On Jun 27, 12:09*pm, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:
"Denis G." fired this volley in news:8b707822-
:

I would now guess a bad SCR or the oscillator circuit went kerflooey,

The couple of cart controllers I've taken apart (after scrapping) didn't
use GTO SCRs, they used really husky MOSFET drivers. *I think GTOs are
still extremely expensive compared to large, paralleled SmartFETs.

Back in the mid-80s, before carts generally had PWM controllers, my dad
and I designed one from scratch, using bipolar transistors (at the time
large enough MOSFETs weren't commonly available). *It ran the motor fine,
but we could never find big enough flyback diodes to protect the
transistors from inductive kickback, and slow switching would overheat
everything. *We faced other impediments, like not having the right motors
for that sort of control.

The first commercial ones were about the size of a LARGE loaf of sandwich
bread, and used banks of TO-3 bipolars to do the switching.

Now they're about 4" square, with lots of IR FETs and big "pellet" style
diodes mounted on TO-220 frames -- even have thermistor sensing on the
heat sink!

I finally elicited from my wife that this problem "comes and goes". *I
think it's time to pop the cover on the controller.

LLoyd

You're more experienced with electronics than I. I'm taking my beer
and I'll watch from the sidelines.