Here's the diagram for the 75 dt400:
I'm pretty sure its not one of those dual source ignition coil setups. It's got the magic black box pickup that apparently is on galactic backorder including
ebay. The 77 and later use a little tiny lighting coil shaped one that's coming out of peoples ears, but not the black one.
I did some reading and noodling on how these CDI boxes work. The more i think about it the more i think the pulser coil is failed/failing. The way the CDI (in this version, i read) works is it uses the amount of voltage coming off the pulser. The slower the rotor sweeps past the pickup the lower the voltage. That means at idle, the CDI sees a low voltage, and retards the timing (i think the voltage amount acts directly on a calibrated thyristor, but i may be wrong). At high rpm, theoretically, the pulser puts out much higher voltage, which tells the cdi that the motor is spinning faster, so it advances the timing. If the pulser coil is failed or failing, it wont put out as much voltage as it should be, and the CDI doesn't think the motor is spinning fast enough for advance, so it goes full retard, which is what the timing light says.
I'm sure i'm over thinking it, but i can see a pulser coil putting out not enough voltage and fooling the CDI into not advancing the spark because from it's data the motor isnt spinning fast enough. What i dont get is that the advance appears to start to move, then snaps back to full retard. I don't get how a thyristor can sort of work, i was under the impression that they either work and work well, or don't work completely and are failed.
Edit: Found the actual service manual for this bike (not a generic clymers) that has an ohm chart for the stator bits and bobs. The source/charging coil is right on spec at 13 ohms or so (spec is 12.5ish). The pulser coil is way off, i'm getting 180 ohms or so, it's supposed to be 90 plus or minus 10 percent. Reading quite a few other threads, noone has one that actually will run with more than 96 ohms or so. Looks like i've got a dead pulser coil, at least. I've got a cdi coming anyway but looks like im up a creek with the pulser coil, yamaha stopped making them in the stone age, and theres not a single one on ebay. Damn.
Edit again: I'm wondering if these are rewindable or not. You can't actually see the windings as it is covered in Magic Black Box plastic, but as long as the wire gauge is correct, and you use the right length, you should be ok, right?