Hornby Thompson A2/2 DCC electrical issue - help!

[doctorsoil]

Hi, I have  just bought a new Hornby A2/2 'Steady Aim'. It worked well on DC, but when fitting an 8 pin decoder and placing on the DCC track, it zooms off at high speed instantly without any control.  An alternative Digitrains sound decoder also did not work (but it and the other chip do with another Hornby model) so i suspect there is an issue with the decoder mounting plate or elsewhere - any clues how to troubleshoot? 

[peterm1]

Carefully check the decoder socket. Best thing is to unscrew it and examine underneath as well as on top. I've had more than one socket with dodgy wiring causing problems like this. It just needs a tiny connection between two pins and you've got a fault. Another thing is that, sometimes the pins on the decoder can touch the chassis when the plug is pushed in to the socket. If so, snip the ends off of the long pins.

 

You can also hard wire as I do.

[Dominion]

I use a volt-meter to test if there is any connection between any of the 8 sockets and its neighbours on all Hornby 8 pin locos now. With the blanking plug out of course. All 8 should be isolated from each other. When I have found a fault it has been a small solder bridge on the underside of the socket. Fixed with a knife or triangular file. 
Good luck with your loco. It is a nice model once you get this issue fixed. Tom

[doctorsoil]

Many thanks for the advice! A thorough inspection, to be confirmed at a less ungodly hour, has strongly suggested pin 1 (orange) had a red wire incorrectly wired to the right-side loco wheel pickup rather than motor+, and pin 8 had a red wire going to motor+ rather than right-side loco pick up. The right-side tender wheel pickup wire was also correctly attached to  pin 8.  So all in all 2 fried chips for my efforts (at  least they were cheap Hornby!). It's pretty shocking how such a mistake could happen at the factory.... 

[peterm1]

Unfortunately, as I alluded to in my post, this sort of mess happens too often. You might be better off doing what I usually do, and strip the whole lot ote and wire it in the right colours.