Diagnosing a mis-behaving decoder

A

Andrew_au

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Loco with LGB 55021 decoder. Was running fine 3 days ago. Started it up today and nothing. Well, nothing except headlight.
  • Headlight toggle works. Whether it is front or rear is somewhat random each time I power it on. Can't change direction - headlight doesn't even switch.
  • Switches connected to track flip correctly, so not a completely busted layout. Other trains are inside being serviced, so can't check them on main layout.
Trying some more things:
  • Took it inside an put it on programming track. Can read CV 1 at least, and that was correct. Shuffles forward correctly when reading.
    • Engine working, decoder at least partially working.
  • Tried driving it on programming track. Same issues as outside (random headlight works, nothing else)
  • Tried another loco on programming track. Works fine
    • Suggests that problem is in that particular decoder, not control system.
Any suggestions on what to try next?

I know its a very old decoder. I'd planned on eventually replacing each of them, but that one had been working fine up to now so was last one on the list.
 
Clean the treads of the wheels..
Clean the backs of the wheels, where the carbon-brushes rub. - Check they move freely..
Check the skates (if present) are clean, and move up and down freely..

This sounds more like a connectivity issue?

PhilP
 
My thoughts as well Phil, possibly track feed worth checking as well. Maybe clean tra ck where feed comes in and see if it works on that bit to joints.
 
OK, what should I focus on for cleaning? Wheels and pick-ups look clean enough, and I know the programming track is good. There is also some connectivity - F0 works (and only F0), and I can access CVs when on the programming track.
 
Andrew_au said:
OK, what should I focus on for cleaning? Wheels and pick-ups look clean enough, and I know the programming track is good. There is also some connectivity - F0 works (and only F0), and I can access CVs when on the programming track.
Click to expand...
Not sure what your control system is, but can you swop that section over to be controlled for a short test run.
 
Greg Elmassian said:
Did you switch your throttle from 14 speed steps to 28?
Click to expand...
As far as I know, all I did was power down the layout then power it up 3 days later. It's possible that a power fluctuation on startup might have reset some CVs, but I find it strange that the loco still has the correct value for CV#1 and still responds to F0, but not to speed commands.

Here's my current plan of approach, using a programming track that can also supply main power:
  • Set the lights to F1 or F2 - confirm that F commands are working
  • Check out the various driving CVs to see if the settings have somehow got switched or cleared.
OK, did that. Here are the results. (For reference, CV#7=13 CV#106=36)
  • Can drive lights with F0 or F1 only. F2 and higher do not work. Tried both regular F2 and fake-serial F2.
  • Lights only work if set to 28 steps (CV#29=2). With CV#29 = 0, I don't get any lights unless I set them to F1 (CV#52=1). This is true even if CV#52=0, which should allow light control without reference to throttle direction.
  • Can read/write all registers in direct mode. Engine shuffles correctly when reading/writing.
Trying a register mode reset. Behaviour did not appear to change.
  • Can still read / write CVs correctly
  • Still non-functional in speed mode 28, but lights work
  • Still non-functional in speed mode 14, and no lights.
  • Oddly, CV9 (motor frequency) reset to 128, which isn't a legal value for CV9
Maybe I just throw up my hands and replace the decoder, but this has me very confused. It's like a very selectively fried decoder.
 
Changing from 14 to 28 is done by the throttle/command stations, no CVs. It looks like you did try this, but you mentioning CV's concerned me.

I forgot the DCC system you are using, so no more comments on that. Also CV9 is weird, you basically got 0 with the high bit on, probably nothing.

I don't think this decoder is worth the effort, but interesting results.

Greg
 
If it is an old LGB decoder, it is likely serial mode and not parallel. Does your digital system handle serial decoder commands?
 
Reading the FIRST post, he had problems much more fundamental than functions:

"Was running fine 3 days ago" so doubtful it is his control system

"
  • Took it inside an put it on programming track. Can read CV 1 at least, and that was correct. Shuffles forward correctly when reading.
    • Engine working, decoder at least partially working.
  • Tried driving it on programming track. Same issues as outside (random headlight works, nothing else)"
Cannot drive loco, that is nothing to do with functions serial or parallel

So, if it responds on programming track, and we can figure he has right address, and the motor burps when programmed, then it is not taking the speed command (again forget functions), so my only guess is wrong speed command from command station, or, watch for it, highly technical word follows: it's broke!

I'd dump the decoder, why bother with it anyway?

Greg
 
All 55021 decoders can run parallel and series. Also they all run 14 and 28 speed steps, and some can do 128 steps.
Check the decoder for speed steps in CV 29, and make sure your system matches that for this loco address. If using an LGB system, make sure you hit f-9 for switching to parallel mode.
 
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