 to 4 Mhz output from the VFO and amplify (2 -4 Mhz range) or multiply and amplify as required. Output is to 3 parallel BNC sockets. Output into 50 ohms (measured on my VOX) is 15V P-to-P at 4.0000 Mhz. For fixed-frequency operation (XTAL mode) the 2-4 Mhz input to the multiplier is generated by a Pierce oscillator using the 6C4 RF Amp stage as an oscillator.

There are 3 switch-selectable crystals; frequency can be trimmed by the HFO-XTAL Freq control capacitor. There is also a fixed xtal 12AU7 Intermediate Frequency Oscillator (3.2080 Mhz in my VOX), one half of the 12AU7 used as the usual Pierce oscillator, the other half as a buffer.

The Power Supply Chassis

The power supply is a conventional choke input type, using a 5V4 rectifier tube, with a B+ output of 300V. An 0A2 tube provides a regulated 150V for the oscillator stages. Transformer and choke are high-quality hermetically sealed types (as you would expect!) This chassis also contains the mixer (6BE6), audio output (12AU7) and BFO (6AQ5) stages. The BFO has sockets for 2 xtals (rear panel selected) and uses a 6AQ5 in a Pierce oscillator circuit.

The Power Supply Chassis also mounts the output meter selector switch and switches for the HFO, BFO, IFO and Beat Oscillator. This chassis is retained in the mainframe by 4 Dzus fasteners and is readily removable to give access to the RF chassis directly below it. A 12-pin Jones connector on the rear of the chassis couples the Power Supply to the rest of the VOX.

Next:

Part 2 - Restoration and Operation Part 2 - Restoration and Operation Part 2 Restoration and Operation