Class 43 (Valenta) TTS decoder causing R3608 HST power car to "run-away"

[DaddyPig]

Hi All, 

I have tried fitting the TTS Class 43 (Valenta) twin pack decoder into my Railroad R3608 Class 43 HST power car. The decoder programs ok (I am using the Hornby Select) but as soon as I increase speed the engine takes off and no longer responds to speed control from the select. It will not stop even when the emergency stop is pressed. I can stop it by programming the decoder again

 

I know the decoder and controller work ok, as i have tested decoder in my Class 66 loco and all behaves perfectly (although the class 66 does not have the extra lighting PCB?).

When I use a normal decoder in the R3608 HST (one with no TTS sound) the loco works perfectly and it also works fine as an analogue loco with no decoder.

 

Are you able to advise as to what is going on please as i have eliminated the decoder, loco and controller as being the issue. This seems like an incompatibility between the TTS decoder and the loco?

 

I did note that the box does not say "DCC ready" although but does have the 8-pin socket ready to accept the decoder and works fine with the basic decoder.

 

I would really like to get this working as I have the twin pack TTS decoder and the spare parts to modify the dummy car for sound.

 

Thanks and Best Wishes.

 

[Chrissaf]

Everything in your post description is compliant with the TTS decoder being affected by 'DC Runaway'.

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TTS decoders have a habit of exhibiting an unwanted feature called 'DC Runaway'. Your TTS decoder may be [probably is] being affected by this. The cause of DC Runaway is usually an electrically noisy path between the controller output and the TTS decoder. Non TTS decoders seem to be less affected by this, which is probably why your loco works fine with a non TTS decoder fitted.

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It is really hard to identify the exact point in the path where this noise might be being generated. It could be just dirty track or dirty wheels, possibly poor quality of the electrical wheel pickups or poor connections within the loco wiring itself. But whatever the cause of the noise, the effect is the same.

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Basically, DC Runaway occurs when the DCC signal is distorted enough by noise such that the decoder loses the ability to synchronise itself with the DCC digital signal. The decoder in essence doesn't recognise the track signal as being a valid DCC digital signal any-more. In your case, when a certain speed is reached.

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In the absence of a valid recognisable DCC digital signal the decoder switches itself into 'DC Operation' mode. The DCC track voltage which is Bi-polar [a form of AC] is fed through a bridge rectifier built into the decoder and the decoder interprets this rectified track voltage as being DC Analogue. But because DCC track voltage is always at full voltage, the decoder interprets this as maximum DC analogue speed and the loco shoots off.

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Once the decoder has switched into this 'DC Operation' mode it doesn't switch back to DCC and becomes uncontrollable using the DCC controller. The power has to be removed from the loco to force it back into DCC mode.

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This issue with TTS decoders is so widespread, that many TTS decoders (particularly factory fitted ones in TTS locos) are now being shipped by Hornby with 'DC Operation' disabled.

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The very well documented solution to this problem is to disable 'DC Operation' in CV29. This requires a controller that can write a value to CV29. Only Hornby Selects with firmware version 1.6 or higher can do this ... see Select firmware upgrade documentation below if your current Select firmware is less than version 1.6.

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It is assumed that your TTS decoder has been given a DCC address between 001 and 127 then I suggest a new value of 2 should be written to CV29 of the TTS decoder. If your current TTS decoder address was to become in the future 128 or above, then I suggest a CV29 value of 34 should be written to it.

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The URL link below will take you to an online interactive CV29 calculator, which should help you make sense of these suggested CV29 values:

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http://www.2mm.org.uk/articles/cv29%20calculator.htm

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The current installed firmware is displayed as the very first number to briefly appear on the Select screen as it is being powered up. Expect to see a number between 10 and 20. At the time of writing, the latest version is 1.6 [16] or 2.0 [20] - both the same. These latest versions completely transform the user experience of the Select controller and add the ability to write (not read) all CVs up to CV255. As well as access to the full F0 - F28 range of functions.

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The upgrade charge is typically £15 plus VAT [£18] plus your sending postage costs. You need to get a 'returns number' from Hornby Customer Services - see 'Contact Us' link at the very bottom of the page.

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If you do arrange to get your Select firmware upgraded, then you will need both the Select version 1.5 full manual and the version 1.6 addendum manual. As the operation of your Select (how to use it) will be completely different in certain areas of operation compared to your current firmware version.

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You can download these two manual PDFs from here:

https://www.hornby.com/uk-en/downloads/view/index/cat/12/

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If nothing appears to happen, check your nominated browser download folder for the files as they can download silently in the background.

