Loco won't run backwards

[TWG]

I have a Roco loco (ÖBB class 38, tender drive) that has a factory fitted Zimo sound decoder.  In the past it has run well.  It still runs well forwards, but when put into reverse it runs for a short way and then stops.  The sound and lights continue to work.  After a few seconds it starts to move but after a few centimetres it stops again. 

 

I took the decoder out and put it in my ESU decoder tester (latest version with small coreless motor).  The motor runs perfectly in both directions.  So I thought that there must be something in the loco causing the problem - the gears binding or a short causing the decoder to shut down.  I detached the loco from the tender but that made no difference.  I dismantled the tender and everything appeared to be running smoothly.  The loco still would not run backwards.  I put the decoder in another Roco loco which uses the same decoder.  The same problem; so the decoder is clearly defective (confirmed by putting a Döhler & Haass decoder into the loco, which works fine).

 

I think that I will have to resort to the Zimo repair service, but I am curious as to how the decoder works in one direction but not the other.  Also why does it work all right in the tester?  It might be because the tester motor is very small and efficient, but the larger current for the big Roco motor is too much for the decoder in its present state (though well below its normal current capacity).

[WIMorrison]

Sounds​ more like a mechanical fault in the 'transmission' to me   

[John K]

The decoder is not the problem here.

John K

[Neils WRX]

Can you try it in DC ?

Does sound like a mechanical problem though.

 

Neil

[TWG]

Thanks for the views put forward.

 

However if the problem is something mechanical in the loco, how do we explain that it runs well in both directions with the D&H decoder?  And why does the decoder cause the same problem when put in another loco, which with its own decoder (of the same type) has excellent performance?  I think that the fault must be in the decoder.

[Phil S]

Phil: The Motor drive from a decoder is via an 'H' bridge .... where  the motor is in the horizontal part, and +ve at the top, and -ve at the bottom,and turned on by transistor switches ...

 

To drive 'clockwise' apply +ve to the top left of the H, and take the -ve connection from bottom right.

To drive 'anticlockwise' or 'to the left' through the motor in the horizontal part ....close the top right switch, and bottom left switch.

 

'Problems' occur when one of the switches is damaged by excessive current/heat,and does not turn off ( a short circuit ),   or does not turn on at all  - resulting in no drive in the affected direction.

 

(There are additional components involved to handle back-emf etc, but that is the basic idea .... turn it on for.longer,and the motor goes faster)

 

Historical Note: ZERO-1 decoders originally used Triacs - a switch which can pass current either way ... and Zero-1 did not rectify the track voltage like DCC decoders do ... it relied on switching the Triac on 'during the forward pulse' or 'during the 'backward pulse' ....

BUT they sometimes failed to switch off when the pulse ended(reversed) ... resulting in much smoke !!!   [ a quick cure was sometimes to scrape off the smd capacitor involved ] ... the problem was avoided by changing to a Thyristor for each direction  

Edited May 12, 2018 by Phil S

[Crosland]

From the description, it's definitely an issue with the decoder.

[shibushe]

Turn the loco the other way round if it still wont run in reverse then its the decoder.

[Crosland]

Turn the loco the other way round if it still wont run in reverse then its the decoder.

 

With DCC reverse is reverse relative to the loco, regardless of which way round on the track. I.E. A kettle in reverse will run tender first regardless of which way it faces on the track. The decoder is still trying to drive the motor in the same direction.

 

It's a complete waste of time in any case, as the OP has already confirmed that the decoder behaves the same in a different loco.

[Phil S]

With DCC reverse is reverse relative to the loco, regardless of which way round on the track. I.E. A kettle in reverse will run tender first regardless of which way it faces on the track. The decoder is still trying to drive the motor in the same direction.

Phil: Normally I would agree with this statement 100%  - except for the strange case of the dog in the night -sorry certain Hornby TTS decoders ! -  these DID behave differently (ie not at all) on certain systems in one physical orientation -until reprogrammed by Hornby -as spotted first by a Canadian member of this forum -and I didn't believe it at the time - but it was the case with 12 of mine!

As to why -I know not.  (Zero-crossing times on the controllers -'ok or not' was the same. Preambles or repeat rate may have differed -I didn't check that.  It is known/accepted that with (some) accessory decoders; they may only program when connected one way