THE TMC VOX VFO PT. 2
RESTORATION & OPERATION
BY JOHN MACKESY VK3XAO
28 MARCH 2000
In The Beginning...
About 15 years ago I acquired a TMC Model VOX Variable
Frequency Oscillator. This came with a Panoramic (Singer) SB12b
"Panalyzor" RF Spectrum Analyzer, and was used generate
the RF signal for the input mixer.
As received, the VOX (Model VOX, s/n 1000) was complete, in good
external condition, and seemed to work OK, except for the heaters
- both open circuit. Further probing suggested that it had been
the victim of a botched conversion from 110 to 220V - the power
transformer had been set to 220, but the heaters were still wired
for 110. This had caused both heaters to burn out. As there was
no pressing need to use the VOX it was seen as a low priority
job, and remained "in storage". In time the Panalyzor
was passed on, to be replaced by one of its more recent
descendants; meanwhile, the VOX hung around in the background,
out of sight, of mind.
I had a bit of a look at it on several occasions, but was always
stymied by lack of information as to just how the heaters were
wired and how the control circuitry worked. I was able to rebuild
the heaters and do the associated mechanical work (some years
ago), but that's as far as it went. Frustrated, I even tried to
give the VOX away, but nobody wanted it! Plundering it for parts
was also considered, but was felt to be ethically unsound. That
all changed recently when (in response to a request to this
group) I received a circuit diagram in .GIF format (http://www.oocities.org/vintagetmc/vox20-330-frschematic.gif).
I now had the missing link!
Mechanical
The first step was removal and disassembly of the oscillator
module. This was kind of interesting, as it's built very like one
of those Russian dolls, an endless series of boxes nested one
within the other, each with a screwed on cover panel. To remove
the assembly, the shaft couplings need to be disconnected from
the front panel controls and the wiring disconnected from the
rear panel terminal strips. Try not to disturb the alignment of
the shafts! The assembly is retained in its box by 4 screws on
the front of the compartment.
Rebuilding the Heaters
The damage to the heaters was not severe - a localized hot spot
had caused each to burn out over a small area. This was bridged,
after which they were checked out, worked fine. As the heaters
run at low temperatures, a short soldered copper wire link was
used. The heater structure is a zig-zag of resistance wire woven
into a coarse fabric mesh, about 5" wide. Each heater is
divided into 2 equal sections - series for 220V, parallel for
110V. Resistance for the outer (150W) heater is 160 ohms per
section, for the inner (18W) heater 1300 ohms per section.
Putting It Back Together
After mechanical reassembly was completed (having first made
notes on which color-coded wire went where AND tagging the wires)
I then produced a drawing of the 220V wiring arrangement. A
19" monitor and PaintShop Pro make a great combination! When
the enlarged section was printed out, I was ready to go.
This was, I felt, one of those jobs best completed in one go,
without interruptions. Come evening, I went into the
workshop/shack, set the VOX up on the bench and selected the
right kind of background noise. In my case, this is a Collins 618T-2 Airborne HF Transceiver,
complete with howling cooling blower, powered by 115V 400 Hz and 28V
DC. Tuned to 8867 Khz USB, listened to air traffic control
from Auckland, Tahiti, Brisbane and San Francisco while I did my
wiring, carefully and meticulously, ticking off each connection
as I went. A couple of hours later, the VOX was finished and
working - apart from the RF output seeming a bit low, it worked
well. One of the major tasks was replacing all the panels, all
retained by numerous machine screws - it's a seemingly endless
chore. Despite being of all-aluminum construction, the VOX is a
heavy (65 Lb) and awkward thing to work on.
Problems
I let it run for a day or so, after which the output rose to a
more reasonable level - I guess this was due to the many years of
inactivity. 3 problems showed up. First, the inner heater wasn't
coming on after warmup - this was solved by moving the outer
heater temp switch (50 deg C) so that it was directly over the
heating element. The effect of this was to shorten its cycling
time and reduce the heat input into the system. The second
problem was moisture forming inside the top (outer) cover of the
module. I suspect the insulation material (sort of a fiberboard)
is somewhat hygroscopic and had absorbed moisture over the years.
After 3 days of continuous operation this has ceased, but I have
the feeling the VOX will need to be run regularly to prevent a
recurrence.
Operation
Frequency stability, as observed with a very accurate digital
counter is not as good as expected, even when thoroughly warmed
up. There's a random medium term drift of several hundred Hz in
either direction, which I suspect may be the 6AB4 VMO oscillator
tube. One further, minor, problem - more an inconvenience, really
- and that's dial calibration. It doesn't seem to track properly,
probably solvable by more messing around with the tuning drive.
Apart from these difficulties (temporary, I hope) the VOX is a
joy to use. With a tuning rate of 1 Khz/turn it's easy to home in
on the frequency of interest, the 100 Khz BEAT indicator works
really well (after calibration), and there's plenty of output for
most purposes.
The Future
Where to from here? Good question. I'd like to get involved with
AM, someday, and the idea of having an all-tube station rather
appeals... Just imagine the kind of conversation you could have!
"Running VFO control here, OM. It's a TMC VOX, 2 to 64 meg,
weighs 65 pounds and soaks up about 200 watts."
Finally...
How old is it? The power supply filter cap is dated 12-59
(thanks, Pyramid!) suggesting a 1960 origin, which ties in with
the companion Panoramic gear. I'd like to thank Denis McCarthy
for his efforts in maintaining these pages, and to Luca Rossi for
posting the VOX circuit and making this all possible.
