[HAM] Motorboating vibrato

[Eric S.]

My new A-100 has the dreaded motorboating vibrato. I believe the problem 
is dendrites in the  vibrato switch. 

Evidence: vibrato/chorus sounds good w/ vibrato tab half way down, but 
motorboats when completely engaged.  Interestingly, in this halfway 
position, one switch turns on vibrato/chorus for both manuals.  In the 
fully engaged position,  each switch controls it's respective manual 
normally.

Question 1- Have I made the correct diagnosis?

If so, I'd like to try 'zapping' w/ 9 volt battery(s) before going 
through the trouble of removing and disassembling the switches. Although 
I've found references in the Wiki and the archives, I haven't found the 
info on WHERE to zap the switches.  Can someone enlighten me?  Between 
ground and where?  Need to unsolder???

Thanks again.
Eric S.