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AO-28 is the model number of the pre-amplifier used in the B-3, C-3, ?RT-3, as well as the A-100 and ?D-100 series HammondOrgans.
The AO-28's in A-100's are mounted upside-down.
To convert an AO-28 between rightside-up and upside-down operation (or vice versa),
two wires going to the swell capacitor must be swapped.
A photograph detailing the wires to be swapped is available at 
http://www.rtclogic.com/images/A100_to_B3.jpg
(courtesy of ?PeterAbrams.)
Frequency Response
Here's plots of the frequency response of an actual AO-28, as measured by BradBaker.
Parts List
?DennisWage has written up a parts list (complete with MouserElectronics part numbers).
You can find it at 
http://www.hammondb3organ.net/parts.php.
The content of this page is Copyright (C) 2000, 2001, 2002 Geoffrey T. Dairiki and
the other authors of the content, whoever they may be.
This is free information and you are welcome redistribute it
under certain conditions; see
http://www.dairiki.org/HammondWiki/opl.html for details.
Absolutely no warrantee is made as to the correctness of
the information on this page.