I said I'd tell you about a signal cabin but I've experienced a strange fault with the Class101 when working on 'Birch Vale'. It's so strange that I think it's worth re-telling on here in the hope that the knowledge may help others with future faults.

Both my S4 layouts have been DCC operated for a long time but with the ability to quickly switch at will to analogue dc. Accordingly the 101 was wired with a decoder and sent to Birch Rail, for which it was always intended, - Bowton's Yard is supposed to be freight only.

It ran well except that at one particular turn-out (there's only 2), it always came to a sudden halt. The same bogie was always in the same place. Because it had suffered pick-up problems, which I thought I had solved, then I suspected I was wrong and there was still a pick-up problem.

I could locate nothing!

My thoughts then turned to the electrics of the turn-out. Where the train always stopped, the motorised bogie was standing on one of the turn-out's closure rails so it was likely that the closure rail wiring had a fault.

To make it easier to use a voltmeter I switched the layout to analogue and tried to drive the train to this stop position, However the fault now seemed to have disappeared. On dc the unit ran OK; on DCC it always stopped at the same place - the fault had not disappeared!

I was now in total confusion, and ended up 'groping around in the dark' for a total of about 8 hours over several days. In the end it was only chance that gave me the required clue.

Birch Vale's analogue controller has an ammeter and a voltmeter built into it and as the 101 ran over the offending point I noticed the ammeter needle flutter. When I slowly manoeuvred the vehicle to the fault position, the needle shot over to 'dead-short'.

Next was to try the DCC again. With the DMU running at normal passenger speed it stalled at the fault position but now I was watching the handset's read-out - "fault switch off" but then, immediately - "fault cleared" and every thing re-set.

At last I knew that with that particular bogie, at that very precise position on the turn-out, there was a short circuit - not a pick-up problem at all.

What must have been happening was that the short-circuit protection in my analogue controller was too slow to operate as the train passed this precise position and the fault time was so brief that it did not show. The NCE DCC system's short-circuit protection is very rapid and shut the system down but train momentum cleared the fault and the DCC quickly re-set itself, but only back to 'train stopped'. So the train stayed minutely beyond the short-circuit position, ready to be off again at the driver's command

The cause of the fault was removed in less seconds than the hours taken to find it!!. A little too much solder on the new pick-up unit just touched the top face of the unused closure rail of the point, and that closure rail is always at opposite electrical polarity to the rail from which the pick-up is collecting. A few file strokes cleared the problem but a couple of coats of nail varnish offer further insulation. NB the nail varnish is very quick drying, needs no brush cleaning, and paint thinners easily removes it.

It all pinched a lot of precious time and was very frustrating but the eventual success is very rewarding.

Model well,
Dave