Topic: my cb750F, 1979, charging system is out

[Hondawggie]

When I rev the bike up to 2000, 3000, etc. RPM or higher, I see zero increase in voltage across my battery terminals -- the battery reads about 12.6 volts, whereas a NORMAL charging system will have that voltage reading across the battery rise to 13, 14 volts when you rev the bike.  Not happening - the voltage across the battery never changes, stays at 12.6 volts.

I bought this '79 cb750f and the seller was the original owner.  He told me he had already chased this 'not charging' problem by doing the following:

- new rectifier/voltage regulator
- new alternator rotor (on the right end of the crank, its the rotating part of the rotor/stator alternator)
- new battery

The parts he installed above were NEW, not used parts.

So I'm thinking it is probably a simple problem like a loose ground somewhere -- any ideas on where I should start looking?  Everything else on the bike is FINE.  It is in 100% stock and original condition -- a time capsule.  Only 20k miles too.   The seller bought it new in 1979 and used it mostly for recreation, not commuting.

[Hondawggie]

Just fyi, I have tried searching the board history on 'cb750f', 'charging', 'charging system', etc. and the search function of the site seems to be broken -- anyone else experience this?

[onepieceatatime]

You have to use the google custom search on the top left, the other search is broken.

If he replaced the R/R before he replaced the rotor, he may have cooked the new R/R.

Were the brushes in the stator cover checked?
If you have a multimeter, measure across the sliprings on the rotor. It should be between 4 and 10 ohms.
Also check between the yellow wires on the stator. you should see the same very low reading across all pairs of yellow wires and no connection to ground from the yellow wires.
Check the R/R like in this link... http://cosky1.tripod.com/imagelib/sitebuilder/misc/show_image.html?linkedwidth=actual&linkpath=http://cosky1.tripod.com/sitebuildercontent/sitebuilderpictures/page16-08.jpg&target=tlx_new

The manual linked is for the CB650, but the DOHC 750 charging system is the same.

[Hondawggie]

Okay thanks -- tomorrow I'll run those tests.  I noticed the google custom search and have been reading about the woes of the DOHC charging system, dang I was about 100% sure Honda was invincible.

Thanks for those tests -- tomorrow I'll run those and hopefully all is well.


He put in new brushes already -- forgot to mention that.
I don't know the sequence of his new parts-swap-in process so I'll check the rectifier tomorrow too.

I'm hoping it's just a connection somewhere that is oxidized.

The styling on this bike is really really nice.  Hope I can fix this electrical gremlin.

[cb650]

Same system as the sohc 650's.  Search them also.

[Hondawggie]

I just finished taking readings of the 5 wires that come out of the stator plate on the right side of the engine at the crank (3 yellow wires and 1 black and 1 white wire).

WITH THE STATOR REMOVED

- 4.3 ohms read between the slip rings on the rotor
- 1 ohm between any 2 of the 3 yellow wires
- 1 ohm measuring the black wire at the connector down to the lower brush (the brush for outer slip ring)
- 1.1 ohm from the white wire at the connector to the top brush (the brush for the inner slip ring)
- and infinite ohms between the white and black wire measured at the connector
- infinite ohms between any of the 5 wires and ground


WITH THE STATOR RE-ATTACHED AND BOLTED TO THE MOTOR
- 8 ohms measured at the connector between the white and black wires (shouldn't this be 4.3 ohms + 1 ohm + 1.1 ohms = 6.4 ohms?)

- 1 ohm between any 2 of the 3 yellow wires


I have the original rectifier/voltage regulator the prior owner swapped out -- it's off the bike and I have my meter, how do I check if this R/R is good or bad?

[Hondawggie]

FUNNY THING:

I thought that if I stick a wrench next to the rotor it would grab on.  NOPE.
I thought the rotor is a magnet?


I tried several things that I know are ferrous (tools, screws, etc.) -- the rotor is not magnetized at all when I put a tool etc. on top of the rotor.

Is that normal?   Or is my thinking about the rotor being a magnet wrong?

[Hondawggie]

Well, I checked out the 'bad' rectifier/regulator he removed -- this is the one that originally came with the bike (he bought and installed a new one then sold the bike when it still won't charge).  He gave it to me in a box when I bought the bike.  There is a new rectifier/regulator on the bike now.  So I went to check the old rectifier/regulator he took off.

The old rectifier part checks out fine.

So if you read my stuff above, you'll see that:

- I seemingly have a good rotor, although it is not magnetized, but that may be a non-issue (?)

- the stator and brushes check out electrically too, although a couple of ohms high on the resistance, about 2 ohms high at the connector

- I have a good rectifier that was removed and was not the problem

- and a brand-new rectifier/regulator that's in the bike now didn't fix the problem.

At this point I'm thinking the regulator's output wire is open circuit.  By that I mean -- the charging system is working fine, but the voltage regulator output is *not* making it to the battery.

