Due to age and materials used in Hammond Organs, a common problem is the growth of tiny metallic hairs on metal surfaces. These hairs can shunt signals to ground causing problems. This happens in the manuals, in the percussion switches and in the preamp itself. This is also a common problem found in the Vibrato System.
This discussion will focus on the percussion circuit.This quick fix process involves the use of a voltage to “burn off” the offending “hairs”. Typically we use an external voltage source such as batteries. You can plug two or three 9-volt batteries together and create enough voltage to do the job. Another source is the Organ pre-amp’s percussion tube V7, the 12AU7a tube on the right end of the preamp.
This is NOT for the faint of heart!
I suggest trying the battery first and proceed below only if that wasn’t successful and you are certain the tubes are good. In fact, you can make sure of that by pressing down a key on the upper manual with the percussion on and no drawbars out. Turn the percussion cutoff pot until you hear a steady tone. If you are able to acquire this tone, the percussion circuit is working and it’s the keying that isn’t. You could also check the voltage on the K terminal. If it shows ground all the time, then the flashing should work for you.
First, take a clip lead and short the GG terminals to each other. This will not allow any loud pops from the flashing to go to your speaker system.
Remove V7 (12AU7a) and connect a wire to the High Voltage Pin 1 with an alligator clip and something like a small screwdriver. This voltage is buffered somewhat from the rectifier. Be very careful not to allow any exposed wire with high voltage on it to touch any metal or yourself!
This is a very high voltage of around 200VDC and can kill! Also, DO NOT TOUCH any grounds in the organ or any other metal parts. This takes a very steady hand.
Mometarily touch the other end of the HV wire to the K terminal just below the percussion tubes. Now, replace the tube and check your percussion, you may have just fixed it!
This information is for educational purposes only and no claims are made that this information will lead to any successful repair. Benton Electronics assumes no responsibility to its use.
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