Hello from Switzerland!

Recently on my 1976 DT250 all bulbs were blown. I noticed that the ground of the regulator wasn't connected correct. After fixing that, I checked the charging voltage at the battery which is up to 9V DC at 4000rpm. That made me think the regulator might be broken or maybe still not working.
So I spent more of time on checking and understanding the electrical system. (I found a lot of wiring diagrams, but none was exactly showing my bike's system).
Later I checked the AC voltage going from the magneto directly to the bulbs and this voltage is regulated. (7.2V maximum) So the regulator is working!
Still, I have some questions:
1) Is the charging voltage of 9V OK?
2) Why is the charging voltage not regulated?
3) Does anyone have a wiring diagram of my model (Europe version)?

Thanks in advance,
Bauer

Question; Was the 9 volts seen with the red wire connected to the battery or disconnected?

Test voltage at battery with battery connected if you haven't already.

Yes I checked with the battery connected.

Have a read through the thread below and then maybe contact Knuth via the big green "Messages" button above (warning, it is slow to load an address) and see if he has the diagram that you need.

yamaha-enduros.com/index.php/forums/74-7...-dt-250-typ-512-1975

Blown bulbs are common on bikes of this vintage. Every one I've owned has come to me with all of the bulbs popped. The electrical system requires a good battery to regulate the voltage. There isn't a voltage regulator that actually controls or limits the charging system output. The rectifier only serves as a check valve between the AC output of the stator and the battery.

As an experiment, I once plugged a 12v 55w headlamp bulb into my KE250 with a dead battery. Stock is the usual 6v 30w. It illuminated brightly and blew by 3000 RPM, despite being a 12v bulb of higher wattage.

I'm wondering if it's a good original type wet cell battery of the right size? I just had a battery go bad and it let the voltage go high because it wouldn't accept a charge properly or something. I'm not really sure what was wrong with it because it wasn't dead, but wasn't strong either. (It was an AGM alarm battery, not an original flooded wet cell battery.)

I read somewhere that since 76 yamaha equipped some bikes with voltage regulators (not rectifier). And mine has one installed. (See attached file) It also works and regulates the AC voltage going to the bulbs. My concern still is the high charging voltage of 9V.

My battery is only two weeks old. It’s a flooded wet cell original modell. So it should be intact But maybe the battery was damaged (overcharged) because of even higher charging voltage as the bulbs were blown.

As a fix; probably a resistor in the charging cycle could be burning some of the exess power....
The 9V seem very high for me but more and more I think the system is not faulty.

I had/have the problem of my bulbs blowing out too, I have a 76 DT400. It had a brand new battery in it so I swapped out the regulator and it still blew out all my lights. After buying three head lights and two each of all the other bulbs I decided to just go with a 12 volt conversion when I can afford the kit. I never checked the voltage but do believe the battery was being over charged as it kicking out a lot of battery acid out of the overflow even though the level was correct. Good luck in figuring out your issue!