My BSA C15 bike has been converted for trials use from a standard road machine, and the circuit (12V) is identical to that shown for a B50MX, as shown on page 195 in Rupert Ratio's book. The problem is that we have now blown two zener diodes on the bike We are having serious difficulties with the electronic circuitry on a BSA C15. This runs for a few minutes, then stops when the zener diode goes short circuit. Without the zener diode there, the bike will run fine, but the coil soon heats up since 30V is now being applied across it. Our next move was to leave out the zener diode - at £30 a time this was getting expensive - but put a 2.5 ohms 100 watt resistor, mounted on an aluminium heat sink, in series with the coil. The bike was now reluctant to start, so we fitted a switch across the resistor to short it out when starting, then opened the switch when the bike started, to bring the resistor back into the circuit. Again this worked fine for a few minutes; the heat sink warmed up but not excessively since one could still touch it. Then the resistor went open circuit and the bike of course stopped.
We are at a loss to understand what causes these failures. Is the capacitor in our circuit the correct one? At the moment it looks like a standard 4700 microfarad 40V type, with the letters BHC on it; is this suitable, or should a special type be fitted? During all this, the rectifier itself has given no problems.
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