That would explain why the following new parts the prior owner put in did not fix the 'not charging' problem:

1) 2 new rotors
2) new brushes
3) 2 new batteries
4) new regulator/rectifier

The only part he didn't replace is the stator, and as I mentioned above, the 3 yellow wires and the black and white wires from the stator measure up FINE.

He's basically replaced the entire charging system and he missed the problem.

It's probably an open connection out of the regulator.

[onepieceatatime]

The rotor does not have permenant magnets, it is an electro-magnet energised through the sliprings.
Based on your testing and results, I would definately be investigating all wiring and connectors between the stator outputs and the battery. Also do not forget about your ground wires when checking and cleaning connections.

[Hondawggie]

The rotor does not have permenant magnets, it is an electro-magnet energised through the sliprings.
Based on your testing and results, I would definately be investigating all wiring and connectors between the stator outputs and the battery. Also do not forget about your ground wires when checking and cleaning connections.

Ahh, that explains the lack of magnetism - I guess I thought the rotor was a permanent magnet.  In my defense, some bikes I've worked on do have a permanent-magnet rotor -- I'm wondering if I'm thinking of my late-70s kz1000.  Not sure.

I believe you're right about the wiring connections.  Tomorrow when it's light out again I'm going through all the connectors and doing visuals and conductivity checks from end-to-end, and cleaning the connections.

Previously I only checked for proper charging voltage across the battery when I revved the bike to 2000-3000rpm.  Now I'm planning to check the voltage at the *output* wire of the regulator, the big red/white wire.

I see on the wiring schematic that for whatever reason, this big red/white wire does not route directly to the battery's  "+"  terminal.    Instead, the regulator's output on the red/white wire goes through the starter relay (why?) 

The starter works fine so I doubt the starter relay is bad.  But I'm checking for voltage right at the regulator's output, *before* it routes through the starter relay.

If I see correct charging voltage at the regulator's output wire then I'll be getting really close to solving the mystery.

[onepieceatatime]

I think some of the older Hondas have permenant magnets, and I know some of my friends have Harleys with permenant magnets too. I think Honda was experimenting with solid state parts on the 79-83 time frame bikes. Earlier models had mechanical regulators, points and condensers, and other electrical system differences.
I am not sure why they did not run the output to the battery directly either, but I'm sure some engineer somewhere thought it was a good idea to do it the way they did.
Good luck, and I look forward to hearing that you found and fixed the problem.

[Hondawggie]

I checked the DC voltage output of the regulator at the nearest point to the regulator.

Since the regulator's output is not wired directly to the "+" battery terminal -- the regulator's battery-recharging voltage output goes through the starter relay (sheesh),  I thought "maybe the voltage regulator is in fact putting out correct battery-charging voltage of 14vdc, and the reason I don't see the 14vdc across the battery at 3000rpm is -- there is an open circuit in, maybe, the starter relay that is blocking the regulator's 14vdc from getting to the battery."

NO SUCH LUCK.    The voltage right out of the regulator is only 12 volts.

So here's what I thought -- "maybe the alternator is not outputting the expected AC voltage to the rectifier.  I'll set my multimeter to AC volts and, with all the harness still connected, check for AC voltage output from the alternator."

I saw ZERO volts AC on all 3 of the yellow wires coming from the alternator. 

Now, a couple caveats.

1) I don't know what AC voltage is supposed to come out of the alternator with the bike running, it's not in the Clymer manual I have.


2) I'm not 100% sure how to set the "+" and "-" black and red probes of my multimeter to try to read AC volts.
- I started the bike and used the choke to keep her revved at 3000rpm

- and I measured between each pair of the 3 yellow wires from the alternator - ZERO VOLTS AC.

- then I measured, one at a time, the AC  between ground and each of the 3 yellow wires out of the alternator.  Again, zero volts AC.

3) Then I thought "maybe the 12 volts DC *input* to the alternator is missing."  NOPE.  With the ignition key on, motor not running, I measure 12 volts DC from the black wire to ground, and 12 volts DC from the white wire to ground.   The black and white wire are each connected to one of the 2 brushes and provide the DC voltage for the rotor.

So right now I'm stumped.   

Yesterday (if you read above) you'll see I checked out the original rectifier/voltage regulator unit -- at least the rectifier part is good -- this unit was handed to me by the seller in a box because he put in a new regulator/rectifier.

So I haven't checked the rectifier/regulator in the bike.  But since the old unit that he took off the bike seems OK, I doubt the new one on the bike is bad.

ANYONE HAVE ANY IDEAS?

[Hondawggie]

still talking to myself, shibbity-bop. I found this that might help me out here IF what Spanner wrote here for a SOHC 1978 cb750f can apply, electrically, to my 1979 dohc cb750f:

"You already confirmed that the stator read 0.2 ohms ( actually 0.4 ) any yellow to yellow, and that the field coil measured 7ohms, so a.o.k. there ( or did I read that wrong ? ). So, back to rectifier IMO. You could look for AC voltage from the alt. ( yel/yel/yel ), but if the rect. is bad then y/y/y may read 0v and the stator still be good !! You would have to have the field coil 'on' ( white/green ) and the 3 yellows disconnected from the rectifier to 'prove'  AC from the stator..... can't be done without snipping wires !!
Time to test the 'new' diodes, I think."

