Michel
Just been taking some photos of my bike for you. Is this the BTH mag / dyno you have?
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If so the original BTH (not Miller) cutout fitted on top of the toolbox. Here is a picture of mine. You can just see the circle in the top that shows the BTH symbol. The cutout can be replaced by a diode rated at 10A to 15A. The original BTH cutouts are extreamly difficult to find and I don't know of any replicas been made. The cutoput just stops the battery discharging back through the dynamo when the dynamo is not generating.
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The orignal Miller headlight switch has 4 positions. Off, Ch, H, L. When used with the BTH dynamo, Off, disconnects the dynamo from the battery. Ch connects the dynamo to the battery through a resistor to limit the charge rate. H switches the headlight on and bypasses the charge resistor to allow maximum charge. L switches the side light on if fitted. The 3 position switch you have is a aftermarket copy which was never used on Vincent's and so will not work for this charging system as it is. There are replica 4 position switches available, I think the VOC spares sell them, but these only have four contact positions on the back which is what series B & C used. The switch you need has seven contacts on the back to allow for switching the charge resistor. You may have to find an original.
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There is a possible alternative. Brian Werrett (in UK) has just completed restoring the Model PS for the club and was shown on the front of June's MPH. This bike also has a BTH with the cutout. Brian is thinking of getting an electronic regulator made by a company which will act as the cutout and also regulate the dynamo output so no charge resistor or switch in the Off or charge position will be required. This regulator will automatically work like the later bikes did, but it will only work on this type of BTH mag dynamo. If you send me your email address in a private message through the forum, I will forward it to Brian.
Regards
Simon
VOC Machine Registrar