>A friend has a problem with his L4B. It works fine on all bands but 20m.
>When he goes to 20m and puts it in transmit mode, the moment he hits the
>key, the FT1 exciter shuts down. Presumably due to high SWR. It has
>worked OK on 20 in the past. Based on what he tells me, I assume
>probably there is a problem in the bandswitch or the pi network input
>for 20m. Before I drive over to help him fix the problem I could use
>some information.
>1. What is the input Z of a pair of 3-500's ?.
C. 65-ohms.
>Will use mfj 259 and
>resistor to check the input.
This should work to check the tuned input without drive.
>2. Is the input circuit a pi network?
yes.
>3. Is it slug tuned or trimmer cap tuned?
>4. In case there is a cap failure in the input, What are the cap values
>so I can take replacements?
What Drake used in the days of tube-type exciters may not be optimal for
solid-state exciters. A Q of 2 - 3 is what's needed -- which means that
original L needs to be less and the in and out caps need to have more C.
360pF would be a good starting point for the input C on 20m. L and the
output C are best determined with 50% to 100% of the normal drive
applied. If the tuned inputs are adjusted with less P, the g-g cathodes
will present a false load to the tuned input. // Since the HV PS is
not up to AØ duty, keying the exciter with 50wpm dits (or a tuning
pulser) would not be a bad idea.
gud luck, Dan
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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