After posting my opinions of the U25C's electrical system, I started wondering if they could be fixed, preferably without redesigning the entire locomotive. They can be, and the changes could be made at the factory with minimum cost and effort.
The biggest problems are that the contacts are too complicated, and the wheel wipers too fragile. The wipers need to be stiffer, so that they'll keep their shape, and stay in contact. As it is, the wheels, when properly gauged, have a lot of lateral motion. This is good on curves, but makes it hard for the thin, soft wipers to maintain contact over the wide range of distances. A thicker, stiffer wiper would work a lot better, and wouldn't require any redesign of the truck, although the upper truck frame mounting sockets might need to be shortened a little, if the wipers are much thicker. I bent the wipers out, but it took several tries, as the wipers tended to flatten again when the wheels were being reinstalled, as they're not springy enough to keep their bend.
From the wheels, the current has to go to the horizontal wheel wipers, to the vertical truck-mounted contact strip, and finally to the circuit board wipers. I soldered wires between the vertical contacts and the circuit board, and that helped a lot. For a factory fix, a wire between the wheel wiper itself and the board would allow the elimination of both the vertical contact and the board wipers, leaving only one contact point, at the wheel.
After these changes my test model runs beautifully, and a little faster, meaning it will MU with my Kato locomotives. It's still slower than the Katos, which many will consider good, but the two do run smoothly together. The increase in speed seems to be due to better contact, as the motor is actually running continuously, rather than relying on the flywheels half the time. The lights showed no sign of flickering.
The changes took about half an hour, and future ones should go faster, requiring only a small soldering iron and some fine wire. If these changes were made at the factory, this would be a much more reliable model, and I wouldn't hesitate to buy more of them.
For comparison, and to go with my Arnold "FA-whatever" test, I dug out four late-80s Hallmark diesels, three U33/36Cs and one U23C, all mechanically identical. These are typical of brass diesels at the time, with truck-mounted worms, U-joint driveshafts, and wipers on the insulated wheels. On these particular models, only two of the three insulated wheels pickup on each truck, but the trucks are insulated from the frames, with the insulated wheels on opposite sides. "Hot" pickup is by a wire attached to the truck mounting screw, giving six hot wheels and four insulated wheels for pickup, five wheels on each side. The wipers bear on the wheel treads, with a wire soldered to the center of the wiper. Not that different from the Arnold U25C, overall, except for wipers on only one side of each truck, and wires in place of the extra moveable contact points. Internally they have a large open-frame motor and no flywheels, a basic traditional "HO brass engine" style drive. No lights, or any other electronics, and a lot of open space, relying on the brass body and frame for weight.
Straight from the box, and not run for at least 20 years, two of the four ran perfectly, smooth, a little noisy, and about the same speed as a Kato. One did short on one of my dual gauge turnouts, so may need to have the gauge tweaked. The third didn't run at all, until I applied test leads to the wheels, found the pair that worked, and ran it for a few seconds. That cleaned enough oxidation off everything that it also ran fine. Notably, it was the only one NOT in the original box, being in a recycled plastic box, but with the original foam. Whether that mattered I have no idea.
The third ran poorly, very similar to the Arnolds, except that, without the flywheels, it simply stopped when it lost contact. After trying a few times to get it to run smoothly with the test leads, I noticed that the wire from one of the insulated side wipers was loose. After disassembling the loco (four screws, maybe a minute), removing the truck (one screw, pull the U-joint apart, maybe another minute, being careful), and resoldering the wire (waiting for the iron to heat took most of the time), the loco ran as well as the others.
This seems to confirm that good wipers and wires work fine, even if they do look primitive. Hopefully Arnold can make these changes.
