Hornby Decoder chip wiring and short circuit problem...

[Pedro1707822406]

Good morning Hornbiers. Only had my track up a week so I’m very fresh to this. I have a problem with a particular loco, the new hornby Eurostar. Its been running fine for 3 days, only had about 2 hours running time though in that and suddenly it creates a short circuit when I apply power to her. I say other locos are fine so I’m happy that it’s this loco. On further examination with body of, I can see the purple wire is disconnected from the chip. It’s a Hornby decoder. Should this wire be connected to the plug? On trying to find out more about this purple wire the fact that it’s ‘function A’ I’m unsure if it’s unconnected for sound or something. Does chip need replacing or is the purple touching something and just needs stowing? Any pointers into what fault finding steps I should make be welcome Thanks in advance.

[Chrissaf]

A disconnected purple wire is totally normal. It arises because the decoder has 9 wires but the plug and socket only has 8 pins, thus there is no 9th pin for the purple wire to attach to. On the Hornby R8249 decoder, the purple wire is the 4th function and switched on and off by F2 on the DCC controller. other brands of decoder using the purple wire may or may not use F2 to switch it. When switched 'on', the purple wire goes negative [0 volts] and can be used to power an accessory. For example a flickering LED for a firebox effect in a steam loco or a cab light in a Diesel. The other side of the LED & current limiting resistor connects to the blue wire positive return voltage. When not in use, the purple wire should be insulated, coiled and stored away within the loco. It should not be left as a bared wire as shown in your photo.

I would be looking more closely at this area [blue box highlight] to see if this bared wire can touch anything it shouldn't.

[ColinB]

It sounds like your decoder has failed, did you smell any burning? What you need to do is take out the decoder replace it with a DC header and check that the loco works on DC. As to the purple wire that is an extra function which you wire something to. The reason it is left as a flying lead is there are not enough pins on the connector to connect it. The important bit is that its end must be insulated to prevent it touching anything and damaging the whole decoder. I normally either sleeve it or paint over it with clear nail varnish. Yours appears to have some copper showing which is not good.

[96RAF]

Possibly two problems there.

1. The purple wire needs to be capped and stowed.
2. The two red wires from the pickups front and rear as pointed out by Chris’ blue box need to inspected and likely resoldered.

[Pedro1707822406]

Thanks you.

I can’t say that I’ve smelt or can smell any burning from chip.

Would I still be able to read the CV if chip had burnt out?

As for trying it on DC, I can’t find blanking plug for the loco, I’ve a stomach churning feeling that it’s been thrown out along with decoder packaging, I think this is needed to run analogue isn’t it? If so can you buy them?

[Chrissaf]

"Would I still be able to read the CV if chip had burnt out?"

Probably not.

Blanking plate [plugs].

Yes you can buy them. Try browsing via this link:

"DCC Blanking Plate"

You should always keep the Blanking Plate when converting locos. They are useful for fault diagnosis as in this specific case. Also, if you ever decide to sell the loco on, you can keep the decoder and revert the loco back to being 'DCC Ready' for the next owner. As with a working DC model there is potentially less chance of the buyer having issues from any resident decoder being installed and requesting a refund.

[Pedro1707822406]

OK Thank you.

I've purchased one to enable me to test on DC, am I correct in thinking it needs to be inserted to enable DC? Also is there a wrong way to install it?

Anyway purple stowed, isolated out of way. Just leaves those red wires although my eyes can’t see anything wrong as such, they are not loose so will ignore that for time being.

I shall report back with findings next week once I’ve been able to test analogue, I’ve ordered a new decoder to for tomorrow, I’ve a feeling with it reading the CV this won’t be the problem though.

cheers.

[Chrissaf]

It replaces the 8 pin decoder plug in the socket to enable DC, it is not polarity sensitive and can go in the socket either way round.

[Pedro1707822406]

Lol I’m looking for the like button 👍

Moderator comment - this forum doesn't have one.

[ColinB]

If the chip had burnt out then you cannot read CVs, but seeing as you have a short I doubt you can anyway. Normally though if the decoder if working you can read CVs.