Hammond A100 percussion not working

[Cuetek]

I have a '63 Hammond A100 that has no percussion function. No sound at all when I select the percussion preset, push in all the drawbars, and turn the percussion rocker switch on and hit some keys. When I pull the drawbars out I get normal play whether percussion is on or off. There is no effect whatsoever when any of the other switches are on, except for the "soft" "normal" switch, which behaves opposite of what it should. The normal playing tones are softer in the normal position and louder in the "soft" position. Anybody have any guesses?

I have zapped away all dendrites and have good readings on K terminal. Have replaced both 6C4 tubes and the 12AU7A tube with no change in behavior.

[Sweet Pete]

There's a good chance you didn't get all the dendrites then.
Zap it again or better still open it and clean the switches.

[Tonewheel]

Pete's right. I had the same thing with My A100, and I cleaned the rockers. Contact cleaner spayed into the tight spots, working the rockers. Careful with things that go boing and fly off into the the dark, and when you get done unscrewing layers of material, you may find that when it is reassembled that you have an extra screw. Or two. Not nice.

I also did the dendrite thing which is well covered in the forum. I used two 9V batteries in series. Then I went to an 18 volt adaptor and rigged that with alligators and wire. When I talked to my retired 80+ Hammond tech, he told of the days when they would use the 300V supply from the Hammond to burn them, and there was a lot of smoke that distressed the ministers! Not interested in that...

It was a welcome sound to hear the percussion.

[David Anderson]

It's not the typical approach, but I've found that the quickest way to diagnose a percussion problem is by measuring resistances (to ground and elsewhere) with the organ turned off.

While most percussion problems are the result of zinc whiskers, they are not the only source of problems. I have encountered a few cases with things such as resistors inside the preamp opening up. I know of one case where a percussion interstage transformer failed.

You can take resistance measurements from an organ with known-working percussion and compare them to the organ with the problem.

[enor]

There is no effect whatsoever when any of the other switches are on, except for the "soft" "normal" switch, which behaves opposite of what it should. The normal playing tones are softer in the normal position and louder in the "soft" position.

This is normal behavior of that switch. In percussion "normal", the non-percussion tones (the regular drawbars) should go softer in order to let the percussion tones thru in all their glory, so to speak.

Many people (myself included) dislike that particular functionality and modify the percussion so that the drawbar volume stays the same regardless of percussion setting.

[Tim_at_Jonas]

Many people (myself included) dislike that particular functionality and modify the percussion so that the drawbar volume stays the same regardless of percussion setting.

Remove the cover from the matching transformers.

Bridge across the bottom-left resistor with a piece of wire.

Put the cover back on.