K1 tuning nonlinearity mod?

[Cathy James]

I recall seeing a reference somewhere to a mod for the K1 tuning pot
circuit.  The mod causes the pot to change frequency about 13 kHz per
turn at the low end of the band (thus giving easy tuning in the portion
where most ops will spend most of their time, and 22 kHz per turn at the
high end of the 170 kHz tuning range (thus giving access to all of the
40 meter CW band, at the price of touchy tuning in the seldom-used portion).

Unfortunately I can't seem to find the references now.  Can anyone point
me to information on this mod?  Does it work as advertised?  I am not
familiar with the exact relationship between the voltage across the
varactor diode and the resulting capacitance, so I can't compute it
directly.  I *have* found some pointers on how to linearize the tuning
action at about 17 kHz per turn, but that is less ideal than the 13/22
arrangement.

Thanks for any replies.

Cathy
N5WVR

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[Mike Morrow-3]

Cathy wrote:

>I recall seeing a reference somewhere to a mod for the K1 tuning pot
>circuit.  The mod causes the pot to change frequency about 13 kHz per
>turn at the low end of the band (thus giving easy tuning in the portion
>where most ops will spend most of their time, and 22 kHz per turn at the
>high end of the 170 kHz tuning range (thus giving access to all of the
>40 meter CW band, at the price of touchy tuning in the seldom-used portion).

The original K1 of five years ago did not have a 100K resistor across the VFO potentiometer, and there were discussions on how to alter the non-linearity of the tuning by placing resistors across the potentiometer.  I wrote in 2001:

> ... I collected data for my K1 with C2 = 120 pF:
>
>(1)  As designed (no resistor from terminal 1 to 2)
>(2)  100k ohm resistor from terminal 1 to 2
>(3)  220k ohm resistor from terminal 1 to 2.
>
>VFO Pot  As Designed   100k Resistor    220k Resistor
>  Turn      Increment       Increment        Increment
>Number    kHz/turn          kHz/turn           kHz/turn
>    1          17.7                  15.9                17.6
>    2          19.8                  15.6                18.0
>    3          22.1                  16.1                18.7
>    4          23.2                  16.6                20.0
>    5          21.7                  18.1                20.2
>    6          18.2                  18.6                18.5
>    7          13.7                  17.5                15.8
>    8          11.6                  16.0                13.6
>    9          10.5                  16.0                13.1
>  10          10.9                  18.4                14.3

R19 at 100K was eventually made a part of the K1 because it resulted in the best consistent tuning rate across the VFO span.

However, Reed/K7FLY wanted exactly what you want, and he had earlier reported the following results with a 68K resistor:

> ... With the 120pf cap and 68K mod, the results:
>
> Freq  Diff
> 0
> 15    15   Finer tuning at this end
> 29    14
> 43    14
> 58    15
> 75    17
> 93    18
> 112   19
> 131   19
> 150   19
> 170   20   Coarse tuning at this end

The results of all this then showed that to get finer tuning at the low end, use something less than 100K.  

Were I you, I'd try paralleling another 100K resistor across the existing R19 (at the VFO pot terminals).  That would give an effective value of 50K, and result in even finer kHz/turn at the low end, and coarser tuning at the high end, than Reed reported.

There is one thing to consider if you do this.  If you have a filter board with 30m on it, then you will suffer from the coarse tuning because the 10.1 to 10.15 MHz band coverage is at the high end.

Personally, I like the 100K option best.  The K1, contrary to many assertions, does not tune too fast anywhere, even with the 170 kHz span option.  The problem with K1 tuning is that the VFO pot shaft (either the supplied pot or the metal shaft version) has very very little resistance to rotational motion, so it is hard to remove one's fingers from the dial without imparting a small amount of undesired alteration to the dial setting.  The best thing I ever did was hand cut some circular thin felt "washers" to place behind the VFO knob to add a very small amount of resistance to rotation.  If you use very thin felt such as can be found at a hobby store, you can add just enough layers to get exactly the right feel.

73,
Mike / KK5F
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