1. Check the battery. If its worn out, it can still have 12V without load but the voltage will dip under load.Try and measure the voltage over the disconnected battery with a 35W headlamp bulb connected to it.

2. If the battery is good, there must be a substantial load on it when the switch is on, so there should be smoke or a very hot component somewhere.

3. Try disconnecting the RC valve servo. If the valve is blocked, it will pull a high current. (check also the PGM for any signs of overheating) Shorted RC valve transistors will pull a high current and melt the rubber surrounding them in the PGM

PS: Andy my rectifier burned out and cooked the battery. Luckily nothing else was damaged. I now have one with cooling fins.
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Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom