[TMC] VOX-3 question

Tue Apr 12 02:05:50 EDT 2022

    I don't know much about the VOX-3 but wonder if the VFO is 
similar to the Collins arrangement. The coil in the Collins VFO 
is wound so that as the slug moves the inductance changes at a 
constantly changing rate. In addition the collins VFO or PTO if 
you prefer, has a "corrector" that moves the slug at a slightly 
changing rate to correct for the slightly non uniform winding. 
When the Collins PTO ages the slug changes (forgot the term but 
its effect on the inductance). While the end points of the PTO 
can be changed the linearity of the calibration does not stay the 
same. Adjusting the corrector plates can make it absolutely 
linear again but its a difficult and tedious job. The method of 
adjusting the end points on the Collins PTO is to remove a turn 
or so of the winding. Of course, that changes the position of the 
slug in the winding and changes the "law" of tuning.
    Now, since the adjustment of the TMC oscillator does not seem 
to require changing the number of turns of the coil I wonder if 
it can be gotten back to a setting where the end points are on 
AND the calibration is linear.
    I also wonder if these oscillators drift out of calibration 
for the same reason the Collins ones do, namely a change in the 
properties of the slug.
    In any case, if the coil is wound using a square law pattern 
and a different section of the coil is used the "law" will 
change. Again, it seems to me that if this is what is going on 
its necessary to change the corrector.
     Note that a similar effect takes place with a variable 
capacitor. If the rotor is not centered in the stator the "law" 
of the capacitor will change. That is, the amount the capacitance 
changes with rotation. When this happens the end points may be 
correct but the calibration will have a bump somewhere. In some 
cases the outer plates are bent to correct this but its best to 
make sure the rotor is centered in the stator. This is exactly 
the kind of error created if a PTO slug is not in the right 
place. Permeability, is the term I was trying to remember.  One 
explanation for the Collins PTO going out of adjustment is that 
the permeability of of the slug has changed with time so its 
effect on the inductance has changed.
    This may have nothing to do with the VOX but maybe worth 
knowing.

On 4/11/2022 4:26 PM, Chris Bolkan wrote:
> Hi TMC lovers!
> 
> I now have the VOX-3 back together and working. Everything seems to
> function more or less. The master oscillator works very well putting out a
> nice sinewave throughout its frequency range.
> 
> Unfortunately, I lost track of the exact relationship of the tuning shaft
> to the counter dial, and of exactly where within the almost inch travel of
> the calibration slug, the movement limits are established by the control
> mechanism for it.
> 
> So, even though I set the counter dial to be correct at 3Mhz, it is off a
> bit at each end and I am not sure if placing the calibration slug in
> another segment of its travel will help.
> 
> I did not mess with the fancy eccentric calibration tracking mechanism
> built into the master oscillator capacitor. All I did there was carefully
> clean and lube everything so it all worked again. I think if there was a
> way to calibrate the end points, it might be pretty close in the middle,
> but I am not sure.
> 
> The other thing that doesn't seem exactly right are the frequency
> multipliers. They all work...sort of.....with reduced amplitude and a funky
> relationship to the output control, where the output control "peaks" the
> output rather than just acting like a level control. The output waveforms
> are not super clean either with differing amplitudes over many cycles
> probably indicating they need to be tuned up.
> 
> I am only starting to dive into this, but thought I would ask if anyone is
> aware of a factory calibration or setup procedure for the VOX when it was
> originally being built. I think if one exists it would be very useful to
> me. The VOX-5 manual I have is very helpful but does not dive into the
> areas I took this thing apart so does not provide what I am looking for
> unless I have somehow overlooked it.
> 
> Thanks in advance if you can point me in the right direction!
> 
> Best regards,
> 
> Chris Bolkan
> KI7NXY
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL