Jooles wrote:

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What would cause a "dreadfully out of tune" and a "bad sound" in certain pieces of music on that organ?

The problem as described to you in regards to a bad contact or bad coil rewind is the answer.

The Hammond tonewheel organs used combinations of the outputs of the various tonewheels to generate the desired notes. The early 96 wheel units simply added the tones together.

If I recall correctly, the X-66 and other "hybrid" Hammonds used the solid state electronics as multipliers to get the desired notes. So any failure of the electronics (dead/out-of-tolerance) would alter the tones radically.

If you were to do an online search, there are several companies out there who specialize in Hammonds and could guide you further. I believe Organ Service Company in Illinois here in the US could even provide a service manual for your organ.

I have a Hammond BCV waiting to be restored myself. [Linked Image]

Good luck and keep us posted!

Stupid should be painful.