[AMPS] Need to pick your brain...

[km1h@juno.com]

On Fri, 16 Apr 1999 15:07:55 -0500 Tom Hammond <n0ss@socket.net> writes:
>
>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.

In this case look at it in reverse. The input cap is for impedence
matching to the 50 Ohm exciter. The output cap and L are the prime
frequency determining elements.

See what happens with 150 pf.

Also remove the slugs and examine them. A cracked slug or one that has
been overheated will throw off all adjustments.

73  Carl  KM1H



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