I've got 2 of these which have been running poorly over points and having come across this post I gathered both courage (dutch) and tools and sat down to work. Did the front wheelset first - took a while whilst I worked out where all the bits that fell out belonged but that's a different story...
Put it back together and it shorted straight away. There is a copper bus arrangement on the inside of the bottom chassis plate which seems, by way of a few "nodules", to be set up to feed solely +ve on either front or back wheel set and -ve on the other. Is this right ? I can't see any form of "split" on the wheels so that only one is picking up current.