Folks:
My MRC Ampack is now cranking along with the finest 1960s technology available. This is the finished design:
Finding that out was a minor lesson in itself, but now everything is fine.
I did reverse the rectifier polarity and ground the center tap and case. The transistor collector is grounded to the tab, which is bolted to a homemade aluminum heatsink, and bolted to the case, with compound at both interfaces. I suppose I could have sprung for the insulated mount, but since I am planning to replace the cord with a grounded one, in order to ground the case, I figured this would be fine.
Control is good. I dug out my bad actors and tried them out. The half-wave pulse power is very helpful, and it seems to help my very worst locomotives the most. My war-weary Life-Like Dockside, with a bent motor shaft, went from a noisy 12 SMPH starting speed to a (still noisy)
2 SMPH.There is a slight surge when switching from pulse to full-wave, but it's not a real problem.
Control was equally good with my remotored Teakettle, whose tape-player motor seems to draw very low current. With pulse power, this crept along at something between 1-2 SMPH.
Motor heating did not seem like a problem. The transistor got warm after all the testing at low throttle settings, but not too hot to touch.
The circuit does bring output to zero, but there is indeed some wasted space at the low end of throttle range, so I might add some diodes to my next attempt, or add them later to this one, as discussed in the other thread.
After some testing with locomotives, I loaded the throttle with a 10k resistor, then the ~10 ohm variable resistor salvaged from the Ampack, and determined that the output was about 0-15 VDC at no load, dropping to 0-12VDC at 1.2 A. This seems fine.
I'd recommend this project to any novice. It was fun and easy, and the cost was low, even using Rat$hack components.
Next project: replacing the selenium rectifiers and adding pulse power to my ancient KF variable-transformer pack.
Cordially yours: Gerard P. President, a box of track and a solid-state Ampack.