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Mechanical CVC's


A mechanical cutout or CVC was the height of technology in its day. They did their job but never with any efficiency. They wear with use and age to the point where they actually are a burden to the charging system or stop working all together.

If yours is faulty, there are 6 options open to you.

1. Fiddle about with it and try and make it work again.

2. Have it rebuilt by a specialist at great expense so it is as efficient as it was when new (see above).

3. Find a 2nd hand unit and set it up to suit your dynamo.

 4. Find a new old stock unit, set it up to suit your dynamo and hope it lasts..

 5. Spend £23 to £100 or more on an Indian made copy of the Lucas RB106, RF95 or MCR2 which will last 2-3 years if you're lucky before it needs replacing AGAIN.

6. Go to the electronic conversions section of this site


My advice is to avoid numbers 1,2,3,4 and 5. 


Regarding number 5, there are a lot of very sub standard copies out there to be had via ebay etc. Indian made copies of Lucas units. All I can say is AVOID them like the plague. I have had a few of these units sent to me for conversion and to be honest I wouldn't entertain fitting a high quality electronic regulator inside such a poor quality unit. 

Here's why they fail in the first place. The case is made from thermo plastic (not bakelite as with original units) that softens when it gets hot. The heat generated by the cheap resistor on the underside together with the heat generated by the contacts arcing when they open and close generates enough heat to cause the base of the copy regulator to deform as it softens. This loosens the screw which holds the assembly of the coils to the base, the cut out contacts close and full battery voltage is applied to the dynamo (trying to turn it as a motor). The dynamo then heats up and burns out, which can result in a fire.

They may look like a new Lucas regulator but their quality is very, very poor. They sell new for around £23 up to an incredible £130. 

You are spending your money on something that looks like the real thing but is even less efficient than an original Lucas unit.


They look like these ones pictured below.

Note the lack of the word LUCAS on the lid. The RB106 copy has a silver sticker on the front.

Don't be fooled into thinking you are buying a genuine Lucas unit or that these will work with any efficiency. They won't.

I tested such a 12v RB106 copy on my test bench with a fully charged 12v battery and the charge voltage was nearly 16v. Charge current was 4.2 amps with no load. Under load the charge voltage dropped to 15.2v. With this regulator fitted you would be constantly topping up your battery as it would boil off its electrolyte. Its internal plates would also warp. If your battery is the maintenance free type it will boil off electrolyte that can't be replaced.

Ideal charge voltage with a 12v system is 14.2-14.5v.

For a 6v system it is 7.2-7.5v.


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