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[Hugh]

Please note question is for a 1957 model Redditch which was originally 6 volts but now 12 for a number of years with no issues. Amp meter registered charging discharging previously. Today checking battery with multi meter for charging voltage so increased revs on bike and reading across battery terminals did not change from 12.6v - pre starting reading. Expecting it to rise as alternator produced output but as said stayed at 12.6v. Wondering then if rotor after 67 years can lose it magnetism. Bike uprated from 6v to 12v but has been standing for some time while sorted magneto (see earlier post this week) but battery seems to hold charge Alternatively could stator ring be no longer functioning effectively (or both stator and rotor failing). Note later British bikes with 12v had Zener Diodes fitted so not sure what parts I need given Lucas will have changed/developed their kit and as said bike now 12v not original 6v although assume as no problems previously the original alternator had plenty of capacity to run at 12v. I only have a simple rectifier. Not sure if this is right in terms of a useful test but if I remove/disconnect battery and read across the disconnected positive and negative wires get a reading of 1.5 volts when revving bike which seems to indicate there is alternator output and rectifier doing its job. Any comments helpful. regards Hugh

[Adrian]

12V needs some sort of voltage regulator as it normally works with the alternator wired to give constant full output, EITHER a zener diode used with a separate rectifier OR a combined regulator/rectifier unit, single phase in this case. 6V worked by switching in extra alternator coils when the headlamp was turned on.

A few years ago I had a '57 500 Bullet whose original 6V 3 wire alternator was still healthy enough to power a 12V conversion, I just joined two of the stator wires together for permanent full output (can't remember which, I'd have to look it up) and ran the output through a single-phase reg/rectifier (a Sparx one with a built-in capacitor), all worked fine. A good 12V conversion also means a simpler wiring loom.

That said, your alternator stator CAN lose magnetism, if the stator is still good it might be worth seeing if you can get it re-magnetised. Also modern reg-rectifiers as well as older-style rectifiers can leak.

If you do need a new alternator and your '57's is the original, replacement with a Lucas RM21 or similar will also require an adapter ring in the primary chain case.

A.