I am repairing another 6403B bell-ringer tender for a friend, and everything seems to have gone well. It worked when I got it, and after rewiring with new silicone wire, it works the same. Then I did a test drive behind my 1666 steamer. The faster it went, the less the bell I heard until there was no bell at all at a moderate speed/voltage.
I rechecked all my wiring against this schematic
I also found this schematic from @David Johnston
which seems to match the one from Dennis.
The only additions I made to the wiring are e-clips attached to each truck (on their mounting posts) with ground wires soldered to each clip and running to the ground connection on the lamp board. The point of that exercise is to improve outside rail conductivity while passing thru switches. I also cleaned the axles to bare metal and cleaned out the axle holes in each wheel to help improve outside rail conduction. Contacts on the lamp board’s thermal switch were burnished a 2nd time (no help), and the lamp was changed from an 18v lamp to the recommended 12-14v lamp, also no help.
So I tried a different engine (1656 steamer) and got exactly the same results.
I set up a quick and dirty test bed consisting of some O27 track and a postwar ZW (the problem also occurs using a ZWC). First, I placed just the tender on the tracks, and tried it at voltages from 6v to 18v – the ringing worked at 6v and only got stronger up to 18v. Then I put the steamer on the same track, held it in place while lifting slightly to let the wheels spin, and voila, the problem returned - the more voltage applied, the weaker the ring, until nothing.
I decided to measure the voltage right at the bell solenoid (referenced to the outside rail) using my analog meter. At initial turn on, the bell rings for a second or so, while warming the thermostatic switch - during that time I see full track voltage (about 12v) on the ringer solenoid. Once the bell ringing settles into a rhythm, the meter shows only about a 5 volt jump each time the bell rings due to the short duration of the "on" time - the needle just can't jump up that fast!
When I put the engine on the track, the ringing tapers off quickly and then pretty much stops altogether, and the 5 volt spikes drop off to what appears to be a relatively steady 2.5 volts, or so.
What are your thoughts on the cause of the weak/no bell with an increase in speed? I suspect the thermal switch has seen better days, but I just can't explain why the problem shows up only with an engine in front of the tender.
I did one final experiment - I placed the tender on one piece of track and the engine on another piece, right beside the tender, then jumpered both tracks together, hooked the ZW to one of the tracks, then applied power. Oddly, the symptoms almost disappeared. I'm about ready for my Proton Pack gun to get rid of the ghosts in this problem child!