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New loco, very odd behaviour


Harlequin
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I have a Bachmann H1 Atlantic which is behaving very oddly.

 

I first installed a Rails Connect decoder and the loco only had binary speed control. That is to say, as soon as the throttle was raised at all it set off at medium speed and varying the throttle did not change the speed at all until I got back down to zero when the loco abruptly stopped. (Note, it did not  set off at full speed...)

 

Now it gets interesting: I have just replaced the Rails Connect with a Zimo MX638D, and it does exactly the same thing... (So it seems unlikely to be a decoder problem.) I have turned off DC compatibility, played with Vmin/mid/max, enabled the full speed table but there's no change to the odd "binary", medium speed running.

 

I'm 99.99% sure that the controller is not the problem: Pi-Sprog3 that has normal full control of all my other locos.

 

Any ideas how I can get this loco to behave normally? Has anyone experienced something similar?

 

Edited by Harlequin
tense
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Here's the MX638 installed:

IMG_20210523_083648r.jpg.462bae043035c11f025ee11f9d991b96.jpg

 

The capacitor appears to be installed by Bachmann as standard. It is connected to the main motherboard. Could the running problems be related to that?

 

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Three options here: 

 

a- return loco to retailer as faulty. 

 

b-  use a multi-meter to test loco wiring without decoder fitted.  Should see connections to motor ONLY from the motor pins on the decoder socket, and the motor pins should not be connected to any other pins on the decoder socket.    From that diagnosis, strategy for repair should start to emerge.   

 

c - rip out any electronics installed by manufacturer and re-wire loco yourself.  

 

 

It might be something in the design of the loco's PCB (though I really doubt that as the design is likely to have been tested).  Its more likely to be poor assembly/manufacture.  

 

The capacitor may be relevant, but you've said you disabled DC running in the decoder, so you've done your side of things.  

 

 

-  Nigel

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57 minutes ago, Nigelcliffe said:

Three options here: 

 

a- return loco to retailer as faulty. 

 

b-  use a multi-meter to test loco wiring without decoder fitted.  Should see connections to motor ONLY from the motor pins on the decoder socket, and the motor pins should not be connected to any other pins on the decoder socket.    From that diagnosis, strategy for repair should start to emerge.   

 

c - rip out any electronics installed by manufacturer and re-wire loco yourself.  

 

 

It might be something in the design of the loco's PCB (though I really doubt that as the design is likely to have been tested).  Its more likely to be poor assembly/manufacture.  

 

The capacitor may be relevant, but you've said you disabled DC running in the decoder, so you've done your side of things.  

 

 

-  Nigel

Thanks.

 

The on/off running and the medium speed running make it feel like a digital control problem rather than an analogue wiring problem but I will check the electrics, of course.

 

I will report back.

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1 hour ago, Nigelcliffe said:

Three options here: 

 

a- return loco to retailer as faulty. 

 

b-  use a multi-meter to test loco wiring without decoder fitted.  Should see connections to motor ONLY from the motor pins on the decoder socket, and the motor pins should not be connected to any other pins on the decoder socket.    From that diagnosis, strategy for repair should start to emerge.   

 

c - rip out any electronics installed by manufacturer and re-wire loco yourself.  

 

 

It might be something in the design of the loco's PCB (though I really doubt that as the design is likely to have been tested).  Its more likely to be poor assembly/manufacture.  

 

The capacitor may be relevant, but you've said you disabled DC running in the decoder, so you've done your side of things.  

 

 

-  Nigel

 

You were absolutely spot on! It was poor assembly, look at this:

 

IMG_20210523_104351r.jpg.c48ff39af08f8540a7e2759c6062783e.jpgIMG_20210523_104424r.jpg.d2b93541d684e3814fb11e992c2068dd.jpg

 

Telltale unused M+ solder pad on top and when you look underneath Motor+ is connected directly to Rail Right...

 

Thanks again!

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I guess that while the wiring was wrong the behaviour could be explained by the decoder keeping the Motor- connection isolated until the throttle was opened. Then it tried to enable M+ and M- but because M+ was also connected directly to the DCC signal some sort of half-rectification took place (or maybe waveform interference?) allowing the motor to run at constant speed.

 

It works fine now that I've fixed the wiring. I just need to get it all back together...

 

Lesson learned.

 

Edited by Harlequin
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1 hour ago, Harlequin said:

Lesson learned.

Hardly fair on you. The fact is that most of the time, a loco fresh out of the box does what we expect. I had a loco about 12 years ago - a Santa Fe 4-8-4 - where the decoder and sound card were in the tender, which had pickups, so just like most these days. The tender sat on the rails making all the right noises, but the moment I plugged the loco in we had a dead short. Turned out it was superfluous solder causing a short on the card with the plug atttached, so I had to use an X-acto saw to cut the requisite channel. At the time it is very disappointing - but sorting it brings a smile!

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Glad you found it.  

 

I'd recommend testing both of the decoders used, because loco faults of this sort (connecting pickup direct to motor) are one way to destroy decoders, either partially or fully destroying the motor output so there is no control.    If either decoder is faulty, you've a claim against the supplier (retailer) for the loss caused by the fault at manufacture.   

 

 - Nigel

 

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