[Amps] SB200 tuned input

[George & Marijke Guerin]

Dear Bruce, et. al.,

I looked up my SB-200 notes and found the following.  The input circuits are
L-networks on 80, 15 & 10, The input circuits are Pi-networks on 20 & 40.
For all the input circuits I found:
    on 80, my SB200 input was good at 4000 KHz, not below.  I moved the slug
in a bit and maybe added 40 pfd in parallel with the stock capacitor to get
a good average from 3.5 to 3.8 MHz.
    on 40, mine was OK.  I am sure I tweeked the slug to best SWR around
7100 Khz.
    on 20 mine was OK as built.  I had a friend with a bad input SWR and
found one of the two capacitors was defective.  Replacing it brought it in
the middle of the band with low SWR throughout.
    on 15 meters, I found the value of the inductor to be low and the
capacitor to be high.  I changed the capacitor to either 47 or 51 pfd (maybe
it was 68 pfd originally?).  On one unit I changed the 15 meter coil for a
20 meter coil and found good resonance.  On a second unit I wound three
turns of coil on a pencil as a form and put it in series with the end of the
coil going to the band switch without the capacitor to ground.  Then I had
enough inductance to minimize the SWR in the middle of 15 meters.
    on 10 meters, I also found the value of the inductor to be low and the
capacitor to be high, again this is an L-network.  I changed the capacitor
to either 42 or 44 pfd and increased the inductance, once by using a 15
meter input coil, and the other time by using 3 turns airwould of #16 or #18
hook up wire in series with the stock coil.  For both 10 & 15, the coil
diameter ID is 5/16" by 1/4" long.  That is 8 mm ID x 6+ mm long in metric
units.
    To loosen the slug in the coil, I had to warm an Allen Key with a
soldering iron or torch just a little bit to warm the slug so the wax would
melt.
    I noted some early 15 meter inputs had a Pi-network, maybe pre-1972 when
I bought an SB-200 kit.  I have no experience with them, except the values
should be proportionally lower than on 20 meters, like +/- 66%.

I no longer have any Heathkit amps, only a couple of cooling fans with
plastic blades.

73,    Good luck,    George    K8GG

----- Original Message -----
From: "Ballew, Bruce N" <bnballew at tva.gov>
To: <amps at contesting.com>
Sent: Wednesday, March 26, 2003 9:16 AM
Subject: [Amps] SB200 tuned input


> As a new amp owner, but with and old amp (SB200). I have other question.
(By
> the way thanks to the all that answered my 1st question. We have it
rebuild
> and are up to testing and hopefully operation.) On the tuned inputs on the
> SB200 I have ran the analyzer on the input and find that the only
coil/tuned
> filter that appears to be tuned any were near the band is 40 meters. I
have
> check the SWR on this also and the any band, again that has a low SWR is
40
> meters.  All the other coils/tuned filters are outside the band area. Is
> this normal for the SB200. If so has anyone tested the inputs to see what
> freq. that they are tuned to. And / or is there a way to tune them.  I
would
> like to make sure that they are tuned where they need to be and insure
> proper operation of the amp..
> Thanks for any help.
> Bruce
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