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The percussion volume level on AO-28s and AO-29s can be adjusted by changing the value of a resistor which is soldered onto the P terminal of the preamp. (You have to remove a cover from the terminal strip to get at it.)

How Hammond said to do it

Here's a not from Service Letter 98:

When installing unit A0-28 (B3,C3,RT3) set Percussion signal level as follows:

  1. Set organ Volume tablet to Normal and with all other Tablets tilting toward the front of the console, press upper manual third C key, pull out first brown drawbar and adjust expression pedal so that 2.5 volts (RMS) appears across G-G pre-amplifier terminals.
  2. With same key pressed, push in the first drawbar (brown), pull out the ninth drawbar (white) and adjust tone control on the amplifier so that .5 Volt (RMS) appears across the G-G pre-amplifier terminals.
  3. Push in the ninth drawbar, set Percussion 'On-Off' tablet to On and unsolder blue wire from amplifier terminal K.
  4. With same key pressed the RMS voltage measured across G-G terminals should be 3.75 +/- 10%. If is it not, clip out existing resistor inside terminal cover on back of amplifier and replace with a different value (between 22,000 and 47,000 ohms) to obtain the above specified voltage.
  5. Tone control can be readjusted to suit.

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