[AMPS] Need to pick your brain...

[Rich Measures]

>
>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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