First, I got the AO-29 amplifier ready for servicing by disconnecting the three wires on the left front of the chassis that supply 120V AC power to the motors in the tone generator. I didn't want the tone generator running while working on the amplifier. This also removed the "run" switch from the circuit, so I made my own on/off switch for the amplifier using a common household light switch, a plastic electrical work box, a switch plate cover, and a few feet of 2 wire lamp cord. These were all materials I had stored away in the workshop from other projects around the house. I hooked up one end of the lamp cord to the terminals on the light switch and the other to the "blue" and "black" wire terminals on the amp chassis. Now, I could turn the amp on and off right where is was working, rather than go around front to the "run" switch.
Next step was to remove the bolts holding the amp down to the cabinet, and I disconnected the linkage for the expression pedal. I also had to remove the green and black wires that go the the switch on the expression pedal. I was then able to lift the chassis up and rotate it 90 degrees onto it's side, with the underside facing me. A couple blocks of 2x4 wood help provide some slack for the rest of the wiring still connected to the amplifier:
I powered the amp up with no tubes, and checked the AC voltages coming out of the power transformer. Filament voltages looked good, and the secondary measured about 760V AC, which agreed nicely with the schematic (380V from winding to center-tap). I turned the power off and inserted the 5U4GB rectifier tube; turned AC back on and started to measure DC voltage points. First thing I noticed was all the B+ voltages were 80-90V higher than the schematic indicated (more on that later). Nothing smoked, and so I decided to go ahead and put the remaining tubes I had available. These 4 tubes were enough to get the manuals, vibrato, and intermediate preamplifier sections working.
I remeasured DC voltages and found them all still 80-90V high. I decided to press on anyway. I disconnected my home-made on/off switch and reconnected the power supply wires for the tone generator motors. Next, I tapped into the intermediate preamplifier with a 0.01uF disk ceramic capacitor at R40, between V4b and V3b (sorry, not the best picture):
The wire connected to the cap goes to a RCA phono plug, plugged into the line input on an old Carvin bass combo amp I own. The ground side of the wire goes to a screw on the AO-29 chassis. With this all set, I started up the M3, and turned on the bass amp. Pulling out a few drawbars, I turned up the input volume on the bass amp and hit a key on the organ....
SOUND! YooHoo!! After a bit more fiddling with levels and the disconnected expression pedal control, I got a pretty decent sound level coming out of the amp, with no sign of hum or other bad sounds. I was then able to verify that the vibrato was working perfectly for both manuals, and all but one key (the highest one on the lower manual) were working fine, too!
Here is a really bad video of my oldest soon noodling around on the organ - basically shot this to prove that it was now making some sounds 8-):
Remember those voltages that were too high? I posted a question about it to the "Hammond Zone" forum on YahooGroups and almost immediately got the answer that it was likely the missing power amplifier tubes that was responsible. Of course! Without the tubes drawing current, and with no regulation to speak of in these old tube amp power supplies, the DC voltage will rise much higher than with the power tubes in place.
Next step is to get some 6V6 power tubes, because the voltage without them is too close to the working limit of alot of the capacitors in this old amp. I will also pick-up tubes for the percussion and bass pedal sections of the amplifier at the same time and see if I can get the rest of the preamp working.
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