Bits arrived from Farnell this morning so have completed the repair.
Fitted a BZV55 series zener, 4.7v 400mW (Farnell 305-0919)
I do not know why the zener had failed, worst case dissipation is only 50mW and it should see nothing like that. The transistor failing would not put additional load on the zener. Also the zener had failed short circuit so that would not have caused the transistor to blow. A mystery.
Fitted a ZXTN2031 (Farnell 108-4421) in place of the failed BC817
There is no mystery here why the original device failed. It is rated at 250mW and even with 12 V on the track will be dissipating 280mW at 16 volts the dissipation will be up to 600mW.
The replacement is rated at 1W however in practice will be less than that because of the very small heatsinking from the circuit board. (I would prefer a higher rating, but this was the best I had) Hopefully will be ok, I ran it for 30 mins on the bench at 16 Volts and it was Ok, but it needs Steve to thrash it round the track for awhile.
A question for anyone with the body off.
The pictures earlier in this thread show the transistor as a 6C which translates to a BC817. The device I removed from Steve's car was burnt so I could not read the marking. Would be interested in what marking is on other transistors.
So final thought. I don't know if people from SCX read this forum?
If they do then there is a major problem with your light board, I would expect you know this by the number of returns you get.
A comment would be nice, failing that if anyone has a suitable name / address I will drop them a line.
Final Final thought. The connections for the lights and the floating motor all just use copper to copper strip contacts. To my eye this looks like "Optimistic Technology". Do people have any problems.
Steve will put the car in the post tommorow.