Greetings:
I'm trying to help a friend (WN0X) do some maintenance on his SB-220.
The tuned input for 80M-20M yields a decent input impedance, but on
10M and 15M it's really(!) way off... to the point that on 15M, the
actual resonant point is somewhere around 11MHz(!) and it's almost
that bad (and in the same direction) on 10M.
The SWR/resonant freq. readings were taken with an MFJ-259 Antenna
Analyzer connected to the RF INPUT of the amp and the amp switched
to the TRANSMIT mode, so the analyzer would 'see' the appropriate
load.
I had the opportunity to measure the input impednace of another
SB-220 last week when I worked on the amp belonging to the Univ
of MO (Rolla), W0EEE. On ALL BANDS, the resonant freq. of the
the tuned inputs was slightly below the band, but the Q was low
enough that the input SWR was no higher than 2:1 anywhere across
the bands... maybe a bit higher at the top of 75M.
WN0X brought the capacitor/inductor combinations for 10/15M over
last night and I checked both the capacitor and inductor values
to see how close they were to being correct. The 10M cap./ind.
components all checked out right on the nose. For the 15M
group, the inductor and the output side of the tuned input pi-net
components all checked out OK, but the 150pF dipped silver mica
cap on the input side measured 75pF... half(!) of what it was
marked to be. It would seem that the resonant freq. of the 15M
tuned input pi-net should have INCREASED as a result of the low
capacitor value, but it still presents a resonant freq. of about
11MHz to the exciter.
Since the tuned inputs on the other bands (80-20M) seem to be
functioning about right (e.g. resonant near-band, and broad
enough to present acceptable SWR to the exciter, I cannot figger
out why the 10M/15M tuned input(s) would indicate such a LOW
resonant freq., esp. in light of the halved input cap. value
for the 15M tuned input.
Any thoughts on the subject? HeathKit used the SAME inductor
for both 10M and 15M tuned inputs, just different caps. on the
input and output of the pi-nets.
Oh, yeah... we HAVE confirmed that the tuned input components
are wired in correctly.
Thanks,
73 - Tom Hammond N0SS
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