If you suspect the tender center rail pickups, there are a few things you can do:
Lubricate/Clean - The rollers rotate around a pin and it can get grungy due to age, old lubricant, rust and so on. I find that using Goo-Gone can break up the crud. I then follow up with CRC 2-26 to clear any further remains and as a fresh lube. Also make sure the wheels don't have a build up of grime. Goo-Gone will break it down. Wipe it off and apply some oil to the intersection of the axles and wheels.
Spring Tension - The spring can become weakened due to age, use, and abuse. Depending upon your abilities, you can replace only the spring, the pickup assembly, the coupler plate, or the truck as a whole.
Physical Deformity - Sometimes the truck is physically deformed, or the pickup assembly's plastic locating tabs have broken off or melted and so on and so on. Inspect carefully and remedy as applicable.
External Issue - The issue may not be the tender at this point a few things to look at, especially if other pre/postwar air whistle tenders exhibit similar symptoms (you can/will test another, right?):
- Dirty Track - clean with Goo-Gone
- Weak Transformer
- Transformer too small for the total load
- Transformer is of a model that does not have the 5V whistle boost circuitry
- The Whistle control contacts need to be cleaned
- Weak rectifier - replace with new rectifier or diode
I know that the above is a dump of lots of potential places for gremlins to hide, but I hope it helps you figure out where your troubles lie.
