NC38S and 10 turn pot

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From: Gary R. Hanson (ghanson@uts.cc.utexas.edu)
Date: Wed May 28 1997 - 17:06:37 EDT


Hey Gang,

Saw the earlier thread about using a 10 turn pot in the 38s and thought
I would throw my 2 cents in on this discussion.

I ordered a 10 turn pot from Mouser (about $17 for the non-plastic
variety) and installed it in the NC38S. It worked fine for about a
day. Varied the voltage from near zero to 8 volts. Then it stopped
working and when I checked the voltage range it ran from 8.04 to 7.93
through 10 turns. I figured it must be shorted out. I took it out and
pulled the 10 turn pot from my GM-15 (of course 15 meters opened up the
next day:-( ) and the NC38S works fine again. It also worked fine with
a single turn pot in the circuit. The strange thing is that when I
check the resistance with the Mouser pot out of the circuit I get the
full range from 0 to 100K. Is the pot bad and can I return it to get my
money back? If I'm getting the full resistance range out of the
circuit, why wouldn't the voltage track with it in the circuit? If it
is really shorted out, shouldn't I get a restricted range in resistance
or very high resistance reading when I tune through the 10 turns?

After putting the GM-15 pot in the cirucuit, I also experimented a bit
with making the tuning a little more linear. [Got some good advice from
my earlier post.] Originally, my first turn range went from 10.094 to
10.112...a bit much for my way of thinking. I added a 15k resistor
across the wiper and grounded side of the pot and that changed the first
turn tuning range from 10.094 to 10.108. I took out the 15k and put in
a 10k and it dropped the first turn tuning from 10.094 to 10.106+. I
plan to try the resistor/diode combo recommended in the last QRPp and
see how that works next. After the first turn with the 10k resistor, I
get about 3 Khz per turn all the way up through turn 9 to about 10.125.
The last turn goes from 10.125 to 10.129. Once I get the tuning a
little more linear, I plan to add a resistor in series from the grounded
arm of the pot to the NC38S PCB board ground to set my bottom frequency
on 10.100

Any body else using a 10 turn pot? Reactions?

I really like the 10 turn pot as it spreads out the signals, though in
some parts of the tuning range, I have to make more than a half turn to
get from the low side of the signal through zero beat to the high side.
I haven't tried the RIT mod, but if you want to avoid thumps and chirps
and don't mind adding 40% to the cost of your rig, I would highly
recommend a 10 turn pot.

72,

Gary KJ5VW


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