Troubleshooting
The percussion gate is triggered by grounding the K terminal on the preamp. The ground comes to this terminal from the 1' busbar (white wire) through the percussion switch. Our old friend metal migration (ZincDendrites) is almost always the cause of percussion problems. This can be easily verified by measuring the voltage on the K terminal of your preamp (blue wire) with the percussion on and the B preset selected. With no keys depressed you should see a positive voltage of 25 to 30 volts. This should go to 0 when any key is pressed. If the voltage is very low or missing, the switch is the problem. Cleaning is the proper cure.
Select the B preset, turn the percussion to ON, NORMAL, SLOW, SECOND.
Measure the voltage on the K terminal with no keys pressed. It should read 25 to 30 volts.
Now press any upper key and the reading should go to 0.
If the voltage on the K terminal seems to be behaving itself, skip the remainder of this section, and see the Trouble in the Preamp section below.
If little (less than 20 volts or so) or no voltage appears at K with no keys pressed, you probably have a short (see ZincDendrites) most likely located either in the percussion switch assembly, or in the upper manual.
Cancel the upper manual presets (but leave the percussion switch in the on position).
If that gives you 25 to 30 volts on the K terminal, you (probably) have a short in the upper manual. Skip the rest of this section.
Turn the percussion ON/OFF switch to OFF.
If you still have too low a voltage on the K terminal: Turn the organ off. Unsolder the blue wire from the K terminal, and the red wire from the M terminal of the AO-28. Turn the organ back on.
If you still have too low a voltage on K, you have a strange problem in the preamp.
Otherwise, you have a short most likely in the percussion switches, but also possibly in the upper manual (associated with the B preset). To further narrow down the location of the short:
Turn percussion back on. Reconnect the wires to the K and M terminals (if you had disconnected them).
(Sanity check: you should be back to having low voltage on K.)
Unsolder one of the white wires going to the left-most terminal of the ResistorPanel.
If you still have 0 volts on K, there is a short in the percussion switches. Otherwise the short is in the upper manual.
To Clean the Percussion Switch Assembly
See Zapping below for an easier (but not 100% reliable) alternative to physically cleaning the percussion switches.
Remove the music rack, the two large screws and two wood screws which secure the drawbar base, place heavy paper (or thin carboard) at either end to avoid scratches and raise the drawbar base 4 to 5 inches. (See also HowToRaiseTheManuals.)
Remove the two screws from the percussion switch cover and remove it. Spray the interior of the percussion switch housing and contacts with a good non-residual cleaner and retest to see if it works.
As only about 1/2 of the contacts are reachable from the back of the switch, it may be necessary to dismantle the entire switch to get results. This is not a job for the faint of heart. Many foul words have been uttered during re-assembly. Some will advise applying copius amounts of D.C. to burn the ofending bits into oblivion. I don't recommend this procedure as severe damage to many components may occur.
Shorts in the Upper Manual
If you have the time, ability and patience, and you are sure that you've narrowed the problem down to a short in the upper manual, the best next step is probably to perform a BusbarLubeJob on the upper manual. (But, be warned: there's no guarantee that this will fix the problem.)
See also ShortsInTheManual.
Zapping:
The quick fix (which also may or may not work) is flashing or ManualZapping. You can zap either the manual or the percussion switches, or both.
Warning: Improper (or even proper) zapping can do permanent damage to your organ! Make sure you completely understand what you are doing before proceding. Also, please read ManualZapping for further comments.
You have several choices of where to zap:
To zap the upper manual assembly, unsolder the white wire from the B preset bundle at the ResistorPanel. Select the B preset, and zap between the white wire and the manual chassis (ground). This will flash away dendrites only in the manual assembly and will not affect any dendrites that might be in the percussion switches.
Unsolder the blue wire from the K terminal on the preamp. Rock the percussion switches (all four) toward the player and flash between the blue wire and ground (the preamp chassis). Rock the percussion switches away from the player, and flash again between the blue wire and ground.
It can also be beneficial to flash the other percussion-related wires on the preamp terminals. In particular, in one case where the percussion disappeared only when the decay was set to "slow," flashing the H, J, M, and N wires (unsoldered) with the switches in both positions solved the problem.
(Others can elaborate here on more choices).
Before you zap, you may want to check with a (good-quality) ohmmeter to make sure there is indeed a dendrite short. (The ZincDendrite shorts typically have resistances a few hundreds of ohms.) If you see a short of very low resistance, you may want to rethink your decision to zap until you've figured out more about what's causing the short.
After you've checked the resistance of the short, take a nine volt "transistor radio" battery (or two, in series), and hook it up across the short for a couple of seconds. Check the resistance again. If you now see a open circuit, rejoice!
If the short is still there, but the resistance has changed (either higher or lower) that's a good sign. Zap again (and again and again, if need be.)
If the resistance of the short isn't changing, I'd try zapping one or two more times, but your prospects are less promising. If you have used only one battery, try two, in series. Sometimes one is not enough, but two are.
Trouble in the Preamp
If the voltage on the K terminal seems to be behaving correctly (high except for when a key is depressed), but your Percussion still doesn't percuss, the trouble is probably in the pre-amp.
Some things to try:
Try adjusting the Percussion Cut-Off potentiometer on the back of the preamp. This probably isn't the root of your troubles, but it's easy to check. You never know if some screw-ball might have mucked it up.
Try swapping/checking the tubes in the percussion circuit. In an AO-28, these are: V5, V6 and V7. (V5 and V6 are are 6C4's; V7 is a 12AU7.) [In an AO-29, the tubes of interest are V7, V8, and V9.]
If you don't have any spare tubes on hand, you can try swapping V5 and V6 [V7 and V8 in an AO-29]. This probably won't fix your problem (since you'll still have a bad tube) --- but if the symptoms change, you can be pretty sure that one or both of the swapped tubes is no good.
If the tubes aren't the problem, you'll have to dig deeper. Reportedly, C31 is sometimes the culprit, but really, it could be anything in the percussion circuit.
