I'm back again with more info about this project.
I decided to examine the input network for the SB-200 and compare the
parts and their values to the ones in the SB-221. VERY interesting!
All of the coils had the same part number EXCEPT the 40 meter coil. SO, to
see if my idea was going to work, I took the values from the 15meter coil
set in the 200 and added the only missing part (a 75pf cap) and away we go,
it worked and tuned up great! HOT DAMN, I'M ON A ROLL!! Of to try
20meters. Checked the values and added a parallel cap to the caps already
in place to duplicate the values of the SB200 and BANG, it tuned up like a
champ with a minimal SWR!! YEAH! OK now on to the big one of 80 meters.
Again added the necessary parallel caps and retuned and all was good with
the world. Last but not least was the most critical band as it was the one
the amp would most likely spend most of its time on, 40 meters.
This is where Murphy decided to pay a visit. Put in the matching values
and got 5:1 SWR that would not tune out. CRAP. Ok, so someone else said
the input impedance is about 200 ohms. Off to the computer to use a program
called Pi Net to evaluate the stuff I had been using...
Using a Q of about 3 to 4 and matching 50 Ohms to 200 Ohms in and out on
each of the other bands I had success with produced values that very closely
matched what I had used for the caps. Using that result, I checked 40
meters and got a value of coil that was just under 2 microhenrys. 1.92 to
be exact. Using the wire table from the hand book and measuring the wire
size on the 40 meter coil came up with something just a hair over .026 which
appeared to be a 22 Ga. enameled wire. Counted the turns and came up with
17 and a half. Ok, so that info used to calculate the coil value gave me a
result that was much too small for 40 meters with the cap values I had used
from the SB200 design so I refigured the coil necessary based on 1.92Micro
henrys with a 22 GA. wire and got something like 21 and a half turns. I
took the old wire off and wound the new coil. Something didn't look quite
right however as the new coil took up way too much space on the form
compared to the original. In fact it appeared to fill the form almost as
much as the 80 meter coil winding does!! OOPS! Don't think I got that
quite right but lets try it out and see!!
NOT! Way out in left field... Damn!
OH, I almost forgot, before I removed the old winding but before I added
the additional caps, I tried to use a old Millen dip meter to check out the
resonate point of the input network... It appeared to have a big dip at
around 11Mhz. and not 7Mhz. that I expected. Hummmmm. Add the new caps.
Try dipping again. NOTHING changed! Yet I could still tune the coil with
the slug and make it change the resonate point but the lowest point I could
find was still 11Mhz.!!! HUH!! Unfortunately I lent my MFJ-249B analyzer
out to a friend who hasn't returrned it to me yet so I couldn't just keep
moving stuff around until it became resonate at 7Mhz. while testing at no
power applied. Had to do it the old Hard way. Make a change, hook the amp
back up again and try again to get the input SWR to get down to something
reasonable. Repeat as necessary, again, and again, and again, etc.
So now I have all but 40 meters working fairly well but am completely
frustrated with trying to get it to match.
DOES ANYONE HAVE THE ACTUAL WIRE SIZE AND NUMBER OF TURNS USED ON THE
SB-200 40METER INPUT COIL!!!
The coil used in the SB-200 for 40Meters is numbered "40-592". The coil
used in the SB-201 is numbered "40-1980".
The 80meter coils in both rigs are numbered 40-591.
The 20meter coils in both rigs are numbered 40-593.
The 15meter coils in both rigs are numbered 40-727.
The 10meter coil in the SB-200 is numbered 40-728.
If anyone has one or more of these 10 meter coils lying about and would
like to see them have a good home, please contact me and quote me a price.
I'll also take a 40-592 SB-200 40meter input coil if someone has a spare one
as well. Then all of this would be moot as I would have it working by
now!!!
OH, BTW, to the fellow who told me about using a toroid wound 4:1 balun
as a replacement, I pulled apart a 4:1 balun designed for wire antennas and
took the ferrite rod balun out of it and tried to use it on the input. NOT!
VERY BAD MATCH! I figured one broadband 4:1 matching transformer is about
as good as another at this 100 watt power level but it still didn't work.
So, back to my original request.
Any ideas? I really want to get this thing finished and off my bench
and back to its owner so I can pack up my buddies C-Line and get it back to
him. (Don't have my TS-930 back from repair yet so I needed to keep the
C-line as a testing exciter!) That's another story.
OK guys. As Paul Harvey would say, "now you know the rest of the story".
HELP!
OH, BTW, thanks to all who have and are willing to lend a hand to this
project. I do appreciate the effort.
Mike Baker K7DD
k7ddmjb@qwest.net
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