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I think one half the bridge rectifier in a DCC decoder may have died. The symptoms are:-

1) slow running in DCC mode in either direction. In other words it wasn't like this earlier.
2) I can read and write CVs and factory reset works
3) Only runs on one direction on DC but the motor will run at full speed.

Am I correct?

I assume that it's not a broken connection in the lead to the 8 pin plug

What went wrong? The decoder was securely fastened with no chance of shorting out. The decoder plug is in a very confined space. Could two of the leads shorted at the plug end?

David
 

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Sounds like you could be right. You could possibly check by putting an ammeter into the circuit, set to DC not AC. If half the bridge has gone then a current should flow in the same direction on DCC regardless of which direction the loco is moving. A fully working bridge will show no DC current when fed from a standard DCC command station.
 

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QUOTE Is this the Zimo one that you tried in the Royal Scot?

Good guess! The "buzzing" noise I mentioned in the review in connection with the gradient load trial became a permanent feature so I had to go back inside the loco (no, no, no, not in there again; I promise to be good!). There was a comment on another thread about the questionable method Hornby use to keep the worm meshed with the drive wheel. The Scot arrangement does nothing to improve that impression.

Somewhere along the way of trying to get the chassis back on, I've managed to damage the decoder by squashing wires or something. If the worm wheel had not disengaged, I would not have had to go back in and there would have been no problem.

I suspect that a bit of pressure in the wrong place on the lump of metal which keeps the worm meshed in the drive gear is enough to allow the motor to move up by a mm or so and disengage the worm. Since the decoder socket sits on top of this lump of metal, there are plenty of ways of providing that pressure through spare wire or whatever.

Edwin: Thanks for the suggestion. I might give it a go.

David
 
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