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Delta P8 Running Only 1 direction, how to diagnose decoder/coil?
 Joined: 05/12/2004(UTC) Posts: 2,976 Location: CA, USA
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Hi everyone, In January I picked up a lovely black P8 with gold bands (similar to a 3099) at a swap meet, presumably from starter set. It has a factory delta decoder. The catch was the locomotive was sold as not running. I assumed oil froze and locked the gears, and took it home. Today I finally pulled it apart, and after pulling a considerable amount of carpet fiber from the linkage   , I realized the wheel quartering was the real reason it was binding, not frozen oil. Fast forward a little longer, and the locomotive was clean and running very nicely in reverse. However, only in reverse. Flat dead except the headlight in forward gear. I suspect I have a bad decoder, but am not sure how to test this? Also, the coil doesn't look the best so I'm not eliminating that possibility either, although the odds are against it. Any ideas what to check via a voltmeter or jumper leads? All wiring is properly connected/soldered, unless there is a hair-thin wire in the coil loose that I'm missing.. Thoughts? Its almost a great little locomotive! I'm hoping the decoder isn't toast though... Here is a decoder image in case useful/helpful.  |
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 Joined: 09/08/2008(UTC) Posts: 1,919 Location: Auckland,
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Hi John,
A real simple way to check the coil/decoder output would be to swap the two wires going to the field coil around. If the loco then runs forward and not backwards, it will be a decoder problem. If it runs backward and not forward, coil problem.
Let me know the result
Cheers....
Mike
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 3 users liked this useful post by mvd71
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Joined: 05/12/2004(UTC) Posts: 2,976 Location: CA, USA
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I guess that was the simple approach indeed! I had realized that as a decoder test, but not as a coil test.
Results: Reverse only after the swap, so its the coil. I have one leftover from a conversion of a similar loco so hopefully tomorrow I'll have a few minutes to get it all installed and working. Great news as a digital conversion would cost what I paid for the locomotive...
I owe you one for all of the help lately! |
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Joined: 09/08/2008(UTC) Posts: 1,919 Location: Auckland,
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No worries, glad it worked out and you have a cheap spare part to fit 👍
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 Joined: 08/11/2005(UTC) Posts: 3,528 Location: Mullerup, 4200 Slagelse
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 I don't get it, a coil either works or doesn't work. Please let us know the outcome of this. Per. |
If you can dream it, you can do it! I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. In case this is not legally possible: I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.  |
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 Joined: 23/07/2014(UTC) Posts: 8,472 Location: ENGLAND, Didcot
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Originally Posted by: 5HorizonsRR  I guess that was the simple approach indeed! I had realized that as a decoder test, but not as a coil test.
Results: Reverse only after the swap, so its the coil. I have one leftover from a conversion of a similar loco so hopefully tomorrow I'll have a few minutes to get it all installed and working. Great news as a digital conversion would cost what I paid for the locomotive...
I owe you one for all of the help lately! One possible reason is the 'centre tap' of the coil may not have both wires soldered very well. It would be worth checking that solder joint is properly soldered by redoing it. You may find the enamel insulation on one wire hasn't been removed properly and has created a dry joint.
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Joined: 08/11/2005(UTC) Posts: 3,528 Location: Mullerup, 4200 Slagelse
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Originally Posted by: kiwiAlan
One possible reason is the 'centre tap' of the coil may not have both wires soldered very well. It would be worth checking that solder joint is properly soldered by redoing it. You may find the enamel insulation on one wire hasn't been removed properly and has created a dry joint.
If it's a double coil, forget my previous post....  Per. |
If you can dream it, you can do it! I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. In case this is not legally possible: I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law. |
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 Joined: 04/02/2011(UTC) Posts: 3,563 Location: Paris, France
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Hi
Indeed it is a Delta decoder with a "normal" Märklin motor: two windings in one coil (one for each direction)
The driving transistors are at bottom of the picture.
Potential issues:
- one winding of the coil is broken/not soldered
- one transistor on the decoder is burned or with a poor soldering
Bonne chance
Jean |
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Joined: 05/12/2004(UTC) Posts: 2,976 Location: CA, USA
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Hi everyone, an interesting development: I replaced the coil with one I removed from my 2701 Orient Express Locomotive, which I know was working when removed.
I popped it on the track anticipating victory, and instead I got nothing- in either direction! Quite the surprise. I tried flipping the decoder to analogue and running on an old transformer, and nothing there either.
