DCC problems with Spectrum Steam locos

[drth2]

I have 4 Spectrum locos: (2) 2-8-0 Consolidations, a K4 Pacific, and a 4-8-2 USRA light, purchased about 5 years ago.  All run fine on DC.  They are apparently DCC ready (though Bachmann gave no info with the loco).  The tenders have a PC board with an 8-pin socket, plus a couple of inductors, caps, etc (anyone have a schematic?).  I recently converted to Digitrax DCC, and am testing locos on my new layout.  First problem:  I am testing non-DCC locos using DCC at address 00.  The 4-8-2 runs fine this way (as do many other locos).  The other three immediately cause the  Digitrax system to ring up a short circuit.  This is strange as all 4PC boards are apparently similar in design.  Any ideas as to why this would be?

This brings up the second problem:  I would like to install a DCC decoder, but am reluctant to try with no info.  Proto 2000's also have a DCC ready design but warn you to use different lights when plugging in a decoder.  Does the Spectrum design have similar requirements or can you just plug and play in a decoder and all is fine?

[Chris350]

I would look to see if one of the tender trucks is reversed.  That might be causing the short. Look also at the engine drivers, one may be grounding out against the frame. The Spectrum 4-8-2 I did a DCC upgrade on was plug and play.  No lighting mods required. The Protos used incandescent bulbs which the DCC voltages burn out.  Most of the Spectrum engines use LED's so you ought to be ok.  The usual mod is to add a resister inline with the bulb to lower the voltage and prevent burn out.

[Jim Banner]

The capacitors that Bachmann uses to minimize radio frequency interference can cause a command station to sense that it is being shorted.  Removing the capacitors normally fixes the problem.

The same thing often happens with ultrasonic ("silent drive") decoders.  This link shows the capacitors in one of the releases of Bachmann 2-8-0s.

http://members.shaw.ca/sask.rail/dcc/2-8-0/index.html

Best bet - install some decoders in those locomotives.

Jim

[drth2]

Thanks, Jim.  Removing the cap did the trick.  Turns out this is indeed an LC pi filter.  I agree that it is probably an attempt to suppress spurious RF from the motor.  I would even go one step further by removing the inductors and replacing them with a solid wire.  I am not sure what effect leaving the pi filter in would have on decoder output design.  This would require some testing that I am not set up to do.  At best, nothing would happen, but it may have some negative effects such as decoders with BEMF control.  It may filter the BEMF signal such that it is no longer as effective as it should be.

[pdlethbridge]

my understanding of the issue is that the suppression caps are integrated in the decoder and are not needed on the Bachmann boards.

[rich1998]

Thanks, Jim.  Removing the cap did the trick.  Turns out this is indeed an LC pi filter.  I agree that it is probably an attempt to suppress spurious RF from the motor.  I would even go one step further by removing the inductors and replacing them with a solid wire.  I am not sure what effect leaving the pi filter in would have on decoder output design.  This would require some testing that I am not set up to do.  At best, nothing would happen, but it may have some negative effects such as decoders with BEMF control.  It may filter the BEMF signal such that it is no longer as effective as it should be.

i have worked with tuned circuits for years. removing the capacitor/s is all that is needed unless you need a little more space, then straight wire. the remaining capacitance of the pc board surface is negligible at these frequencies. most people just remove the capacitors.
lex

[Jim Banner]

The filters are quite effective at removing RFI.  They are required to meet regulations in some parts of Europe where people live much closer together than we are used to.  I have some older German locomotives from the 60's and even back then they had RFI filters.

Leaving the RF chokes in place is not problem, not even with BEMF control.  Their inductance is just too small to have any effect at that low a frequency.

Jim