Tuning back EMF

[djconway]

I have several (a lot) locomotives that are now pushing 20 years old all DCC equipped, all speed matched to where they run fairly well together with a max speed of about 70 smph.  I am beginning to notice that after some of the locomotives run for about 20 - 30 minutes there is a noticeable slow down.  Since most of these locos have Digitrax back torque compensation (back EMF) equipped decoders I am thinking that tuning back EMF would help. 
I'm in agreement with Peteski that the Digitrax manuals are somewhat lacking when it comes to explaining things, and am looking for a little assistance in getting the back EMF set up in these locos.

Another question is that since 80% of the locomotives are older Atlas and Kato built could the faster speed motor be generating heat, causing the loco to slow down?

[mmagliaro]

Since these locos all worked well before, and the are now pushing 20 years old, I would be much more inclined to suspect the motors before messing with the back emf settings.  20 years is a long time for an N Scale motor, if you've been using these engines regularly over those 20 years.  You can pick up the eBay replacement Atlas/Kato motors really cheap off eBay these days, even though you may find that 2 or 3 out of every 10 don't run very well.  Sometimes, they ALL run okay.  It's not as reliable as paying retail and buying new motors, but then, these are selling for $1.50 to $2.00 a piece.

I guess the first two things I would do:
1. clean out the commutator slots in 3 motors in consist and see if they work better
2. blindly put new motors in the three engines and see if the problem goes away.   

But I think eliminating worn motors is the first step in this diagnosis. 

Are you able to hook an ammeter into your power supply and see if the current climbs as you run the engines, to a much higher level by the 20 minute mark?  That would be another indicator of motor problems.

[peteski]

To add to Max's suggestion (which is right on target), I would also recommend full disassembly of those locos and thorough cleaning/lubrication.  You didn't mention anything about whether those locos were regularly serviced or how thorough the servicing was.

Also, before replacing the motors, it might be worth to thoroughly clean and lubricate those old motors. They might also need new brushes.

To clean motors, I first remove the brushes, then I throw them (along with the loose brushes and springs) in an ultrasonic cleaner filled with naphtha.  After drying them thoroughly (naphtha is lighter fluid - flammable) I reassemble the motor and lubricate the bearings (since naphtha removes the oil from the bearings too).

[djconway]

Thanks for the replies guys.

I was heading down that tare them down and clean them track.  It just seemed a few tweaks of programming code vs disassembly would be a quicker path.

[peteski]

Thanks for the replies guys.

I was heading down that tare them down and clean them track.  It just seemed a few tweaks of programming code vs disassembly would be a quicker path.

Um, software workarounds to fix hardware problems are usually destined for a failure.  They'll bite you in the end.

Remember my first rule of DCC: make sure your model's "hardware" is finely tuned before installing DCC.