My AT3 was not charging so while replacing the left side crank seal, I have cleaned up the armature and replaced the brushes which were worn down to the minimum mark. However, the battery shows 11.7 V at tick-over and when I rev it - ie no change.

I did notice that the magnets on the stator seemed pretty weak eg they wouldn't hold a small screwdriver. What should I expect?

If that is the problem, any suggestions how to fix it? A quick search didn't find any stators for sale.

Thanks

Anything to help in the service manual?

Can't see anything (but that doesn't mean there isn't, of course)

There are no permanent magnets on the AT stator Chajoma, This is designed as an electromagnet .... Yamaha has 12 volts DC from the battery switched to the copper wire coils that surrounds the iron cores of the stator ....then they become magnetic...even a large screwdriver will stick to it ... 
 If the electric starter button can spin the motor over they are likely OK
 The service manual gives the ohm values of the two coil windings in the stator to see if it is defective . There is two large copper bands and two very small ones. I’d test the ohm values of each to see if they are within the specifications 
 If so next step is to do the tests listed for voltage regulator. 

 

Hi DaveHunter

Thanks for the sterling advice. I have now been through all the tests in the manual and done all the cleaning and checking possible. The key test is to earth out one side of the supply to the regulator and then check the voltage across the regulator supply at 2000 rpm.  When I did this first I had around 1.2V (should be 14V) so I stripped everything down, checked the brushes were moving freely and not earthing out. I gave the commutator another light sanding and a good clean and put it all back together.  I checked the field loop and it was spot on at 4.8 ohms. Then I had 4.5V. When I did a test across the battery with the engine running, I could see a small voltage increase when I revved it but only from 11.7V to 12.5V - before I was seeing no change

I took it out for a run to bed the brushes in and when I got back the charge to the battery was no different. Finally, I found the starter button lead (I had removed the button when I replaced the electrics - it was very rusty and really not needed) and earthed it out.  The motor rotated twice and then stopped. I took out a plug and tried again and it happily span the engine for around 10 seconds before I stopped.

I am now baffled. Any other ideas?

Thanks
Paul

I guess it wouldn't spin on the starter except for twice over compression as battery hasn't got enough grunt from not properly charging. Lift the lid off the regulator box & observe the cutout cutting in just above idle. If it doesn't then nothing is charging. If it cuts in above idle then put your finger under the charging top arm & hold up against it & if motor then labors watch the lights on the wall & see if they get brighter--be careful & don't over do it--. Bottom line is with a volt meter across the battery it should obviously rise to 15+ or whatever, an obvious rise. A normal system you can audibly hear the motor slightly struggle as the charging starts at about '1200rpm--that was a guess' but just above a slow idle.
Another thing i've done for a brief second, was manually close the cutout & you'll see the motor kick slightly as it tries to turn it into a starter. Not recommending that at all--but i've done it more to see if there was a connection right through to the fields & brushes, armature etc.

I might a chance this weekend to take the covers off mine & see what’s happening with a properly working voltage regulator at idle. Here is my understanding of how it works. Voltage regulator gets 12 volts from the battery then through its contacts sends that 12v down to the smaller set of coil windings to make the iron cores magnetic, The brushes conduct the electricity generated through separate wire back up to the voltage regulator. Once sufficient voltage is reached the contacts open and no more 12 v sent down to coil windings. 
Electric starter I’d put meter on big green wire at starter and see what voltage is it’s getting while cranking. Also a meter with one side on battery positive + and one on big green terminal while cranking will show how many volts are lost through wire and solenoid 

Luckily--'for me'--they never gave any trouble back in the 70's except needing brushes once in a while.

Here is some random observations of my properly working AT voltage regulator. Sometimes when fixing things it is helpful to know what is normal. Picture attached.
The two relays are marked 1 & 2 with a white marker.  
 NOTE — This bike has not been started in about a month so battery has lost some of its charge 
 Key off / left side the points are open, 10.2 volts at the bent bars under the points
              / right side points closed
             / no voltage anywhere else
 Key on / no change
 
Kick start it/  green neutral light flickers with each kick as battery is so low but it starts
 Points on left side now close completely, 14.2 volts now present through those points. Voltage stays at 14.2 even when reving motor. 
 Right set of points are touching but not closing completely / constant-arcing and buzzing as they keep regulating voltage at 14v to recharge battery . Little guys are arcing like brushes on a electric motor. They never open enough to touch the top point above them. I’d assume they are the ones sending voltage down to the electromagnets.

 

There will be an adjustment screw & lock nut behind the charging one to increase tension which increases max voltage charge. The harder it charges to more the motor struggles. Good idea to leave alone unless necessary.