[Amps] L4B Problem

[dan hearn]

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 ?. Will use mfj 259 and
resistor to check the input.
2. Is the input circuit a pi network?
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?
5. Is the input band switch driven by cable or chain from the plate
bandswitch? Might have a indexing problem.
  Any suggestions about what to look for will be appreciated.

73, Dan, N5AR