Glad to hear things are working better. I do find it hard to believe that the motor switching is working with that triac problem. Make extra sure that both stacks have fast and slow speeds. Maybe you got lucky!
Combo preamp III schematic is on this page http://www.captain-foldback.com/Lesl...bo_preamps.htm
OK you found a problem in the combo preamp which makes that suspect. The stereo mono switch is a simple switch that either mixes the two input signals into one, or keeps them separate. Some 11-pin Leslies have two separate channels - a rotary and stationary. The 860 only has one so mono is the way to go if you want to mix two channels - or else channel 2 will get sent off to a unused input pin in the single channel Leslies.
Low volume on channel 2 can be because of the bad switch or even a bad filter cap or othwr faulty component on the preamp channel 2. I would get the mono stereo switch sorted out first. The schematic will explain how it works.
Anyway I am very happy the Leslie is up and running. Those resistors can get hot so I wouldn't worry too much about that unless you start noticing problems in the power amp stage. Enjoy!
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Oh, something else you should be aware of ...
The speed switches work a little strangely, but it is normal. The second switch changes speeds from fast to whatever the first switch is set to. The first switch changes from stop to slow. If it is in the stop position, the second switch will change speed from fast to stop. If it is in the slow position, the second switch will change speed from fast to slow.