Model Forum / General / Railroads / February 2009

The round black thing on the left side of the circuit board is a
trim-pot that has been adapted for use as the throttle control for the
powerpack. That, in fact, is the potentiometer - or one of them.

At the bottom of that round black area you can see three black squares
with little legs that go through the circuit board. The two outer legs
are a fixed resistance and will measure some value depending on what
is printed on the potentiometer body. Read there and see if there are
some numbers like 103, or 10k, or 100k or something like those. When
you do, I'll tell you what the outer leg measurements should be.

The center leg is the wiper of the potentiometer and that measurement
from the center to either outer leg will vary depending on where the
knob is turned. From the limited view of what you have photographed,
it looks as if you have a simple unregulated rectified circuit with
one leg of the bridge rectifier variable. This is not a good
powerpack. You'd be spending good money to repair something you can
buy in a thrift shop for a dollar. Toss it and buy a better one.
Seriously.
--
Ray

   You didn't point out which component you're speaking of. I'm going
to assume it's one of the two potentiometers at the top center of the
photo, one of which appears to be attached by wires back to the power
pack. It appears to be a carbon film pot.
   It *may* be bad, or not. The way to check is with an ohmmeter across
the two outer terminals. The center terminal is for the wiper. If it
reads infinity or some other very high value across the two outer
terminals then it is bad. Normally for this application you might have
something between 1kohms and 10kohms.
   However, I also see, on the circuit board, a black, 3 terminal
device almost exactly in the center of the photo that is likely either a
transistor or SCR. Assuming that power to the track is actually being
controlled by that device, if it is shorted you would have the "always
on at full speed" problem that you describe. If you have a multimeter
with a diode check function, and you know how to use it, you can
determine if that is shorted and the cause of your problem.

And next a sharp picture of the circuit board alone. Both sides.

If the original potmeter was blown, and the new mounted blew too, then
perhaps the error is somewhere else.

Shorted transistor perhaps?

Do you hava an ohm meter ?

Klaus

That is a cool site. I see I will be spending some money there. Started making a
list already.

Thanks for the site.

Chris

When you replaced the potentiometer (aka pot) did you replace it with
one with the same resistance value?  That makes a big difference in
how the device will work and if it'll "smoke" or not.

Most electronic components (including pots and rheostats) have some
sort of identification imprinted on them.  Pots might have something
like "A2K" or "502B" or some other number/letter combination.  The
number is its total resistance and one of the letters indicates
logarithmic or linear taper.  You should replace the burned out pot
with one that has the same properties.

I also never heard of "potmeters".  Ohm-meter is used to measure them
as they are just variable resistors.

Peteski