Many BEMF decoders have functional issues with Radio Frequency Interference (RFI) filters that are installed across the motor power leads or terminals. The observed problem ranges from very poor slow speed performance to just erratic motor control. NOTE: The RFI filters do NOT effect DC operation.
1) What locomotives have these RFI filters? You will often find these RFI filters on any European Manufactured locomotive or a locomotive designed to be sold in Europe. The need for the filter is driven by European Union Laws governing radio interference issues. These filters are NOT NEEDED in the USA. That said, for the interest of keeping manufacturing cost low (1 version of the product), these RFI filters are shipped anyway in products destine for other country outside the European Union such as the USA.
2) What are the parts involved? It can consist of parallel capacitor(s) and/or series inductors on the motor leads.
3) Where are these parts found? These filters can be placed directly on the motor terminals, on a PCB board or a combination of both. Look at the bottom for some pictures of these parts installed in various locomotives.
4) What is the minimum I need to do? You should cut off and remove the capacitors at the capacitor wire lead level. Simple operation to perform. Do not assume destroying the capacitor itself will work for if improperly done can actually create a short circuit which at worse can damage your decoder when you attempt to run the engine.
5) What about the inductor? Between the two parts involved, capacitors and inductors, capacitors are the ones that cause the most problems. Ideally the inductors should be removed too for they still have some effect. However, cutting them out creates another problem for the inductors are wired in series with the motor. If you cut them, the motor will stop working. You must take the additional step to replacing the inductors with a wire jumper and that will involve more work including soldering. The decision is up to you.
6) Does this modification violate the manufacture warrentee? It depends on the manufacture. Bachmann allows the modification as stated on their website's FAQ on DCC.