>
>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.
? A Pi-network (which is two L-networks in series) is typically resonant
slightly below the operating frequency. A 21MHz tuned-input Pi-network
that is resonant at 21MHz, will not work acceptably at 21MHz. At
24.9MHz, maybe.
>
> ..........
>Any thoughts on the subject? HeathKit used the SAME inductor
>for both 10M and 15M tuned inputs,
? The 10-15 m coils' powdered iron slug gives a wide range of L.
Unfortunately, the Q of all the 220's tuned inputs need to be increased
to approx. 2 (XC1=25 ohms) to make the SB-220 compatible with solid-state
radios. This means that each C-in and C-out needs to be increased, and
each L needs to be decreased. . . The fly in the ointment is that
solid-state radios use bandpass filters which introduce various
reactances at the output. Thus, there may be bands where XC1 will need
to have a reactance of other than 25 ohms. (For info on reworking and
adjusting the 220's tuned inputs, see Nov. - Dec., 1990 *QST*, "Circuit
Improvements for the Heath SB-220 Amplifier" // orig. manuscript
available on Web site)
>just different caps. on the input and output of the pi-nets.
>
? To increase the Q to approx. 2, turns need to be removed from the
inductors where backing the slug does not reduce L enough to do the job.
.
- later, Tom
. . . and it's prudent to oil the fan motor every two years.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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