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  1. #1
    CB750 New Member donuts's Avatar
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    Question 1975 Supersport - diagnosis of (yet another) charging issue

    Heya CB750 fam! I recently bought my first bike (a 1975 750F0 Supersport) and I'm loving it. It came with a few projects, and I found yet another one when my bike died about 30 minutes into a ride. I'd like your help on what I can do to get my bike running well again.

    Tl;dr, I tested the stator coil, field coil, and battery and from what I can tell the only tests that failed were: voltage across the battery at high RPMs and the output voltage from the stator coil at high RPMs. This leads me to believe that the stator coil is at fault. I would love a second set of eyes on the steps I took before I possibly spend $$$ on replacing it.

    This is the first bike I've ever tinkered on, and I'm relatively new to motorcycles in general. So I'm sure I'll get some terminology wrong, or overlook something obvious...

    After doing a bunch of searching through this forum and sohc4.net I found a few guides that helped me narrow down my problem. I mainly used Mike Nixon's guide at the motorcycle project, augmented by a couple of other troubleshooting guides for charging systems slightly different than mine, including this one from Honda Chopper and the DOHC guide from cb750.com.

    For starters, I'm working with a brushless three phase alternator, a combined regulator/rectifier, and a new battery.

    I started off returning to my bike after a full night on the battery tender. I then took DC voltage measurements across the battery at various RPMs and expected the voltage to increase from ~12V at idle to 14-15V at 5k RPM. However, I only saw a constant 12.3V as I was cranking the throttle from idle through 5k RPMs:

    • off - 13.3V
    • idle - 12.3V
    • 2.5k RPM - 12.3V
    • 5k RPM - 12.3V

    In some of the guides, there was mention of a magnetism test. So I took a lightweight paperclip and dangled it by the alternator cover. It was attracted to the bolts fastening the field coil to the cover whether or not the electrical system was engaged (key turned off/on), or whether or not the engine was running. It was never attracted to the alternator cover (aside from the field coil bolts).

    Stator Coil tests

    On to some stator coil tests. The first two tests (resistance and voltage measurements) were taken with the stator coil disconnected from the main harness. The output voltage was the only failure here, both the resistances and insulation tests matched expectations.

    I took the resistances between the three yellow alternator wires, expecting something like 0.2Ω. I got a consistent reading of 0.7-0.8Ω for each of the three pairings. This seems well within reason according to what I've been reading.

    Next was measuring the AC voltage across the three yellow alternator wires. I was expecting all of them to be equal, and about 50V AC when at 5k RPM. I found all of them to be equal, but all with a very low voltage: 0.3V at idle and 1.5V at 5k RPM.

    I wasn't able to find any continuity between the alternator wires and the cover, frame, or ground. So at least the insulation checks out.

    Field Coil tests

    The field coil tests were straightforward, and all agreed with the expectations for resistance and insulation.

    The resistance between the white and green wires for the field coil was 7.8Ω, close enough to the expected 7.2Ω.

    I wasn't able to find any continuity between either the green or white wires to the cover, frame, or ground, as expected.

    Next steps?

    So, is this data enough to confirm that the stator coil is at fault? Are there other tests I should run? Is there any way to diagnose the problem in the stator coil and repair/clean it?

    Thanks for joining me down this rabbit hole, and happy riding

  2. #2
    CB750 Guru amc49's Avatar
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    You need to read field coil volts while running. The stator will make zero power with no field. The field coil can read ohms fine but if reg is not putting volt into it again stator shows nothing. Your paperclip thing was a waste of time, you need a LOT more attraction than that.

    FYI, the 13.3 volt at battery is fake and right after you charged it. It's called surface voltage, or volts the battery has not had enough time to soak into plates fully so it stays on surface to read artificially higher than it should. A true battery volt cannot be higher than about 12.85 once the battery sits to soak all up.

  3. #3
    CB750 New Member donuts's Avatar
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    Thanks for the reply! I just ran a couple of tests with the field coil, and youíre right Iím definitely not getting any field.

    I had two tests, a charging test (where I put an ammeter across the main fuse with the fuse removed), and trying to stick metallic things against the alternator cover.

    I then tested two test scenarios, one where the field coil was getting voltage from my regulator, and another where I bypassed the regulator and created a jump wire between the white wire of the field coil and the negative terminal of my battery.

    In both test scenarios, I saw no increase in current (which would indicate charging), not any magnetic field around the alternator cover.

    The second bypass scenario was to rule out a faulty regulator. So I think now all signs point to needing to replace the field coil?

  4. #4
    CB750 Guru amc49's Avatar
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    I'd bet the reg is at fault there. The field coil is simply a long loop of wire, if it tests as not shorted or grounded it cannot be bad.


    'I bypassed the regulator and created a jump wire between the white wire of the field coil and the negative terminal of my battery.'

    Wrong. The white wire is the regulated plus side of the field, jumped like that you have just removed the field coil from the circuit, there can be NO charging there. Normal power flow there is reg supplies variable power to white, it flows through the field coil to a ground on the other wire. Your jumper merely grounded both ends of the field. It can't work like that.
    Last edited by amc49; 05-29-2020 at 06:31 PM.

  5. #5
    CB750 Addict Medyo Bastos's Avatar
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    Iíd swap your regulator before the field coil.


    Sent from my iPhone using Tapatalk

  6. #6
    CB750 New Member donuts's Avatar
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    Thanks for the clarification @amc49! I just realized I never posted a followup.

    After correctly jumping my field coil to my battery I was able to verify that my field coil was energized and found a healthy amount of AC voltage from my stator coil. This pointed back to a busted regulator/rectifier, and after replacing it: I now have a working charging system.

    Thanks to everyone that helped me out on this!

  7. #7
    CB750 Guru amc49's Avatar
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    Rock on player....................

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