The BSA A7-A10 Forum

Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: berger on 31.03. 2025 19:48

Title: carbon brushes
Post by: berger on 31.03. 2025 19:48

what length should the dynamo brushes be and has anyone got a figure for the resistance of the coil or any other info on a 12v unit?

Title: Re: carbon brushes
Post by: trevinoz on 31.03. 2025 22:06

The brushes when new should be 0.497".
12V field coil could be anything, depending on supplier.
I have seen 10 ohm from suppliers. I wind my coils around 11 ohms.
Most problems are with the armature, rarely with the field coil.

Title: Re: carbon brushes
Post by: berger on 31.03. 2025 22:35

thanks trev that rules out the brushes what do i look for with the armature apart from a bad com that might need a skim.

Title: Re: carbon brushes
Post by: trevinoz on 31.03. 2025 22:59

I voltage drop each coil on the armature and test for earth.
To drop test you need to set the armature between two contact points on the comm and apply a low voltage, usually a couple of volts.
Read the voltage across each adjacent pair of segments around the whole comm. You will need to rotate the armature to do this. I set the meter to millivolts, each pair should be around the same.
If you get a low reading, you have a short circuit. Zero is a complete short.
High reading is an open circuit. With an open you will get pairs with zero reading then a high which is the faulty pair.
Good luck.

Title: Re: carbon brushes
Post by: berger on 01.04. 2025 11:48

thanks trev i have good brushes in and got a mate to read the charge at 2000rpm with 40watt bulb in the headlamp and i do know the last time i checked this it was charging well and now it was showing a discharge until i gave it 3000rpm then it was jumping about from 12 to 12.5 v on a cheap digi meter and the ammeter did show a charge, with your good information i think i will take it off and let MWAS have it because he has all his expensive electrical reading equipment from when he was at work,  then i might or should know if anything is not right. cheers.

Title: Re: carbon brushes
Post by: Rex on 01.04. 2025 18:08

As old dynamo output voltage varies with rotation a cheap DVM isn't much good for accurate measurements as it'll flicker and vary too much.
You need an old AVO Mk8 or similar.

Title: Re: carbon brushes
Post by: berger on 01.04. 2025 19:07

Rex because my meter is basic digi  this is why brother  [ MWAS  ]   will end up with it he has megga and other stuff from when he worked so he's got another job once i take it to him