'80 cb750k Charging system troubles

[nickajeglin]

So, the new bike isn't charging.

I checked the stator, all three leads continuity to all the other leads.
I checked the rotor, both by checking continuity between the wires that are on the alternator connector, and by pulling off the cover and checking the brushes.
Is it true that the rotor in this bike isn't a permanent magnet, but has a field excited in the windings by those brushes?
Since both of these checked out ok, I moved on to the regulator/rectifier.

Continuity between all the yellow wires, and green one way, but not the other.
Continuity between all the yellow wires, and red/white the opposite way, but not the other.
My small ability to read schematics tells me that this is how it should be, is that correct?

All the connections look great, but since I don't have an ammeter I can't really check like the manual says, by hooking one up in series.

I put a voltmeter across the battery terminals though, and reved it a bit, and there wasn't really any response, all the way up to 5000 rpms. So I'm assuming that means that something is bad on the charging system. (plus.... the battery dies after a while of riding, so that's a red flag.)

Now that I'm away from the bike and thinking a little more systematically, I'm realizing that whatever is sending an excitation current off to the rotor could be malfunctioning, which would make the alternator not work.

Now wait, maybe I'm backwards here..... Does the rotor get an induced magnetic field, which is transfered to the stator, which then charges the battery, or is the stator induced, the rotor picks it up, and then charges the battery? It has to be the first, right? That's why the three phase yellow wires go to the rectifier. rather than the wires from the brushes.

Grr. I don't so much love electrical problems.

Thoughts on what I may be missing? I'm pretty inexperienced, so I often miss stupid simple things.

[nickajeglin]

Ok, there's some black and white wires going to the brushes from the regulator/rectifier.

I tested them with the bike off and at idle, they're sending ~7v at 1.15mA down to the rotor, so now I'm pretty stumped.

I guess I haven't checked the connections from the regulator/rectifier to the battery itself, so there is still a chance that the regulator half could be malfunctioning and not actually sending any power through. Do they often fail closed like this, which is to say fail by not sending any power, or is it usually that the regulator doesn't uh, regulate, and will let to much through.?

[nickajeglin]

Now I put a different regulator/rectifier on off of my friend's parts bike, and the charging situation has actually gotten worse. ~11.5v at idle, running up to ~12.3v at 5000rpms.

I checked the frame ground, it's in a pretty inconvenient place, but I felt around, and it feels like it has no corrosion what so ever. In fact all the connections look great.

Any ideas? I'm pretty lost here.

Could a bad battery really be the only problem here? It charged off of my cheapo 12v charger ok, so how could it not charge off of the bike if the system is ok? And I'm under the impression that the charging system really should be showing ~14v even at idle.

Edit: I put a voltmeter between the yellow wires coming from the stator and the battery ground, and got like basically no volts, there should be tons, right? Does this imply that it's something in the alternator rather than the regulator/rectifier? Can the stator be bad when the yellow wires all have continuity, and aren't shorted to ground? And how could the rotor be bad if it's getting power from the reg/rect, and it has continuity through the brushes?

[nickajeglin]

Ok, I've been pulling my hair out all day on this, but I finally came upon this post:

http://honda-cb750-s.456789.n3.nabble.com/Charging-td3270817.html

In there cheesehead mentions that the Ohms between the two copper rings that the brushes ride on on the rotor should read ~4 ohms, he said that his read 1.1 so it was bad.

Mine reads .1 ohms, so it must be dead eh? I assumed that if there was continuity between the two rings I was good, but we must be looking at an internal short here I suppose.

Am I on the right track?

[icerigger]

You're on the right track. The resistance between the rings of the rotor must be between 4 - 6 ohms. Any less and the windings are shorted out and the rotor has to be replaced.

The sad side effect is that the regulator will try to send current to the rotor to excite the stator in excess of its capacity and will frequently fry itself trying to accomplish the impossible.

If the rotor is shorted, check the regulator to make sure its still okay.

[nickajeglin]

Ahh, thanks for the verification. I think that both of my regulators are ok, the yellow to red/white and yellow to green checks out, I have been unable to find a spec on how much power should be sent down to the brushes, on my current regulators it's like between 7-9 volts iirc at somewhere around 1 amp. Does that sound right? Edit: that's 1ish mA, not a whole amp

[nickajeglin]

Just tested my buddies rotor, it starts at 0 ohms,  then over a few seconds ramps up to 4 or 5. Does that sound ok?

[icerigger]

Some digital ohmmeters take a few seconds to settle on a reading. Sounds good.

[icerigger]

Checking the output of the regulator to the rotor is not going to give you any usable results as it depends on the voltage across the battery. However, the fact that you get anything at all looks promising. Try a good rotor and measure the voltage across the battery at 3000 RPM. 14-15 V is what you're looking for.

[nickajeglin]

Woohoo, I put a new rotor in, and it charges perfectly now!

Thanks icerigger, and sorry for writing a novel here, but it helps me to clear my head and keep track of what I have and have not tested. Anyways, now on to the carburetors, choke, and whatnot.

The choke is this jury rigged monstrosity with hose clamps and what not, it makes it really hard to find a good spot to keep it idling nicely. And since the carb slides are sticking (I think), and it doesn't want to rev down, I keep diddling with the choke and idle screw, and never finding a good spot. (and burning my forearm in the process)

[icerigger]

Excellent!

A lot of weird idling down problems on the DOHC are cause by the carbs out of sync. Sync them up before you mess around with the idle and/or pilots.

[nickajeglin]

Should I tear them down/clean/replace gaskets before I sync? I can't imagine they will stay synced through that process. Also do you have an opinion on what tool to use for syncing?  The bike store has one with dials,  but I think it's pretty expensive.

[TOOLS1]

Cycle-X has the gauges for $59.00. http://www.cyclexchange.net/Carb%20System%20Comp%20Page.htm They will pay for themselves the first time you use them.
TOOLS