[TMC] VOX-3 question

Tue Apr 12 11:24:20 EDT 2022

Thank you Richard,

The VOX uses a slug tuned inductor (L-301) for "calibration" but I don't
know exactly what that means. It is a front panel control. I suspect it is
intended to be used with the 100KC calibrator to make the tuning dial
exactly align with the zero beat from the calibrator. There are also three
variable capacitors in parallel. Two are driven by the tuning knob. One of
those  two (C-301 I think) is directly driven by the gear reduction
mechanism and the other (C-302 I think) tracks an adjustable metal band
"cam" that fine tunes the frequency tracking to the dial counter. I believe
the third (C-303) is a trimmer that may have been used to set one end
point along with the coil (L-301) used to set the other end point. That's
what I am thinking anyway.

It's pretty complicated. I am told it was calibrated at the factory in a
custom made oven with slots and holes to access all of the adjustments
while it was running at stabilized temp. I am hoping the procedure exists
somewhere or someone can remember exactly how it was performed and what
components were used for particular adjustments so I can try and figure out
how to get it as close as I can without screwing it up. Fortunately I made
no changes to the factory settings when I removed it for lubricating. I
don't think I can realistically perform the original factory procedures
even if I can obtain them, but the procedures would inform me as to what
does what so I can do the best with what I have.

I'll keep tinkering and report back if I figure anything out.

Best regards,

Chris
KI7NXY

On Mon, Apr 11, 2022 at 11:06 PM Richard Knoppow <1oldlens1 at ix.netcom.com>
wrote:

>     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
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