STEP BY STEP PERCUSSION TROUBLE SHOOTING BY TIM PADRICK:
The problem is most likely in the manual or in the percussion switch
assembly.
Test 1:
Power the organ. Depress the B preset key and turn percussion the
tablet to the OFF position. Using a VOM/DMM, measure between ground
and the preamp's K terminal (blue wire). You should have 30 Volts
(plus or minus a few).
Turn the percussion tablet to the ON position. There should be no
change in the voltage reading. If it drops substantially, there is
likely a dendrite problem.
Press any playing key on the upper manual. The voltage reading should
drop to 0 as the percussion sounds and decays (if the above was ok).
If not...
Test 2:
Turn the organ off.
Locate the terminal block on the back of the manuals that has white,
green, yellow, yellow, red, orange, red, and brown wires. Connect an
Ohm meter between ground and the terminal that has the white wire.
With the percussion in the OFF position, you should have about 0 Ohms.
With the B preset key depressed and the percussion tablet in the ON
position, you should have about 120k Ohms. If it is substantially
less, there is likely a dendrite problem.
There is a short procedure for "zapping" the dendrites with DC
voltage. It can repair the problem for a while, but dendrites will
continue to grow and the problem will eventually return. However, if
it clears the problem it is easier to do this a number of times than
remove the manuals to get at the buss bars.
This procedure requires a DC voltage source (9 volt battery may work).
Using alligator clips, attach the ground terminal to a chassis ground
point. Attach another clip to the positive terminal.
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Locate the white wire that comes out of the back of the drawbar base behind the right end of the percussion switchbox and goes to the left tab of the terminal strip on the back of the upper manual (wires are white, green, yel, yel, red, or, red, brown - there's a resistor on there too) . Turn the percussion tablet to the ON position. Zap the wire. Turn the percussion tablet to the OFF position and repeat. If it does not work, try a higher zapping voltage.
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Check your meter (or just play the organ) to see if you have the
correct results. If not, try again, touching the lead to the terminal
for a bit longer than briefly. Recheck.
If this does not clear the problem you need more voltage. You can tap
off the Leslie kit (or one of the tube socket pins) and get about 290 Volts from it. Be "real careful" if you do this - don't short it to ground!
If this does not clear the problem, remove the wooden cover from the
drawbars, remove the fasteners for the drawbar & switch "chassis",
remove the percussion switch box, and remove it's bottom cover.
Remember that white wire? Trace it into the switch box and to the
percussion switch. Find the wire that this gets connected to when the
switch is in the on position - I think it is black. Turn the switch
off. Touch your DC voltage source to this terminal. This is what
finally cured the one I was working on.
