The resistor panel is a small phenolic terminal strip mounted on the back of the upper manual in Hammond B-3's and similar organs. It contains the resistor network responsible for mixing the various harmonics for the pedal tones. It also has three terminals which are used to patch the 4th, 5th and 9th wires from the B preset bundle to the wiring harness running to the percussion switch assembly.
There's a picture of a A-100's resistor panel at:
http://theatreorgans.com/hammond/faq/files/bri-pics/73291_24.jpg
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