On my Vision Line Big Boy, I've been chasing down a problem with weak smoke output for the whistle and blowdown. I've read the other threads on this topic but none of the suggestions/tips helped. This included things like repacking with new batting and presoaking it, replacing the 8 ohm heating element, and verifying correct operation of the NTC thermistor. For some reason, I also noticed that anytime I switched the smoke units on, there would be a faint buzzing sound from the loco. I couldn't tell exactly where it was coming from though.
After staring at the smoke units and studying the schematics for the loco, I noticed that both the whistle and blowdown smoke are powered from a board separate from the main RCMC board. It is the SFC3 board and it sits below the Railsounds board (1st pic). Upon removing the SFC3 board, the main filtering capacitor (200uF, 35V) fell off (pic 2). It looks like it had cold solder joints. Hmm -that would explain the buzzing. The SFC3 board has it's own 8A full bridge rectifier (GBU802). On the output of the rectifier is where the main filtering capacitor sits. If that cap is missing, the DC voltage will be poorly regulated. Note that the SFC3 board does not use traditional triacs to drive the smoke units. Instead it uses a power MOSFET for each smoke unit heating element (IRF7341). The MOSFET switches the GND side of the heating element on/off as needed. But the switching is done when a DC voltage is applied to the FET's gate to source pins. However, without a solid DC voltage on the board, the FET will not operate correctly.
My soldering iron tip is too big to fit in the tight spacing on that SFC3 board to re-solder the existing capacitor. But I had a 470uF 35V leaded capacitor. So I bent it over and soldered it in place (pic 3). Lastly pic 4 shows the SFC3 board reassembled in the loco.
After reassembly, I was able to get abundant smoke output from both the whistle and blowdown within a minute of powering up the loco. Woohoo -problem solved ! :-)
If anybody else has problems with anemic smoke output on their Vision Big Boy whistle and blowdown, it's worth checking that capacitor to see if it's loose, or reflow the solder joints if you have a tiny soldering iron, or replace it with a leaded capacitor as shown.
