alphaindia4oscartango

finally after all the years running a stock 20A I’ve done the voltage stabilizing mod and the varactor tuning mod with a 10turn pot, and I think I’ve worked out the L7 windings now to get into the phone section of the band. But as a result, the band spread is only about 25khz.
I remember doing a tuning mod on a sw40+ Using a 10turn pot and the bandspread was small. There I added capacitance in the VFO and get nearly 100khz band spread with a 10turn pot.
I’m thinking that by increasing C39 on the Bitx20A I should get more bandspread?  Has anyone done this? I’m I going in the right direction? I’ll try it nevertheless but I didn’t want to reinvent the wheel. Any guidance? Thanks!
72/73 de Chas ai4ot

John Backo

In varactor tuning, your goal is to maximize the capacitance
variance from one end to the other.

Therefore, the real questions are two: what varactor is
being used, and what is the voltage range it is operating
under.
Note that most datasheets show varactor variance at
best linearity and maximum variance at a particular
voltage range, usually the lower end. So the first thing
is to determine the voltage range. Next, figure out a resistance
range that will exploit that. Usually, the voltage range is about
8v, and the full resistance range is about 10-15k.

It is a relatively simple matter to use 2 pots in series, one of
a higher value to set the frequency position, and one of a lower
range to fine tune. The only real advantage of a 10-turn pot is
to get better gross resolution, but a 10k and 1k regular linear
pot in series will accomplish about the same thing.

As to adding capacitance, that is what the varactor is doing.
It may be that more capacitance is needed to change the
base frequency of the VFO. Usually experimentation is
required, though a rough estimate can be made by figuring
out the total LC ratios. Where to add that capacitance
is another question: probably it will require some in the
main tank and some in the DC isolation of the varactor circuit.

For instance, I modified the VFO for the BITX20 v.3 as follows,
following the example of the BITX20A:

Q5 is a 2N3904; the two main VFO caps are 470 pF polystyrene.
The L4 coil is a T68-6 with 40t, measuring about 9 uH free-standing.
One of the secrets to a stable VFO was paralleling that coil with a
180 pF s.m capacitor. Main tuning is with a 365 pF variable cap,
in series with 2 parallel 220 pF caps, and then into the VFO circuit
of 5-45 pF range-finding cap in parallel with a 220 pF s.m. cap.
The basic tuning range is about 2.895 - 3.295 MHz. The original
test tuning range without the variable cap and a 220 pF in its place
was 2.95640 MHz to 2.965445 MHz (varying the 5-45 pF cap).

For fine tuning, I added a 78L08 regulator running to a 10-turn pot
with a 1K resistor to ground. That ran through a 200K resistor to
a MV2109 (to ground) and then to a 100 pF npo cap leading back
to the main oscillator. This gave a full range of about +/- 25 KHz for the fine
tuning.

So, in essence, the varicap mod added fine tuning. The main VFO
remained the principal tuning for the rig. It works well and is quite
stable.

The IF is 11.092 MHz, not the 10 MHz of the original BITX.

All of this information comes from my original pages of builder's
notes form Leonard's manual. I can publish the pages if you want,
but they are a bit hard to read if you didn't make them up originally.
The rig tunes from about 6.95 to 7.34 MHz.

Hope this helps.

john

Charles D. Smith

I’m using the 78L08 and mv109. I may revert back to the capacitor and fine tuning pot. The real problem I had was stability, which I have now.
72/73 de Chas ai4ot

On Apr 23, 2018, at 9:54 PM, John Backo <jabac@...> wrote:

In varactor tuning, your goal is to maximize the capacitance
variance from one end to the other.

Therefore, the real questions are two: what varactor is
being used, and what is the voltage range it is operating
under.
Note that most datasheets show varactor variance at
best linearity and maximum variance at a particular
voltage range, usually the lower end. So the first thing
is to determine the voltage range. Next, figure out a resistance
range that will exploit that. Usually, the voltage range is about
8v, and the full resistance range is about 10-15k.

It is a relatively simple matter to use 2 pots in series, one of
a higher value to set the frequency position, and one of a lower
range to fine tune. The only real advantage of a 10-turn pot is
to get better gross resolution, but a 10k and 1k regular linear
pot in series will accomplish about the same thing.

