DCC Problems [A4 sound loco not responding to DCC commands]

[sgtbiro]

Hi all i bought a railroad A4 locomotive with a sound chip on board a couple of years ago i ran it a few times without any issues then due to circumstances it got boxed up and stored in a warm dry place, when i have now come to run it, it just runs as soon as you put it on the track and wont stop no matter which button you press except the emergency stop any ideas anyone.

Cheers Kevin..

Mode note - title amended to aid future searches

[Howbi]

Sounds like DC runaway where the decoder thinks it is getting a constant DC voltage instead of the DCC command. You will need to put the controller into programming mode and change CV29 to a value of 2. This will prevent DC running. Which controller are you using?.

[Fishmanoz]

I agree with Howard but wonder what has caused this now versus when run previously.

It could just be dirty track, wheels or pickups, including pickups not properly tensioned on the wheels. So I suggest you check all of these.

More worrying, have there been any recent short circuits on your track while the A4 was on it? These can affect decoders making them more susceptible to DC Runaway.

Are you using a track bus or just DCC Point Clips? If a bus, is it fitted with snubbers. Snubbers damp out damaging high voltage transients when shorts occur.

Howard’s suggesting to set CV29 to 2 assumes it was on 6 (often the default value) in the first place. What you actually need to do is subtract 4 from its current value. And I’d write 8 to CV8 to first reset the decoder before anything else. You’ll have to re-write your loco ID after this as resetting puts it back to 03.

Finally, a word on getting a better experience on here by using informative titles. A large number of the topics here are DCC problems, so your title tells us little. Title I would have used is “A4 Loco Not Reponding to DCC Commands”. With that title, more relevant replies are likely.

[96RAF]

@Fishy - Brian is Howard.

@Kevin - a couple of questions.

DC runaway seems likely and this reminds me of the old unreliable R8215 decoder, which could lose its memory overnight and go back to default settings.

As you seem to have no control it is difficult to confirm but I was going to ask if the loco was responding to its original address or had it reverted to default 03. The option is to read the address on a programming track if your controller is so capable, else either perform a decoder reset as suggested or simply refresh the original address.

[Fishmanoz]

Oops, sorry Howard.

[Howbi]

Not a problem, Fishy. sunglasses......HB

[sgtbiro]

Hi all thanks for all the replys i have just noticed that the decoder is getting really hot so hot you cant touch it without burning your self does this mean the decoder is broken.

cheers kevin..

[Chrissaf]

In this reply, I am assuming that the original loss of control issue has not been fixed and that the heat comment is just providing additional information.

If we then also assume that the sound decoder is a Hornby TTS and not an ESU Loksound [as found in Hornby XS coded locos]. Then faults with the decoder H Bridge motor control output circuit are far too common in Hornby TTS Decoders. The H Bridge contains four semi-conductors in an H formation that control the motor speed and direction.

Semi-conductor failures could feasibly create an output condition where the electric motor is given full speed with a complete loss of DCC control. This type of fault could certainly account for the observed high temperature heat.

This H Bridge failure can easily be invoked via a simple case of poorly soldered wires (at the factory) touching where they are in close proximity on the underside of the 8 pin decoder socket. A situation can then arise where the touching wires short the decoder motor H bridge output circuit to the DCC track voltage. This will instantly damage the H Bridge semi-conductors. If this socket wiring fault is present and not fixed, any replacement decoder will also be instantly damaged. So this wiring fault possibility needs to be checked for prior to decoder replacement if only to eliminate it as the source of the original failure.

These H Bridge decoder faults are not repairable and would need the decoder to be replaced.

[ColinB]

If it is too hot to touch it is fair to say it is probably broken. I am surprised it is not smoking, when my ones have failed that is what happens. In one case I thought I had fitted a smoke generator. Generally the next symptom is it puts a direct short across the track. As it did work initially it could be it didn't have an insulation on the decoder, with the moving around the PCB has moved and is now touching something metallic. The other issue I have had with them is cooling, sometimes if you "shrink wrap" them then there is not enough air to keep it cool, but from your description it was probably factory fitted. Finally it could just be a bad device, I think I remember reading somewhere that most electronic devices fail in the first 50 hours, which is about right for your loco. Unfortunately in my experience, TTS decoders are not the most reliable of things, the latest ones I have bought seem better.