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: