Dead Short in loco

[Frisco Larry]

I have a Kato PA in which I installed a decoder.  After running about 1 to 1.5 hours, it came to a screeching stop and had a dead short in the circuit board.  I doubled checked the installation and replaced the decoder board.  Again, after about 1 to 1.5 hours of running, bam! dead short again.  I have put the same type of decoder in about 30 other Kato E's or F's or PA's with no problems, so I don't think it is me.  I can't imagine that I would have two bad decoders that both got installed in the same loco.  Thus I don't have a clue as to what the problem could be.  Anyone have any ideas?

[peteski]

Larry, you need to be more specific:  What's shorted?  Does the short exist between the rail pickups, or is the short between a rail pickup and a motor lead?  Or is the short across a function output and the common positive (or some other type of a short)?

Also, what decoder are you using?

How exactly are you determining that there is a short?

[mmyers]

Are you sure it's a short? I've had the same thing happen while running this type of mechanism. The old original runs and decoders (DN145K's) didn't seem to be a problem. Usually it's not a short. The decoder will still be fine. The motor contacts lose contact. The best solution I've come up with is soldering the motor contacts to the board. Then install the little gray clip.

Martin myers

[davefoxx]

I had a Kato F40PH go dead on me a few months ago after running flawlessly for quite some time.  I noticed if I pushed down in the shell, it would run.  So, I parked it.   This weekend, I took it out, soldered the contacts to the decoder, and it's running flawlessly again.  Thank goodness, because I missed it.

DFF

[Frisco Larry]

The decoder is TCS KD08-A in both attempts.  The motor tabs are soldered to the pads on the decoder board (as are all my other E's, F's and PA's).  If you set the loco on the track it trips the circuit breaker killing the entire track.  I have no idea where on the decoder board the short is and have no idea how I would begin to determine that.  The motor runs fine if I remove the decoder and touch the motor tabs with DC power.

[peteski]

Ok, that is a bit more descriptive. 
I am going by what I see on http://www.tcsdcc.com/public_html/Customer_Content/Installation_Pictures/N_Scale/Kato/PA-1/PA-1.html
Take the shell off.  Is the short still there?

If yes, take 4 strips of paper and slide them under the flat power pickup strips which touch the tops of the truck's sideframe metal pickups. Basically, you are isolating the pickup strips which run under the decoder from the trucks power contacts.  At this point the short should be gone (but test the loco anyway).

If the short is gone then the location of the short is narrowed down to the pickup strips and/or the decoder.  Since the decoder sits ina plastic clrddle, the most likely possibility of a short you described (across the tracks) is where the power pickup strips are routed over the exposed solid-metal frame. Examine all those areas carefully to see if there are slivers of metal connecting between the power pickup strips and the metal frame.

This photo (from the TCS website) shows where the pickup strips run very close to the metal frame. If there is a metal debris in that area or if the pickup strips are bent down, they will cause a short though the frame.

[mmyers]

OK, so you definitely have a short. Check the kapton tape on the long brass contact strips. Look where the motor tabs are bent around them. That's the most likely area for that design to short. The kapton tape should be wrapped so that the joint is NOT on the outboard side. It should be continuous around the outboard side of those long strips.

Martin Myers

[peteski]

OK, so you definitely have a short. Check the kapton tape on the long brass contact strips. Look where the motor tabs are bent around them. That's the most likely area for that design to short. The kapton tape should be wrapped so that the joint is NOT on the outboard side. It should be continuous around the outboard side of those long strips.

Martin Myers

But if the Kapton tape was damaged then the motor leads will simply be shorting to the rail pickups. That would probably blow up the decoder, but it shouldn't cause a dead short across the track pickups.  But it is worth checking, nevertheless.

[mmyers]

He said the circuit board shorted out. I assumed he meant the decoder and not the whole damn system. Thanks for correcting me. Don't know how I would have slept tonite.

[peteski]

He said the circuit board shorted out. I assumed he meant the decoder and not the whole damn system. Thanks for correcting me. Don't know how I would have slept tonite.

I suspect that you would agree that the original problem description was a bit vague. Leading to assumptions. Which means there were multiple possible causes of the problem.  I'm glad that you will sleep well tonight. 

However I am still not convinced that shorting motor leads to the pickup strips (through a break in the Kapton tape) will cause the DCC booster to sense a short and shut down.  I have never encounter this type of a failure.  I suppose that if a certain pair of transistors in the H-bridge motor driver became shorted, that might be seen as a short across the rails (assuming that the rectifier bridge is still intact).

[mmyers]

Shorting them should not cause the booster to trip. But is the booster tripping? We are both speculating because there is not enough information in the original post to make a conclusion. My assumption is based on the word "circuit board". Yours assumes the booster is shorting.
Need more info from the original poster.

Martin Myers

[peteski]

Shorting them should not cause the booster to trip. But is the booster tripping? We are both speculating because there is not enough information in the original post to make a conclusion. My assumption is based on the word "circuit board". Yours assumes the booster is shorting.
Need more info from the original poster.

Martin Myers

Larry's second post mentions "If you set the loco on the track it trips the circuit breaker killing the entire track."  To me that indicates that the booster shuts down.  But I'm in total agreement that we need more detailed info.

[bnsf971]

Check for shorts across the wheelsets. I've had a couple of Es and PAs where the axle half shafts were slightly too long, and touched each other through the gear. It was intermittant, and drove me crazy until I found it.