I posted questions at the Hammond Zone yahoo group and Keyboard magazine forums and got some great advice.
Using this advice, I tested the wires that connect the scanner to the vibrato filter box and the switches. These all tested good.
There appears to be evidence that someone has (poorly) resoldered these wires in the past:
Here is an image of the scanner in the A100. It is the round thing on the left:
Googling the issue led me to learn about zinc dendrites that tend to grow in some of the electronics in these old organs.
I used a continuity tester to see if there was a connection between any of the brass screw connectors on the scanner and the scanner case (which is grounded). As you can see in the video below, there indeed was a connection on one of them.
Since the connections proved to be sound, my options were to either open the scanner up to clean or try the "zap" method to fry the zinc dendrites that tend to grow in these old organs.
On the Web there seem to be two lines of thought: zapping is too risky and zapping is fine (with proper precautions).
I really didn't want to open the scanner up, since there are fragile parts such as the oiling wicks and the axle.
After reading about the zapping method, and learning more about how the scanner works and what connections it has, I decided I was comfortable trying it.
The idea is that you apply a voltage across the dendrite, effectively turning it into a fuse which you "blow". In my case, I used a pair of 9 volt batteries in series to achieve an 18 volt source.
I used an alligator clip to connect the positive post of the battery to the scanner body:
With a wire connected to the negative post of the battery, I touched the brass screw on the scanner that allowed current through when previously tested with the continuity tester. There was no spark or pop etc. Continuity test failed after this, proving the dendrite was no longer conducting electricity.
After disconnecting the battery, I turned the organ on and the vibrato was back to its old self!
Here's a video I shot of the sound and testing for the dendrite:
And here are the photos I took during the operation:
https://picasaweb.google.com/davyyd/FixedA100Vibrato#
