Walthers P2K 2-8-8-2 motor drive problems

[CNR5529]

I have been looking over @Paradise275 's walthers Y3a with a QSI sound decoder that was acting up. Sound would work, steam chuffs would rev up with the throttle, but no movement would occur.

I took the motor out to test it with DC, and nothing would happen. Cleaned out the commutator, and the motor came to life. I then looked at the mechanism and noticed it was caked with heavy grease, so I tried to remove some of the gunk and put a small amount of lighter oil, to help reduce any viscous damping and excessive torque on the motor. I reassembled the loco and tested it only to find that it would run in reverse, but not forward...  Still the sound worked in both directions.

I then tried reversing the wires from the little board under the cab that go to the motor. This time the locomotive ran forwards, but not in reverse (albeit with the lighting opposite, as it was not reprogrammed with the motor wired backwards). hmmmm   

So the problem is not the motor anymore, as it can run both forwards and backwards, and it is not the wiring harness between the tender and the loco, again as it can run in both directions. I would thus deduce that the decoder is not sending a motor drive signal in one of the two directions.

This one has me stumped, any ideas welcome! @peteski 

[peteski]

Nice troubleshooting CNR5529!
I agree that what you are describing now is a problem with the decoder.  Decoders use an H-bridge (4 MOSFET transistors) to supply power to the motor.  Something like this:


Each diagonally opposite pair of transistors supplies power to the motor but with opposite polarity (so the loco can run forward and backwards).  If one or 2 of those transistors used for running the motor in one direction is burned out you will end up with the loco only running in one direction.  I suspect that at some point the gunked up motor might have developed a short which blew up a transistor or two in that H-bridge.

But I would really hate to see that decoder replaced wit another brand decoder.  I have the equipment to replace small surface-mount components and if you guys want, I'm willing to try to fix the decoder if the loco is sent to me.  I'll contact QSI to see if they'll give me specs for the components needed. If not, I'll see if I can identify them myself, from the markings on their cases.  The total cost of those components will be most likely less than $10.

[CNR5529]

Each diagonally opposite pair of transistors supplies power to the motor but with opposite polarity (so the loco can run forward and backwards).  If one or 2 of those transistors used for running the motor in one direction is burned out you will end up with the loco only running in one direction.  I suspect that at some point the gunked up motor might have developed a short which blew up a transistor or two in that H-bridge.

Thanks Pete, I figured you might have an idea what is going on here. Dead transistors seem like a highly plausible explanation. I proceeded to remove the decoder and package up the rest of the loco to hand back to the owner so he can get in touch with you and/or Walthers, and guess what I saw on the underside of the decoder?

(been trying to upload a few pictures but the folder seems to be full... will upload again as soon as I can)

Looks like something got hot, melted the insulation on some wires, fused the wire harness together and some wires had fused to the one chip that now has a hole in it! This is the wire harness in the tender, that is under the decoder.

Anyways, like you said, it would be a shame to loose the QSI decoder over 10-15$ worth of parts, so now I will leave the decision making up to @Paradise275.

Thanks again!

[peteski]

Thanks Pete, I figured you might have an idea what is going on here. Dead transistors seem like a highly plausible explanation. I proceeded to remove the decoder and package up the rest of the loco to hand back to the owner so he can get in touch with you and/or Walthers, and guess what I saw on the underside of the decoder?

(been trying to upload a few pictures but the folder seems to be full... will upload again as soon as I can)

Looks like something got hot, melted the insulation on some wires, fused the wire harness together and some wires had fused to the one chip that now has a hole in it! This is the wire harness in the tender, that is under the decoder.

Anyways, like you said, it would be a shame to loose the QSI decoder over 10-15$ worth of parts, so now I will leave the decision making up to @Paradise275.

Thanks again!

That confirms my suspicions.  Like I said, I'm willing to tackle the decoder repair - at this point there is nothing to lose. I only need the decoder sent to me, but it might make more sense to send the entire tender (to fix the wires too).  If Paradise275 wants to take that route of course.  I'll  be off at the N scale convention this week - probably won't be checking here often.

[CNR5529]

The full folder problem had prevented me from uploading these photos earlier, but this is what was found under the decoder:

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There is a crater on the right most square chip.

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The insulation on these wires had melted and started fusing together. Some copper was exposed.

[peteski]

Yeah, those 8-pin chips are the motor drives (each is a half of the H-bridge).  I could definitely try to identify and replace the one which released the magic smoke, and take care of the wire harness.

[jdcolombo]

Hi guys.

I'm on the road and won't be home until Friday, but I might have an operational decoder for a Walthers 2-8-8-2 in my parts bin.  I replaced one with a LokSound about a year ago.  Not sure I still have it, but I'll look on Friday.  If I do still have it, would be willing to sell for a small amount ($20) plus shipping. 

John C.

[Paradise275]

Thanks John.
I'd be more than happy to take you up on your offer.
It would save a lot of grief and aggravation.
Send me a PM to let me know how to pay.

Rick

[peteski]

John,
I consider the QSI Revolution decoder to be a really good quality decoder (both for motor control and for sound quality). I'm curious as to why you replaced it with ESU.

[jdcolombo]

Thanks John.
I'd be more than happy to take you up on your offer.
It would save a lot of grief and aggravation.
Send me a PM to let me know how to pay.

Rick

OK.  I think the decoder in the photo below is the one for the 2-8-8-2.  If this looks right, e-mail me directly at jdcolombo at gmail and we'll figure out how to get it to you.

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John C.

[jdcolombo]

John,
I consider the QSI Revolution decoder to be a really good quality decoder (both for motor control and for sound quality). I'm curious as to why you replaced it with ESU.

I changed the decoder out for two reasons.

1.  The huge honkin' caps took up room in the tender that could better be used for a nice 11x15 or 13x18 speaker in a separate enclosure.  Which is what I put in my 2-8-8-2 after pulling out the QSI and replacing it with a LokSound Micro.  The dual 15mm speakers in the tender lacked an enclosure, and were pretty poor speakers to begin with, so the sound was simply too "thin" for my tastes.

2.  I decided that life is too short to have to remember (or look up) different manufacturer's CV schemes for programming things, and the QSI system was one of the more complex ones out there.  Since I've got ESU LokSounds and LokPilots in all my other locos now, this "outlier" simply became an aggravation I didn't need.  And the ESU 2-8-8-2 sound file is excellent, unlike some of their other generic steam files (soon to be replaced, I hope, by new files with loco-specific recordings done by Matt Hermann).  The result of the transplant was better sound and escape from the CV hell foisted upon me by different complex decoder brands.

John C.