DCC short circuit - faulty decoder?

[windmills]

I have run my Bachmann Eddie the engine Class 66(00 gauge)continuously for 4 hours recently.....then found it would not respond to the NCE handset (ie I could not stop it). Having removed it from the track, I then put it back on to discover it was still running. Several days later, on a different layout it was still running when placed on the rails. I placed it on the programme track to discover there was a short circuit. With the body removed (and lights disconnected) I carried out a thorough test of all wiring using bulb and battery but could not find any trace of short circuit. Programme track test is still showing short circuit......this leads me to think that the decoder may have developed a fault......could this be so, and what would be the cause? Decoder is 21 pin Bachmann plugged into the socket on the loco. (All my other locos run normally using the same handset on the same track).

[Grovenor]

The programming track will report a short circuit just because it sees the current from the motor running. However the decoder should be resetting itself when its taken off power. So something is wrong. Check the instructions to see if one of the CVs will give it a factory reset. If so then disconnect the motor and try programming the reset. Without the motor you won't see any acknowledge from the decoder or be able to read back any CVs but the programming may still be effective. If not then the decoder needs replacing.

Keith

[noiseboy72]

Check for a short between the track pick up and motor - one side shorting will be enough.

 

If the fault developed during the 4 hour running, it may well have fried the decoder as well.

[windmills]

The programming track will report a short circuit just because it sees the current from the motor running. However the decoder should be resetting itself when its taken off power. So something is wrong. Check the instructions to see if one of the CVs will give it a factory reset. If so then disconnect the motor and try programming the reset. Without the motor you won't see any acknowledge from the decoder or be able to read back any CVs but the programming may still be effective. If not then the decoder needs replacing.

Keith

Thanks grovenor and noiseboy for input....I have disconnected the motor (brown and orange wires) from the decoder but still getting short when I place the loco on the programming track. What I have discovered is that I get a short (bulb and battery test) between decoder pin 19 (motor right) and both rails of the track. No short on pin 20 (motor left). Would this indicate that the decoder is faulty?

 

Even though I disconnected the wires from decoder to the motor (or have I???), programming track still shows short...I cannot progress beyond this point to get a CV reset to factory default.

 

What concerns me is, if I fit another decoder will I eventually (or sooner) experience the same problem as I do not know what has caused this decoder to blow (if that is what has happened).

[noiseboy72]

It does sound like the decoder is faulty. It might be worth checking for any foriegn body shorting out the board (bit of swarf or loose cable strand), but a short to both rails indicates the driver chip may have failed.

 

As to whether a new decoder would go the same way, It is difficult to tell. A higher powered type might well be a good bet, as possibly the amount of heat generated is causing the board to fail on long running sessions.

[windmills]

It does sound like the decoder is faulty. It might be worth checking for any foriegn body shorting out the board (bit of swarf or loose cable strand), but a short to both rails indicates the driver chip may have failed.

 

As to whether a new decoder would go the same way, It is difficult to tell. A higher powered type might well be a good bet, as possibly the amount of heat generated is causing the board to fail on long running sessions.

 

Stripped the entire loco assembly - no obvious cause of short. Reassembled and swapped decoder from another Class 66....tested all Ok, programming track also gave clean bill of health, so looks as if Eddie's decoder has definitely failed. Will buy a middle of the road replacement and see how that fares. My other Class 66s (3 of them) have all run longer periods than Eddie the engine without problem....so maybe just a duff decoder from the start?

 

Thanks again for the advice.