LCGB Forums

Are all brands of AC 12 volt regulators compatible with Varitronic kits. Bulbs are blowing again on same system. Second replacement regulator to fail after about a thousand miles use roughly, since replacing with same small original regulator. What would cause the part to fail . Wired up as Varitronic diagram shown. Copper braided 3 point earth cable on strut, mag, regulator. Thanks

These are shunt type regulators. They either use a very heavy Zener diode to dump the excess voltage or or a zener controlled thyristor. Either way, the excess power is shunted to earth.
With powerful magneto's, this excess can overload the regulator and burn it out, usually resulting in an open circuit. Therefore, no more regulation, which leads to your bulbs blowing.
The simplest way around this is to always make sure you run with your main lights on, so the regulator isn't having to dump all the power from the magneto to ground. It then just needs to dump the excess after your lights have taken their fill.
Its a good idea to always have your lights on anyway. So..... Win Win.

Apparently the BGM regulator works with the varitronic set up.

Thanks. I had a read through Cambridge Lambretta notes and they do mention this kit relies heavily on good earths to work correctly. Looking at a couple of wiring diagrams that varied in detail ( I had used white paper diagram ), yellow version was showing coil to be earthed on postion 2. I have not got a earth wire coming off the coil apart from the blue earth wire coming in via regulator. Have I balls up here, not fitting a earth here ?

You do need to separately earth the coil. Good earths on the varitronic are a must. It'll cut out at high revs if not..don't know if that will have any impact on your immediate issue. Sounds like the rectifier isn't rectifying.