Thursday 2 October 2008

Goofy blows a decoder

It had to happen, though its taken a while. But I blew up a DCC decoder yesterday.

I hard-wired the wagon with AJ couplings, and tested with a multimeter. All seemed fine. So I pushed some connectors onto the wheels and the decoder malfunctioned. Much prodding with multimeter and the DCC programmer followed, but the chip was dead.

Head scratching for several hours followed, until I realised a potential short circuit route; if the suspension of the wagon is compressed on one side only, a wheel rim can touch a W-iron. The W-iron base is used to solder the AJ wire. The AJ wire is pushed by the operating cam, and the operating cam is connected to the function output. BANG!!. The answer, by touching the wheel against the W-iron I could have put track current onto a function output wire.

Lessons learned:
a) Should have put the blue common onto the "dangerously exposed" bits of the mechanism, rather than the function output. Better still, should have wired it without a blue, but used half-wave back to the track (red/black) as in a NEM651 socketed decoder. (Check decoder supports this wiring before doing it!).
b) Should insulate EVERYTHING so it cannot short.
c) When fitting AJ couplings, should ensure that the coupling is not directly soldered to a metal W-iron, otherwise a short can go the length of a train. Same must apply to metal buffers attached to metal underframes.

Changes to the test wagon:
1) Lots of insulation added; the operating cam for the uncoupler is now rubber coated, the cam bracket has insulating tape between it and the W-iron. Insulating tape added where memory wire might short against W-iron it crosses. Plasticard added to prevent AJ wire shorting across.
2) Checking whether chip will support half-wave wiring (ie. don't use blue, instead connect to either red or black).

And finally, I will be sending the FL2 back for exchange under TCS' very generous Goof-Proof replacement scheme. (Replacement received in 48 Hours, fitted into wagon, all now working well done Bromsgrove Models for the prompt service)

(signed) Goofy.

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