Author Topic: Faster in one direction - decoder failure mode?  (Read 285 times)

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jagged ben

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Faster in one direction - decoder failure mode?
« on: March 07, 2021, 12:59:20 AM »
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Haven't seen this before and thought I would put it out there in case anyone can provide knowledge.

Whilst speed matching my locomotives this afternoon I noticed that I have one that goes a lot faster in reverse than in forward.  Like, 2-3 times as fast.  Naturally I first suspected something mechanical, but when I pulled out the decoder and swapped in a DC lightboard and tested on DC, the issue completely disappeared.  I verified all this with a stopwatch.  (And it's a Kato C44-9W but I think you'll agree that's irrelevant.)

The decoder is a slightly older Digitrax DN163K1B, the one with the 3mm LED that they replaced with the DN163K1B.   Upon taking the shell off and doing some programming and testing, I had a couple failed read-backs in JMRI and then at one point the loco started moving and ended up off the end of the programming track.  (i.e. not just the little jerk you often get each time a CV is entered).   Upon further investigation I realized that I had failed to put any Kapton tape on this install like I usually do.  I rectified that issue, put the loco back on the track, and it wouldn't respond to the command station at all anymore, even though on the programming track things were reading back fine.  Eventually I reset the decoder (CV8).   That got it to respond again on address 3, but ... the speed issue is still there.

Anybody run into this before?  I suspect that one of the motor driver chips is shot, and that I should just replaced the decoder.  But I thought I'd throw it up here in case anybody ever saw this before.


MK

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Re: Faster in one direction - decoder failure mode?
« Reply #1 on: March 07, 2021, 08:49:06 AM »
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Haven't experienced it or heard anyone who has.  Is this a new out of the package decoder?  I would use a magnify glass and check to see if there are any bridged solder joints.

I once had a brand new Digitrax decoder for a Kato Dash 9 that went full speed (in both directions) as soon as it touches the track.  Not exactly your issue but I found a bridged solder joint on one of the chips near the middle of the board.  I'm guessing it's one of the motor driver chips.

peteski

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Re: Faster in one direction - decoder failure mode?
« Reply #2 on: March 07, 2021, 01:09:06 PM »
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The motor driver (H-bridge) in DCC decoder is made up from 2 discrete dual MOSFET packages.  Each has eather 6 or 8 leads (not a single chip).
I have seen individual transistors in the H-Bridge fried, that always results in the motor running in only one direction (totally dead in the other direction). I have never seen the failure model you described.

Many better decoders feature froward and reverse trim, becuase most models exhibit slightly different speed forward and revers due to mechanical issues.  I don;t' think Digitrax decoders have that feature. But if you verified that the model in DC runs equally well in bot directions, then I agree that the decoder itself seems to be the suspect, especially since it also behaves strangely otherwise.
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jagged ben

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Re: Faster in one direction - decoder failure mode?
« Reply #3 on: March 07, 2021, 06:32:14 PM »
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...I don;t' think Digitrax decoders have [forward and reverse trim]. ...

They do.  But both of those CVs were set to zero.  And I believe they only work with the speed table, which wasn't enabled.   I'm currently just using basic mid-max volts speed control for speed matching.