So, I guess its the chip? Its just strange that now the second lead/direction would go bad. Reading with the voltmeter we have power into the chip at the solder points, and out via the light bulbs. I doublechecked all soldering points through the loco and chip and all is ok.
The last question- does anyone know how to take a voltmeter reading on a delta chip for power at the motor leads? (meaning the solder point where they exit the chip for the motor) Do I measure voltage across the two leads? Or one to the lead and one to the black wire with choke to the motor plate? Either way I don't seem to be getting voltage under throttle, but I'm guessing I'm also not measuring at the proper points.
Thanks in advance! I have a backup delta chip I found this morning, but before I go installing I need to understand what exactly went wrong and ensure the locomotive/motor itself is ok. |
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Joined: 04/02/2011(UTC) Posts: 3,563 Location: Paris, France
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Hi Originally Posted by: 5HorizonsRR
So, I guess its the chip? Its just strange that now the second lead/direction would go bad. Reading with the voltmeter we have power into the chip at the solder points, and out via the light bulbs. I doublechecked all soldering points through the loco and chip and all is ok.
Difficult to say. There are equal chances that one winding broke during the operation or the delta module went wrong. One possibility is that the "electronic cooking" during the module manufacturing process went wrong and other soldering on the PCB are not good. Originally Posted by: 5HorizonsRR The last question- does anyone know how to take a voltmeter reading on a delta chip for power at the motor leads? (meaning the solder point where they exit the chip for the motor) Do I measure voltage across the two leads? Or one to the lead and one to the black wire with choke to the motor plate? Either way I don't seem to be getting voltage under throttle, but I'm guessing I'm also not measuring at the proper points. While it is ok to verify continuity using a multimeter while the power is off, I strongly advise not to do any measurement while the power is on unless the Delta module is in place , you have very thin tipped contacts and a very steady hand. My advice (1) check the continuity between each of the two motor wires and the big transistors (drivers) on the module and (2) measure the voltage between one of the motor wires (the green or the blue) and the return (black wire normally) The likely schematics (not sure as multiple variants are out there) of your decoder is here http://www.persmodelrailroad.net/d6603.htmlOriginally Posted by: 5HorizonsRR Thanks in advance! I have a backup delta chip I found this morning, but before I go installing I need to understand what exactly went wrong and ensure the locomotive/motor itself is ok. Only you would know. If your soldering is clean (using an electronics soldering iron with temperature control), rapid and tested for short circuit then it is probably OK. Another possibility is that your Delta module soldering is not good (some bad contacts between surface-mounted components and the PCB) Jean |
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Joined: 09/08/2008(UTC) Posts: 1,919 Location: Auckland,
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Mmmm..... That is odd.
As Jean has suggested, continuity testing with power off. I would test from where the coil is attached to the decoder, to the point on the moto face plate where the two wires coming out of the coil are joined in common.
One other thing to double check is the brushes, I know I have forgotten to install them more than once on conversions.
Unless you've had an oopsie, it seems strange that the decoder would have failed on both motor outputs all of a sudden
Cheers....
Mike
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Joined: 05/12/2004(UTC) Posts: 2,976 Location: CA, USA
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Thanks for the tips Mike and Jean! So I decided I'd rather solder on the new decoder (known to be working) then dig into a coil. I soldered it up, and surprisingly no motor! Then came the continuity tester, and sure enough there was a coil issue. Pulling the locomotive apart again, the problem was whatever coating marklin put on those late-production coils- the ones with a brown finish. In particular my problem was on the small fused end lead going to the motor contact/cover. I had to really scrape the coating stuff off after a touch of carburetor cleaner on a q tip. Once that was off I soldered it back to the motor cover, and we were in business! Well not quite, I then had to quarter the wheels, and then repair a bend in a drive linkage. Talk about a lot of work for a $30 locomotive! But, it was a fun project and she runs shockingly well for a delta locomotive. Here is the fully- assembled culprit now. Thanks for the help!  Edited by user 14 April 2020 17:40:08(UTC)
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4 users liked this useful post by 5HorizonsRR
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Joined: 09/08/2008(UTC) Posts: 1,919 Location: Auckland,
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Hi John,
Great to see you got it nailed in the end. And you still have a good delta decoder 👍
Cheers....
Mike
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1 user liked this useful post by mvd71
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Delta P8 Running Only 1 direction, how to diagnose decoder/coil?
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