MTH 20-20149-3 short

One thing to check for that's happened to me twice now, is a piece of ballast gets stuck in the axle housing causing the short. It melted an axle on one wheelset, with a handle of a Z4000 feeding power yet did not blow the quick blow auto fuse connected to the rails.

Of course, make sure the axles are installed the right way!

I've also seen the truck's power wires rub the frame where they enter it.

Try CRC on the pickup rollers’ spindles. Dirty rollers can do this. Just a thought, but may help.

I put a video up on the arcing.  The wheels are pristine.  The rollers are pristine.  The track is clean and a 100% isolated section on the bench powered with just 18v from a lionel brick.  I cleaned both anyway the rollers and wheels anyway.  I've checked for red pickup wire rubbing against the frame and even added electrical tape around the wire and where it might cross the frame.  Still arcs.  I disconnected the two white connectors that I believe are the output of the AC/DC converter boards that send power to the lights.  The arcing was still there.  I took off all of the spiral wrapping and checked for signs of missing insulation between black and red wires.  Found none.  Cannot see where the center pickup or any wiring to it could be physically touching anything on the truck to ground it to the outside rails or even the frame, unless it is inside the trucks.  I can't believe the arcing is normal.  In this setup, however, it hasn't tripped the breaker on the brick.  The original owner (fellow forumite) reached out to me and confirmed it was powered on at purchase to check the lights.  They lit.  Then it went onto a shelf and never moved nor was ever powered again until I put power on it last weekend.  He was even willing to let me return it.  But its now a puzzle to me.  I suppose the next step, if no one here has any ideas, is to dis-assemble the trucks, but I don't see what that will do.

I was surprised how hot the heat sinks got.  But that seems to have nothing to do with the arcing.  Looking for ideas.

Short FOUND!!!???  After complete disassembly of the trucks and finding nothing, I took them out of the equation.  I checked for continuity and found after removing the trucks, I had a hard power to ground short now.  I applied voltage and had no lighting for that half of the engine.  The other half worked.  taking off the CV board, I found this on the bottom side (couldn't see it while it was installed).

Anyone know the p/n of this assembly?  I assume its a CV board of some type.  Anyone have one?  MTHpartsandsales doesn't have an exploded parts list for this engine yet.

Is this a cause of the short or a result of the debug?

The board you removed, appears to be a AC to DC power converter.  If I'm reading the numbers right, the 3 prong device that was attached to the heat sink is a KA7806 voltage regulator.

Out of curiosity, where are you finding a short?  Are you sure that the continuity reading you're getting isn't indirectly being measured through one of the semiconductors?  Which setting on your meter are you using?

What has me questioning this, is in this frame of your video, the spark appears like it's coming from the other direction.

@VADarthDad posted:

Short FOUND!!!???  After complete disassembly of the trucks and finding nothing, I took them out of the equation.  I checked for continuity and found after removing the trucks, I had a hard power to ground short now.  I applied voltage and had no lighting for that half of the engine.  The other half worked.  taking off the CV board, I found this on the bottom side (couldn't see it while it was installed).

Anyone know the p/n of this assembly?  I assume its a CV board of some type.  Anyone have one?  MTHpartsandsales doesn't have an exploded parts list for this engine yet.

Is this a cause of the short or a result of the debug?

Offhand I'd say that the board bottom connections somehow contacted the chassis  I've got one in my parts box if you need one.

Thanks!  But while it’s apart I think I’m going to do your incandescent to led w/220 ohm swap. I reassembled with just one CV and no short.  MTH indicated its AG-0000016. It’s $20.  I assume one working CV could handle the 8 LEDs vs 2 CVS with 4 incandescents each.  

@VADarthDad posted:

Thanks!  But while it’s apart I think I’m going to do your incandescent to led w/220 ohm swap. I reassembled with just one CV and no short.  MTH indicated its AG-0000016. It’s $20.  I assume one working CV could handle the 8 LEDs vs 2 CVS with 4 incandescents each.  

I'd go for the LED's as well.  Unless you provide a solid heatsink (bolted to the chassis), no way that little CV board with the 6V linear regulator handles more than one or at the most, two 6V incandescent bulbs.  In free air, 240ma through a 6v linear TO-220 regulator with 18V track power into the board in will cook in not too long a time unless it has a big heatsink.  That's dissipating around 5 watts, the free air package dissipation of that part is around 1 watt.  The heatsink-to-ambient thermal resistance in air for a TO-220 package is approximately 70 °C/W.  That means with 60 milliamps out of the regulator and 22V DC in (18VAC * 1.4 = 25.2 volts into the regulator), the regulator case is over 100C.  Long before it got anywhere with four times that current, it would have gone into thermal shutdown.

OTOH, for LED's, you can use a 470 ohm resistor, for instance, and a white LED, that will dissipate 0.2 watts, and four of those are no problem for the CV module with no heatsink.  The LED will be at least as bright as that incandescent dissipating the 0.2 watts.

Short resolved. Converted to LEDs.  Driving all LEDs with one CV board and it doesn’t even get warm.  Previous two CVs got hot to touch driving 4-6 incandescents each. Next time. Need to use small gauge wire.

I’ve had a few of these boards fail driving too many incandescents.  I upgraded the regulator to a 2 amp version.