Help with SB-200 input circuit

[Multi-Volti Devices]

I think most people will support this comment, and that is that from a
solid state rig perspective, yes the input circuit does stink. It was
intended for a tube rig with an adjustable pi-network output stage,
which was much more tolerant/accepting of swr.

I think AG6K did an article on SB-220 mods that addressed revamping the
input stage for solid state rigs. I think the premise was designing new
pi-net inputs with higher Q (something like 3-5, and the original may
have been lower, if I remember).

Murray

Jim Mullen wrote:

> Hi and thanks for reading this.
>
> I have an SB-200 that will not present a good load on 10 and 15
> meters. The
> input coils seem to have no effect when tuning them. Checking the
> setup
> with a dip meter, reveals good, in-band dips on 80, 40 and 20, but a
> real
> strong dip on about 28 and 35 MHz when the band switch is set to 15 or
> 10
> meters.
>
> I pulled the entire coil & switch assembly out of the amp, just
> sitting on
> the bench, I get a very good dip meter dip at 28 MHz.
>
> Am I chasing my tail here, because the circuit stinks, or am I missing
> the
> obvious? Beats me!  My best guess at this time is the whole assembly
> is
> self-resonant at around 28 MHz, and is swamping out the tuning ability
> of
> the individual 10 and 15 meter circuits.
>
> The switch and all contacts look fine, coils are in good shape,
> capacitors
> test OK, and are the right value. Coils and switch are wired
> correctly.
>
> I'm driving the amp with a Kenwood TS-440. Amp tubes aren't the
> greatest,
> but shows about 500W key down on 80, 40 and 20.
>
> Any help or ideas would be greatly appreciated.
>
> Jim Mullen - WA1ZUH
> 144 Tower HIll Road
> Brimfield, MA  01010
> (413-245-3228