There is no cam in the old big boy. You will have to program the decoder for syncing the motor power to the chuff like the older Soundtraxx decoders. You can install a cam if you would like. That's a lot of work to get 8 chuffs a wheel revolution. I get mine close enough. No one ever notices how many chuffs a rev I get.
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
The only way to come close to synchronized sound without a cam on one of the wheels is to use voltage sensitive chuff. SoundTraxx decoders have this option, but I don't know about Digitrax because I don't own any of them. And the chuff rate can never be accurate without a cam.
On G-scale equipment small magnets can be glued to the insides of wheels or onto axles to trigger a reed switch for the chuff rate, but with HO scale this is very difficult. Since the decoder and speaker must be mounted in the tender, you may be able to devise some type of cam on one of the tender wheels. The SoundTraxx web site has documentation concerning wheel cams and I think they sell such cams for locomotive installations but I don't recall ever seeing one for a tender wheel.
A non-articulated two-cylinder prototype locomotive had 4 chuffs per drive wheel revolution, but a Big Boy's chuffs were not always in sync because each engine (set of drivers) was exhausted separately. Since tender wheels are so much smaller, only one chuff per wheel revolution would probably be close enough.
I just bought a digitrax DCC decoder with sound for my older HO big boy. The manual says something about the CAM input wire. The big boy doesn't have a CAM wire or at least I dont see it. Does anyone have any experience with the older big boy as far as syncing the chuffs for the DCC system?