Heres a closer look at the "tab" im referring to on the far right set of terminals. The terminal the white connector is screwed to appears to have a "tab" of sorts connecting it to the terminal to the right of it. I assumed it was to connect multiple connectors without them "stacking up" too tall for case clearance??
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You can kind of see it in the close up picture.. below the white connector standoff is the "tab" that connects the two posts together.
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Luke

I'll keep my head down for a bit as i don't know sorry. Mark & others will be back.
Actually i've got to go & check the output of the generator on my old Fergy 135 tractor. Lucky its diesel & once started needs no DC power. But i think to test that i just connect the two leads together that exit the Gene then hook a meter between that & earth--did it last time with a halogen 50watt headlight bulb & blew at revs by memory. Think one wire if fields & one brushes [or a brush]. So on that theory can you do similar by joining [guessing] the reddish--orange to one of the brush terminals--not the earth brush--then connect a dc meter to that bridging wire & earth then run it up.
Big green must be starter so ignore that.
Hope i'm not suggesting a short circuit & a total guess on my part--be careful.

From your initial diagnosis It sounds like a current / ground problem . Not enough amperage getting to the starter.

Start working from the battery down, clean and check solder connections from batt to regulator, regulator to starter. Most important is a good ground, battery to frame and frame to engine . Check for bad solder connections on all wires, partially broken wires, like only one or two strands. just enough to give a good voltage reading but not carry current/amps. check all plugs for oxidation, you might want to cut back the tape covering the wires and check their condition.

The only thing that kills a generator is broken or shorted windings, then there is rewinding, talk to your stater shop.

I used to chase down gremlins on British cars, fun things like having the wipers come on when you honk the horn always came back to poor grounding or oxidized connections.

Good luck,

JC

Here is a pic where I've labeled the wires.  The heavy light green at front is the starter wire (M).I don't have these systems "memorized", I've always been able to find the problem following the manual steps in order and completely though.  Try using the 1969-71 AT1 manual in Tech Library...  same basic generator system and the earlier manuals often have more details.  There is also a 1969 Yamaha "shop guide" in the tech library that goes over general theory and can help.I might have time later to look at the steps and results you've posted...  busy weekend though.  



Here's something from the 1969 Shop Guide that I find helpful:

MarkT,

Thanks for confirming the terminals. I had assumed that was the case based on the shop manual I downloaded but as you said it’s sort of vague in spots. I mainly didn’t want to damaged anything without confirmation. I have the system pulled apart currently and I’m taking a bunch of meter readings with photos to show my process incase I’m doing anything wrong someone can chime in and show me. It’s very possible I’m misunderstanding something during my readings.
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Ill update the thread shortly. 
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Thanks, Luke

So I dove back in this morning. Heres my process with photos up to where Ive stopped to take a break.
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Started by lightly repolishing the armature and cleaned off the slight brush dust that accumulated during the testing process. Ive included a close up of the grove that was pointed out yesterday. The armature has no play in it though and spins true which makes me feel a little better about the potential crank bearing problem that I was worried about. Not definitive but im glad its not super worn. I proceeded to test the armature.
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TEST #1 – Checking the armature insulation.. RESULT – No continuity between the iron core and the commutator. I even went around the entire commutator, testing each pad independently just incase
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TEST #2 – Checking continuity between the commutator and the top field coil pads (I don’t know what these are commonly referred to so forgive my ignorance).. RESULT – No continuity, same as before went around the whole thing testing each pad individually to be safe
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TEST #3 – Checking resistance between the field coil pads (?) and the iron core .. RESULT – 00.2ohms with meter on 200ohm setting
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TEST #4 – Checked continuity of the field coil between terminals M, A, and F .. RESULT – Continuity between all three terminal posts, which seems to indicate the coil should be in good shape still. I also double checked the field coil brush insulation and confirmed the positive brush is properly insulated from the yoke. (No pictures of this one, as it was pretty clear cut)
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TEST #5 – Field coil to yoke resistance .. RESULT – While probing between the field coil and the inside face of the yoke I found continuity and had a reading of 00.2 ohms. Should there be continuity here? I assume so since all other tests following the manual seem to be going well up to this point.
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Are there anymore tests anyone would recommend while the dynamo is torn down to this point? Next plan will be to run the Dynamo test laid out in the manual with the jumper wire, I just want to double check that I shouldn’t be testing anything else first before I put everything back together.
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Thanks for the help up to this point. Please let me know if im on the right track!!
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Luke

You seem to be adding your own tests?  If not, what does the manual say the reading should be? 

(The fact the the starter works somewhat indicates armature is probably fine.  Pay close attention to the "mica undercut" though.  Important to get that right.)

One thing you need to check is for grounded positive brush while it is installed in holder like the pic in manual.  Pic shows stator removed from armature, don't remember if you need to do that.  But removing brush and letting it hang in space doesn't tell you what you need to know. 

MarkT,
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Sorry I am most definitely adding my own tests.. mainly due to my inexperience with this system. I figured I’d over deliver with information incase someone noticed a problem from my metering that I overlooked. 
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I will double check the undercut and verify it’s correct. Then I will reinstall the stator and check the positive brush again. Won’t the positive brush show continuity with the ground while installed and touching the commutator if the negative brushes are also touching it simultaneously? If not perhaps I’ve overlooked something very obvious and misunderstood the setup.
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Thank you for your consistent replies!
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Luke

Please review manual and what I said. They test brush with stator removed from bike going by pictures.

MarkT,
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Sorry for the confusion. I may not have been clear before or posted results but I have done that test already off the armature and on the bench. There is no continuity between the positive brush or brush holder and the yoke. In the previous post you mentioned “Pic shows stator removed from armature, don't remember if you need to do that.” which is what prompted my question for clarification. 
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Additionally I checked my mica undercut and to the best of my ability with my calipers it looks like I’m at 0.7mm in depth. Which should be in the acceptable 0.5mm-0.8mm range.
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Im going to reassemble and do the dynamo test.
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Luke