As to adding capacitance, that is what the varactor is doing.
It may be that more capacitance is needed to change the
base frequency of the VFO. Usually experimentation is
required, though a rough estimate can be made by figuring
out the total LC ratios. Where to add that capacitance
is another question: probably it will require some in the
main tank and some in the DC isolation of the varactor circuit.

For instance, I modified the VFO for the BITX20 v.3 as follows,
following the example of the BITX20A:

Q5 is a 2N3904; the two main VFO caps are 470 pF polystyrene.
The L4 coil is a T68-6 with 40t, measuring about 9 uH free-standing.
One of the secrets to a stable VFO was paralleling that coil with a
180 pF s.m capacitor. Main tuning is with a 365 pF variable cap,
in series with 2 parallel 220 pF caps, and then into the VFO circuit
of 5-45 pF range-finding cap in parallel with a 220 pF s.m. cap.
The basic tuning range is about 2.895 - 3.295 MHz. The original
test tuning range without the variable cap and a 220 pF in its place
was 2.95640 MHz to 2.965445 MHz (varying the 5-45 pF cap).

For fine tuning, I added a 78L08 regulator running to a 10-turn pot
with a 1K resistor to ground. That ran through a 200K resistor to
a MV2109 (to ground) and then to a 100 pF npo cap leading back
to the main oscillator. This gave a full range of about +/- 25 KHz for the fine
tuning.

So, in essence, the varicap mod added fine tuning. The main VFO
remained the principal tuning for the rig. It works well and is quite
stable.

The IF is 11.092 MHz, not the 10 MHz of the original BITX.

All of this information comes from my original pages of builder's
notes form Leonard's manual. I can publish the pages if you want,
but they are a bit hard to read if you didn't make them up originally.
The rig tunes from about 6.95 to 7.34 MHz.

Hope this helps.

john

kc0wox Leeper

I've modified many 20a's using the directions on my web page. http://golddredgervideo.com/kc0wox/bitx20amods/varactormod.htm The key is the varactor. I would guess that the one you have doesn't give a 400 pfd variation in capacitance.with an 8n 0r 9 volt variation. What is the varactor diode that you are using? I have used the NTE618 and it has a capacitance range of 20pfd at 8 volts and 420pfd at 1.2 volts. Also acceptable is a MVAM108 or MVAM109 Let me know what you are using. Send me your email address to kc0wox@... and I'll get back to you by email.
Leonard
kc0wox

Charles D. Smith

MV109 ... email is ai4ot@...
Thanks Leonard! Your videos helped me to understand so much when I first started with the bitx radios.
I’ve got a 6:1 reduction drive on the cap so I do have a fairly smooth tuning range of about 400khz, plus the fine tuning always puts me right on frequency easily. I don’t “need” varactor tuning or a 10 turn pot. (It’s nice on the cw rigs but not “necessary” with ssb signals being further apart usually.)
72/73 de Chas ai4ot

On Apr 23, 2018, at 10:38 PM, kc0wox Leeper <kc0wox@...> wrote:

I've modified many 20a's using the directions on my web page. http://golddredgervideo.com/kc0wox/bitx20amods/varactormod.htm The key is the varactor. I would guess that the one you have doesn't give a 400 pfd variation in capacitance.with an 8n 0r 9 volt variation. What is the varactor diode that you are using? I have used the NTE618 and it has a capacitance range of 20pfd at 8 volts and 420pfd at 1.2 volts. Also acceptable is a MVAM108 or MVAM109 Let me know what you are using. Send me your email address to kc0wox@... and I'll get back to you by email.
Leonard
kc0wox

kc0wox Leeper

Charles:
A mv109 varactor is not the same as a mvam109. The mv109 ihas 26-32pfd capacitance and a mvam109 has 10-420pfd capacitance.
That would make a big difference in the tuning range.
Leonard

KM4TRT

Hi Leonard,
I was having trouble with that setup until talking to you. I had the mvam 109 but didnt remove the zener because I thought it was already at 9 volts. After talking to you, I removed the resistor and replaced the zener with a 7809 and now it
band spreads from 14.15 to 14.37+(I don't do code but I know to remove or add turns on the coil if i want to).  Great Hack-thanks for your help.  I hated that tuning cap.
Andy KM4TRT