Max,
I operate with DCC and use the old MRC powerpacks only for testing non-equipped locomotives. I wanted to use a PWM control because the old powerpack rheostat output produces a high start speed and cannot be turned down enough to test run at a crawl speed. The voltage remaining on the rails at stop is not a problem since it is not enough to move the loco, but Is the hum created by the PWM harmful to the motor?
I have gotten the 150 pF capacitors and will install them if your test is successful. If it is necessary to replace the pot and some other parts, it may be better fot mè to just buy another controller. It looks like many inexpensive models are available.
I totally agree. I would not bother replacing multiple components. There are plenty of PWM controllers on eBay that operate at higher frequencies that won't cause the hum problem. I doubt that the motor sitting there humming at 264 Hz at throttle off will cause a problem. The amount of power being dissipated in the windings is extremely low, so there should be no heat or brush wear.
Now that I understand what you are trying to do, I still think it is important to use a higher frequency PWM generator than the one you have. DCC decoders use a much higher frequency PWM to control the motor, and I assume you want to do a comparable test.
I am going to test that circuit with various components, and put its output on a scope, when it arrives. But meanwhile, I might be able to just cobble the same thing together on a breadboard. I have 555 timers, and the other parts, lying around here. I just want to make sure that the circuit actually oscillates and produces a nice square wave at the higher frequency, and looking at it on a scope is the only way to know for sure.
