"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