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  1. #1
    CB750 Enthusiast everready's Avatar
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    1979 CB750F not charging

    Hello all,

    So, I've been working on this for the past few day and I'm stumped. I disconnected the stator wires from where they go into the R/R. With the 3 yellow wires I got 0 voltage with the engine running. Also I could not detect any magnetism from the rotor with the engine running. Would this point to a bad rotor? The rotor slip ring resistance test passed as well as the continuity test on the stator. Basically, no voltage is making it back to my battery.

    Really confused,
    Al



    Edit: when the bike hits 4-5k rpms it starts missing. I guess the coils can't keep up with demand. I would think that this means the rotor is shorting out at higher rpms. Is that a correct diagnosis?

    Editx2: How does the rotor become magnetized? Is it from the brushes?
    Last edited by everready; 03-20-2018 at 09:30 AM.

  2. #2
    CB750 Enthusiast everready's Avatar
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    I went back and started from the beginning. The spec for the Ohms on the slip ring is 3.6-4.4 ohms. Mine reads 2.7. So, I'm guessing that the resistance is too low causing the rotor to short out and prevent it from creating a magnetic field which in turn would send AC voltage out from the stator. Does that make sense? BTW, the stator resistance test passed but I get 0 AC voltage (engine running at 3k rpms) on any of the three yellow wires coming from the stator.

  3. #3
    CB750 Guru amc49's Avatar
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    Makes plenty of sense, the most common way these fail, the rotor. The regulator supplies power to the field (rotor, through the brushes) to magnetize it just like a car. When the rotors short the field drops to lower the power made at stator too and then the reg ramps up the field trying to get back the lost charge, it commonly then fails the reg next from overheating doing that.

    2.7 ohm is a dead rotor likely. Part of the winding is shorted to lower the resistance, the field will then be too low. With a shorted rotor you still should be getting at least some partial stator power, not getting any at all may mean the reg is dead too. I'd start with a rotor first.

    Yes, low battery power will make the ignition begin to miss at some point.

  4. #4
    CB750 Enthusiast everready's Avatar
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    AMC,

    Thanks. I'm probably just going to replace both at the same time. Mo money, mo money.................

  5. #5
    CB750 Enthusiast everready's Avatar
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    Update:

    So, I applied power to the brushes (black and white wires) via a direct connection to the battery. When I start the bike it produces 20-30 volts AC at 3k rpms from any 2 of the 3 yellow leads. That leads me to believe that the stator is good and the rotor and R/R are bad. Would you concur with that assessment?

    Addendum: I also checked DC voltage at the battery (at 5k rpms) with the brushes energized. The battery voltage did not increase so that would confirm my suspicions that the rotor and R/R are indeed bad.
    Last edited by everready; 03-21-2018 at 04:00 PM.

  6. #6
    CB750 Guru amc49's Avatar
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    Uh, that my friend is a REALLY good way of toasting parts......................direct short of battery since you have no amp control there and if rotor wasn't bad before it may be now. I would NEVER do that.

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