R3854X LMS Duchess of Kent

[Jeditommo]

I purchased the above model with a pre-fitted decoder, it would seem that the decoder has blown as the train no longer responds yet works on dc with a blanking plate fitted. I was wondering if it is still just simply a matter of replacing the decoder or is it in some way different having come pre-fitted. Any help appreciated.

[Flashbang]

If you want the same then you will need to obtain a Hornby R8249 decoder. You of course can also fit a non Hornby 8 pin decoder if wished, so long as the replacements physical size fits into the space available .

Are you 100% sure that the original decoder has blown? Have you tried resetting the decoder via CV8 value of 8 this will reset to address number 03. Or just tried the loco with decoder fitted on address 03?

[Jeditommo]

Yeah tried to reset it but it constantly says it can’t read the decoder, also when I place this loco on the track none of my other trains work, once I remove it the others all work fine again

[96RAF]

Another example of where a decoder test rig is invaluable. You can prove the decoder away from the loco, which in turn proves the loco is good or likely the root of the fault.

[Brew Man]

I can vouch for that. I just used mine today to verify a dead decoder that I only bought recently.

[Fishmanoz]

I would certainly agree some form of decoder verification is needed. Given Tommo says no trains work when the suspect decoder is fitted and the loco is on the track, the loco + decoder is likely to be causing a short circuit. This could be either the decoder itself (input stage bridge rectifier etc failed short) or decoder pins in the plug shorting to something. So I think more needed to verify the decoder itself is dead.

Good start would be trying another decoder.

[ColinB]

I had exactly this same issue with a loco I bought second hand. When I posted the fault someone mentioned that there were some early Hornby decoders that failed this way. You could try resetting the decoder, I did have one where someone had managed to load an address of 2. I suspect though, it is the same fault as I had. If it doesn't recover after resetting the decoder, buy a new one.

[Jeditommo]

Well I certainly think it’s a decoder issue and am going to replace it, as it came prefitted I am a little confused as it the decoder has the pin socket but the rest is wrapped in black plastic which is then in some way taped down, is it a matter of just pulling it off the tape and then replacing it. As a novice regarding this I do not want to do more damage. Have posted a photo of what it looks like inside.

[Jeditommo]

Clearly it is decoder issue as when on the track the wires get instantly red hot and the black cover on the chip seems to have melted in parts. I now need to remove the chip but it appears to be fixed down by some tape or something, is it safe just to pull this up to be able to fit a new decoder.

[RDS]

@Jeditommo

I am sorry I have deleted your picture by mistake. (My fault)

Could you please repost your picture (The large version).

[Chrissaf]

The black tapes do two functions, the tape holding it to the chassis is just there to stop the decoder from flying around inside and touching something it shouldn't. The black cover is for electrical isolation to make sure that the electrical bits on the decoder do not touch something else inside the loco that has electrical voltages on it.

However, you also have to be mindful that the decoder needs to have air flow for cooling, so the tapes should not be so tight that the decoder components on the printed circuit board are devoid of air flow.

If the decoder has to be removed, then just peel away the holding tape so that the decoder is free, but still has its black cover on it. Then just unplug the decoder from its socket. In your case, the photo shows an 8 pin plug and socket.

Note that with the decoder removed, the loco will not work on a DC controlled track unless a DC Bypass Blanking plate is used to replace the 8 pin decoder plug in the socket.

The 8 pin decoder plug is highlighted in the red box below. It is this plug that is pulled out of its corresponding socket to physically remove the decoder from the chassis.

Your text description would infer that the decoder has been damaged and is creating a 'short circuit' theoretically this should have shut down the controller. The cause of the decoder failure needs to be investigated else plugging in a new decoder may possibly damage that one as well. You should ideally fit a 'Blanking Plate' I mentioned and test the loco on a DC Analogue power supply, before plugging in a new decoder.

EDIT: I have just spotted in your original starter post that you have already fitted a 'Blanking Plate' and tested the loco on DC.

[Jeditommo]

Thanks for all the info will certainly follow your advice.