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TIP: As a newbie poster on the forum, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.

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See also – further TIPs on how to get the best user experience from this forum.

https://www.hornby.com/uk-en/forum/tips-on-using-the-forum/

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Note: Forum has a BUG at present that prevents making the URL links above clickable and also prevents any images being posted. Copy and paste the links above directly into you browser navigation (go to site) box.

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[DaddyPig]

Hi Crissaf,

Thank you so much for the quick reply and the details within.

Thats so helpful and I will see about getting my controller off to Hornby for upgrade. I know they are on "15" as i sent them back a few years ago for upgrade.

The beahaviour it is showing certainly makes sense with your explanation. I did check all the wiring and pickups as i had to repair a couple of dodgy connections. All connections seem ok and continuity/resistance seems good to all wheels.

This will give me an excuse to get the oscilloscope out and have a little play too if the wife will let me have a break from DIY! 

Will be great if we can get the sound working as my 10 year boy loves these HST's and hes running them with some old coaches of mine from when i was a kid 30+ years ago. Ive got some spare bits to try and get sound working in the dummy car if sucessfull but one step at a time I think

Thanks again for your advice.

[96RAF]

Select can only operate locos on address up to and including 59.

Have you checked your track power connectors - power clip or power track are digital compatible, i.e they do not have capacitors installed. Flip open the cover to check and if found just snip the capacitor out.

[Chrissaf]

In terms of noise generation. Personally I would be focusing on the wheel pickups. The transference of power and thus the DCC digital signal is totally reliant on the quality of the physical contact between pickup and wheel. As the wheel rotates the pickup wiper has to rub against a moving contact area. Any ingrained dirt on the wheel pickup areas passing by the pickup contact can produce electrical noise pulses. Whilst the loco is moving slowly these pulses comparatively to higher rotational speeds will be few and far between allowing enough of the DCC digital signal [nominal frequency is 7Khz (7,000 cycles per second)] to pass through and be decoded. At higher rotational speeds it is possible that the increased rate of these noise pulses is enough to lose the DCC synchronisation and trigger 'DC Operation'. It is only a theory, but it does fit in with your observations regarding loco speed.

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This will give me an excuse to get the oscilloscope out and have a little play too if the wife will let me have a break from DIY!

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As you have a scope ... look for ringing on the leading edges of the DCC signal or general distortion. A perfect signal should be a very clean square wave.

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This link below gives some example DCC waveforms both clean and with various amounts of ringing (one is a particularly severe ringing example at 60 volts peak to peak). Scope screen 7 is in my view very useful as it shows how fitting a 'snubber' can really provide some effective DCC signal conditioning. I use 'snubbers' on my DCC layout and rate them highly.

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http://www.wiringfordcc.com/dcc_waveforms.htm

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'Snubber' theory link provided for additional info.

https://sites.google.com/site/markgurries/home/dcc-general-best-practices/wiring-planing/snubbers-rc-filter

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Remember though, that the track waveform may be perfectly clean. This will be difficult to achieve, but you really need to be looking at the waveform on the input to the decoder (red and black wires). Might be possible with a 'rolling road' set up.

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EDIT: Rob makes a very good point about the track power connectors. If you are using either R602 or R8206 or R8201 then none of these are compatible with DCC and need to be either modified for DCC operation or replaced with R8241 or R8242 DCC friendly track connectors. The modification is simple to do and as Rob has described.

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The capacitors in the DC Analogue power connector products cause DCC signal distortion. This distortion affects different brands of decoders in different ways and has proven on this forum to be the cause of control issues on Hornby TTS decoders in particular.

 

[DaddyPig]

Hi Chrissaf and RAF96, thanks for the replies and additional information.

My controller is soldered direct to my test track as i had issues with the spring clips worn out but this is a DCC track connector.

I have started the ball rolling with returning the controller for upgrade but the guys at Hornby are backed up so may be a 3 weeks + before they can accept the return.

In the mean time i have used it as an excuse to buy a rolling road test bed so i can check the waveforms with the scope and make sure all pickups are solid.

Ive been meaning to get one of these for a while as have a couple of locos in need of some TLC and this will make life easier. My kids (boy and girl) will love doing this too as I like to keep them involved and playing with tests tracks, multimeters and scopes helps keeps their interest in the hobby too.

Thanks again to you both for your help as you have given me a great place to carry on troubleshooting!

 

[Chrissaf]

Thank you for the feedback. If you have not yet purchased your rolling road, may I suggest putting this one on your 'short list' to consider.

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https://tencommandmentsmodels.co.uk/product/rolling-road-oo-ho-compact/

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