As this conversion is one of those that does not require dismantling the loco, I had not needed to gain access to the interior. The first problem with the investigation was, as is so often the case these days, getting the thing apart. Each manufacturer seems to have their own design ideas.
The body was the easy bit, undo four screws and the body come off with a bit of gentle persuasion. The next job was to figure out how to get the bogies out. Consulting the diagrams helps. Unplug the pickup wires from the PCB (printed circuit board). There is a clip holding the top of the gear tower in, which needs to be sprung off.
DSCF2369.jpg
Then the drive shaft needs to be disconnected. At one end there is a speaker and this plus its support moulding will need to be removed. It should then be possible to remove the bogie assembly. This proved to be enlightening.
DSCF2361.jpg
The pony truck pivots on the beam to the right although the hole in the pony truck is a slot, which means the pony truck is effectively uncontrollable, so yes, making a new one is definitely the order of the day. The area under the center drive axle has a peg on which the inner unit pivots and four lugs.
DSCF2360.jpg
These are intended to limit the side play of the inner unit before the outer frames turn with it, which is quite a cunning design.
The top of the frames are flat apart from three small ridges to the left, top and bottom of the pivot to reduce friction. Sorry about the poor picture quality, but I don't fancy taking the loco apart again just to take another one.
DSCF2362.jpg
The frames on the real thing could only pivot relying on the springs to deal with any track variations.
The model attempts to deal with this by allowing the inner bogie to rock relative to the frames.
Above the center axle can be seen the pivot bar with the central pivot point and the outer crescent shaped bearers. These allow the inner bogie the rock for and aft, but there is no allowance for twist as both bogies are the same. This could be dealt with by removing the bearers on one bogie only and placing a washer the same thickness as the bearers on the pivot point.
DSCF2367.jpg
The two shiny patches either side of the screw head are where the lugs on the underside of the outer frames make contact.
Measurement over the the inner bogie at this point showed it was 6 thou or 0.15 mm less than the spacing between the lugs. This may not seem much, but is enough to allow the inner bogie to turn appreciably relative to the outer frame. My solution? I slackened the screw and inserted a 5 thou sheet of plasticard into the join before re-tightening it.
DSCF2368.jpg
This leads me to my final conclusion as to why I have been having problems.
DSCF2366.jpg
The axles are located in place by some quite large phosphor bronze bearings. These are visible in Johns original pictures.
Whilst I had the bogie out I tried a few tests. The center axle is quite rigid as it is securely held by the retaining plate. When the unit is placed on a flat surface, there is a distinct tendency for the majority of the load to be on the center axle meaning less weight is borne by the outer axles and any high spots in the track unload the outer axles increasing the chance of derailment.
Solving this is not going to be an easy fix. It has crossed my mind to use a one of the sprung chassis conversions intended for a class 40 or 45 loco, but don't know which would be most suitable, but then there is the fact that this is a low priority loco, totally unsuitable for my layout and I would rather spend my modelling time on higher priority projects at the moment.
Regards
Tony.
You do not have the required permissions to view the files attached to this post.
Inspiration from the past. Dreams for the future.