Hey forum members, looking for an expert on Dynamos and voltage regulators. My bike is a DT125B (1975) and I cannot get my dynamo to charge the battery. I am particularly confused about the dark green wire (ground from the regulator) as it does not ground to the frame on it's position on the insulator block. We believe it'll be a simple fix, a wrong wire or a faulty regulator. Ive looked up and down the forums and can't find the answers to my particular questions. If anyone knows how to make sure a regulator is working and could supply information and pictures about the dynamo wiring it would be greatly appreciated, thanks!

The service manuals in the Tech Library have excellent step-by-step troubleshooting and diagnosis.

The earlier (AT1) manuals sometimes go into the theory in more detail than the newer manuals. System is basically the same from 69-76.

I still don’t understand these systems completely but did learn some from fixing mine a year ago. Printing off the pages from the service manual and having them handy when working on it will help you.
The small green wire allows the voltage regulator to ground the generator windings allowing electricity to be produced. It is not grounded if generator is producing less than about 10 volts & it is not grounded if battery is fully charged. There are two small resisters in the voltage regulator I suspect they allow some continuity in the grounding circuit. If that green wire is grounded out and RPM is over 1800 things burn up -
Picture shows insulater block separated from backing plate, it is just to keep wires separate, no hidden connections in behind it.
In order to generate electricity you must have a good battery , the magnets are not permanent magnets so using the electric starter applies 12v to them. I had an old one here that was no good so took it apart to see what was in there. Picture shows how they are arranged & I used metal filings to check for magnetism, they would still attract filings after power shut off but that ability dropped off in time
The test procedures are in the service manual, hard to tell what’s going on for a problem without putting a meter on it and getting some numbers, unless it’s something simple like worn brushes

If anyone wants to read a little more theory of the generator and starter it can be found in the Tech Library Service Manuals section "1969 Yamaha Shop Guide" starting on page 84 of the PDF (page 76 of the manual)

That section explains the generator part of the system. The following section explains the starter part.

Following the testing procedure in the AT manuals has always worked for me... going step by step... not skipping over things.

+1 on the Service Manual diagnostic process. I used the '70 AT1B Service Manual in the Tech Library for my guide on my '72 AT2 (the starter dynamo setup is the same through the years). Following the steps, I first found that even though the electric start worked, I had no voltage output from the dynamo when doing the "Full Field" step. Replaced the brushes, and then I had voltage output when Full Fielded. Next following the steps led to inspecting the Voltage Regulator, and while doing those tests, with the voltage regulator cover removed I found one of the contact boards melted, beyond repair. Had to source a good, used Voltage Regulator. Bottom line is you can do the same using the Service Manual steps & a voltmeter.

I just downloaded the CT1 and AT1 manual and I'm going to try the more simplified voltage regulator test I found. As well, refill my battery with distilled water. I'll see what happens from there!

We figured out that the green wire grounds the regulator, but it's isolated on the block and does not ground out (tested with a meter). Is it supposed to be connected solidly to a ground?

Also, the circled black wire is confusing me. Is it supposed to connect to the orange wire (points) or to the dark green (regulator ground)?

After doing the regulator test as per the manual, I'm getting no voltage. No voltage at the white wire (positive brushes connection to the reg.), no voltage on any brushes and no voltage at any wires. Even tried grounding the dark green, grounding the black and connecting black to green. So it's not the regulator yet because I'm not even getting any voltage from the dynamo.

Ever try an epsom salts solution in the battery when it runs low? I've done that a few times, and it freshens it right up. Gets you by easily until you can get a new battery.

The circled black does go on with dark green, before hooking it up test with ohmmeter between that circled black and white, should be around 5 ohms.

The voltage produced when running is DC not AC, make sure your meter is set properly. I used one of those simple little 12volt pilot lights with one lead grounded to a case screw and one on white wire terminal while sorting mine out. The bulb gave a easy visual while checking things with motor running the spinning parts are dangerous