Problem with DZ126D decoder

[Mike C]

 Couple days ago I bought a used brass loco for a good price  . HOn3 C25 2-8-0 . It ran nicely on DC good control and real quiet . Switching it to DCC wasn't too difficult and by itself runs really well . But , when I hook it to a train going up or down grade it surges really bad  , going up it picks up speed really fast and spins the wheels for a couple inches then settels back to the correct speed only to do it several more times . Downhill it surges forward then settles back down only to do it again , again ....... I've been adjusting the BEMF but just can't get it to run correctly . HELP  .....Mike

[Mike C]

 Um DH126D  decoder 

[peteski]

Um DH126D  decoder 

DZ, DH . . . I'm pretty sure that both (and many other Digitrax decoders) use the same microcontroller chip with the same programming inside.

Does the loco have a coreless motor or standard 3- or 5-pole motor?

Also, are there any RFI suppression components in the motor circuit?  Usually it is 1 or 2 capacitors in parallel with the motor and 2 chokes (coils) in series with motor leads.

[Mike C]

 Well...... I pulled the boiler off again today . Then for the he!! of it I put a small weight on the frame and pulled a few cars arround . And guess what , it ran perfect . So I set the boiler on the frame loosely and it ran pretty much OK except for a few incidents . So I completely tore apart the motor gearbox and tender connections looking for a short . Didn't find anything so put it together again . Still couldn't screw the boiler to the frame .GRRRRR .  Turned out to be a very simple fix . The motor solder pads were making intermittent contact with the brass backhead . It wasn't causing a short , but when one of the tabs would make contact the motor got full DCC voltage and would surge ahead a few inches until the pad lost contact and slowed down . Insulated the boiler and backhead put it back together and it now runs great . Sucks when the simple stuff get you !  So next a visit to the paint shop and then a sound decoder .....Mike

[Mike C]

DZ, DH . . . I'm pretty sure that both (and many other Digitrax decoders) use the same microcontroller chip with the same programming inside.

Does the loco have a coreless motor or standard 3- or 5-pole motor?

Also, are there any RFI suppression components in the motor circuit?  Usually it is 1 or 2 capacitors in parallel with the motor and 2 chokes (coils) in series with motor leads.

This was a Westside model from the late 70's so I'd guess a standard 5 pole definately not a coreless  . It did have some kind of home built chuff generator but I ripped that out when I first took it apart .....Mike

[peteski]

I guess the motor type doesn't matter as you got it fixed already! Nice!  I was looking for something that would explain the surging, and some decoders and coreless motors do not play well and behave strangely.