A100 with a hum

[CousinGJ@aol.com]

   I recently purchased an A100. There is a small problem in the percussion
circuit. I'm hoping someone can help.  There is a low frequency hum that is
always present (B1 played on the keyboard with the fundamental drawbar pulled
is the pitch of the hum). It completely disappears when percussion has been
employed. More specifically, I select the B Preset key of the upper manual
and turn the percussion On/Off switch on, no drawbars need be pulled. The hum
continues until a key is played on the upper manual. The percussion harmonic
will be heard appropriately and with its decay, so too will the hum decay,
and remain silent until the depressed key is released. Then the hum returns.
When other preset keys are selected, the hum is always present. I feel the
problem is in the percussion section of the pre-amplifier. Should I start by
bringing in the pre-amp to a repair shop, or do I need to have a service call
to my home at a considerable cost?  Also, I have a Leslie 147 and want to
adapt it to the A100. I understand I should use accessory kit #7271. Is this
something that just requires a few solderings and carefully following
instructions, or is an experienced technician needed?  I will appreciate any
help.  Thanks, George