Hello All,
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How old are your grandfather’s model trains (approximately)?
What scale; N, HO, O (AKA, “Lionel”)?
The problem might not be with the controller but with the locomotive- -which is a common problem, even with newer locomotives.
Controllers (rheostats) are pretty robust units unless they were damaged- -short-circuited- -at some point.
Do you have a multimeter to test the controller?
An inexpensive digital multimeter from your local or big box hardware store will suffice. (Approximately $20.00-$25.00.) Analog meters might be cheaper but are more difficult to use.
A set of alligator clips helps.
Set the meter to DC Volts.
Put the test leads into the alligator clips and clip them to the “Variable DC” posts. You might need to unscrew the posts to get the clips to bite.
Turn on the controller.
Don’t worry about the “Direction” switch at this point.
Turn off the “Momentum” switch.
When you noch up the controller you should see the DC Voltage rise to the maximum specified on the back of the controller. EG: “Output 20 VDC.”
You will see a “+” or a “-” symbol next to the DCV on the LCD screen (polarity).
Switch the “Direction” switch to the other side.
The “+” or “-” symbol should change to the opposite symbol (polarity).
If you don’t get a reading or only a reading when the “Direction” switch is flipped to one side or the other, it’s the controller.
If you do get a reading it’s most likely the locomotive…
Which is another “can of worms.”
Hope this helps.