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Author Topic: BITX20A Varicap Tuning Mod  (Read 4223 times)
AD6KA
Member

Posts: 2237




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« on: March 17, 2012, 11:48:29 PM »

Hey All:

I have built the Hendricks BTX20A 20m SSB
QRP kit rig. It works quite  well, and I get 10w peaks out
of it. BUT...it has a limited tuning range of about 60 KHz
because it uses a one turn polyvaricon.
You can see the complete-with-frequency display
and "custom" enclosure here:
http://www.qrpkits.com/bitx20a.html
There is also a 17m version but I have not built one.
I was lucky enough to buy mine a while ago when they
ere still offering "Board and Parts Only" kits, not
forcing you to buy their enclosure and their digital
frequency display.

There is a simple mod out there to convert the BITX20A
to using a Ten Turn Pot and a MVAM109 Varicap (or NTE618)
and a couple other simple parts to both make the tuning easier
AND increase the tuning coverage of the rig.

A friend helped me out with a very similar tuning
mod for the Radio Kits UK MKARS80, an *excellent*
and very well priced 80m SSB QRP Kit Rig.It too
used a Polyvaricon tuning scheme and some smart
guy also came up with a ten turn/varicap diode
tuning scheme to ease tuning AND *greatly* increase
tuning capacity (From about 170 KHz to over 300 KHz).
Another terrific thing about the MKARS is that it COMES
SHIPPED with a nice 16 X 2 LCD digital Frequency Display.
http://www.radio-kits.co.uk/mkars80page.html

Note to U.S. QRP Kit Makers: Get With The Program!

Has anyone tried the Hendricks BITX20A varicap tuning mod?

I have heard some good things about it,
I wonder if there are any pitfalls or things like instability
to watch out for?. Yes, I know there is a Yahoo Users
Group for this rig (actually more than one as there are two versions
being marketed, one in India, one here in the USA by
Hendricks' company "qrpkits.com".

Would like to hear from and correspond with
anyone who has experience with this mod.
TNX ES 73, Ken  AD6KA/5R8GQ
« Last Edit: March 18, 2012, 12:01:13 AM by AD6KA » Logged
AD6KA
Member

Posts: 2237




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« Reply #1 on: March 18, 2012, 03:01:30 PM »

Well I went ahead this morning and did the varactor and
ten turn pot tuning mod on the Hendricks BITX20A.
The results were spectacular:

The tuning range went from about 14250 to 14300 KHz
to 14090 to 14345 KHz.
Can't ask for better than that,
and the ten turn main tuning pot makes the included
"fine tuning pot" just about unnecessary.

I am one happy camper. I can't believe that
such a ridiculously simple mod can make such
a terrific difference in performance....but it sure does!  Grin

If you have a Hendricks BITX20 you MUST
do this tuning mod. You'd be a fool not to.


I will admit that I got lucky with where the
edges of the tuning ended up, with the Caveat
that after the mod, you might have to add or
remove a few turns of wire off of L7 in the VFO
to put your range where you want it. Luckily,
I did not need to

The mod schematic can be found here:
http://golddredgervideo.com/kc0wox/bitx20amods/varactormod.htm
Note: Be sure not to confuse it with the other BITX
version mods. This one is for the Hendricks BTX ONLY.

Please write me if you have any questions or suggestions
about this or any other BITX20A mods. Next I am going
to add an N3ZI digital frequency display.
http://www.pongrance.com/ddfc-cc.html
My email is good at QRZ.
73, Ken  AD6KA
« Last Edit: March 18, 2012, 03:03:07 PM by AD6KA » Logged
K4DSB
Member

Posts: 3




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« Reply #2 on: August 03, 2012, 01:17:14 PM »

I too have the mod and its great!  Depending on the diode you use (some versions of varicaps can actually span 300pfd's with a 9 volt change!  Mine was over 350pfd.. I can now tune from 14.110 to just over the top of the band 14.360 Mhz  I love my mod on the Bitx20A


K4DSB
Dean
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