I found the above in a thread for a 1978 cb750f charging problem, at:
http://forums.sohc4.net/index.php?topic=93865.0


THIS IS THE PART THAT CAUGHT MY ATTENTION:
"You could look for AC voltage from the alt. ( yel/yel/yel ), but if the rect. is bad then y/y/y may read 0v and the stator still be good !!"


Today I saw around 12vdc on both the black and white wires leading from the regulator/rectifier to the brushes.  And I've already ohm'd out the stator and rotor and they read fine.

And I saw exactly what Spanner said above -- y/y/y wires all read 0v AC.

But here's what I don't get.

If the rectifier/regulator on the bike is bad -- then why do I even see the 12.1 volts DC on the output red/white wire of the regulator?

[onepieceatatime]

So here's what I thought -- "maybe the alternator is not outputting the expected AC voltage to the rectifier.  I'll set my multimeter to AC volts and, with all the harness still connected, check for AC voltage output from the alternator."

I saw ZERO volts AC on all 3 of the yellow wires coming from the alternator. 

Now, a couple caveats.

1) I don't know what AC voltage is supposed to come out of the alternator with the bike running, it's not in the Clymer manual I have.


2) I'm not 100% sure how to set the "+" and "-" black and red probes of my multimeter to try to read AC volts.
- I started the bike and used the choke to keep her revved at 3000rpm

- and I measured between each pair of the 3 yellow wires from the alternator - ZERO VOLTS AC.

- then I measured, one at a time, the AC  between ground and each of the 3 yellow wires out of the alternator.  Again, zero volts AC.

I have read somewhere that it should be 50VAC between yellows. I have not tested the voltage on a running system though to confirm.
+ and - make no difference with AC, either lead connected to each test point is fine.

[onepieceatatime]

still talking to myself, shibbity-bop. I found this that might help me out here IF what Spanner wrote here for a SOHC 1978 cb750f can apply, electrically, to my 1979 dohc cb750f:

"You already confirmed that the stator read 0.2 ohms ( actually 0.4 ) any yellow to yellow, and that the field coil measured 7ohms, so a.o.k. there ( or did I read that wrong ? ). So, back to rectifier IMO. You could look for AC voltage from the alt. ( yel/yel/yel ), but if the rect. is bad then y/y/y may read 0v and the stator still be good !! You would have to have the field coil 'on' ( white/green ) and the 3 yellows disconnected from the rectifier to 'prove'  AC from the stator..... can't be done without snipping wires !!
Time to test the 'new' diodes, I think."

I found the above in a thread for a 1978 cb750f charging problem, at:
http://forums.sohc4.net/index.php?topic=93865.0


THIS IS THE PART THAT CAUGHT MY ATTENTION:
"You could look for AC voltage from the alt. ( yel/yel/yel ), but if the rect. is bad then y/y/y may read 0v and the stator still be good !!"


Today I saw around 12vdc on both the black and white wires leading from the regulator/rectifier to the brushes.  And I've already ohm'd out the stator and rotor and they read fine.

And I saw exactly what Spanner said above -- y/y/y wires all read 0v AC.

But here's what I don't get.

If the rectifier/regulator on the bike is bad -- then why do I even see the 12.1 volts DC on the output red/white wire of the regulator?

0.4 ohms should be fine on the yellows. 4-10 ohms is good on the sliprings. What reading does your ohm meter show with just the red and black connected to eachother?
You can test the diodes in the R/R like at this link.
http://cosky1.tripod.com/imagelib/sitebuilder/misc/show_image.html?linkedwidth=actual&linkpath=http://cosky1.tripod.com/sitebuildercontent/sitebuilderpictures/page16-09.jpg&target=tlx_new

If you had the connectors plugged in when you were testing the output of the r/r, the 12.1 VDC on the red/white wire was your battery voltage.

[Hondawggie]

To fix this not-charging issue I needed to understand how it's supposed to work and I finally found it (NOT in the Clymer manual.)

HERE'S THE BRIEF EXPLANATION -- see the details below

- regulator uses the black and white wires sent to the rotor to produce a magnetic field to recharge the battery

- regulator can VARY the size of that magnetic field with the black and white wire

- regulator slowly grounds one of the black and white wires when the battery needs charging

- as the regulator drops one of the black and white wires to ground, the other wire stays at around 12vdc

- now that the black and white wire have a different voltage level, that produces the same voltage difference in the rotor - a 'voltage drop' -- and that voltage difference on the rotor grows the magnetic field strength and the 3 yellow wires from the stator produce an AC voltage to recharge the battery.

THE KEY TO THE CHARGING SYSTEM
- the black and white wires from the regulator to the rotor induce a voltage drop on the rotor that varies in size depending on the state of the battery's charge
- bigger voltage difference on the black and white wires = larger magnetic field to charge the battery
- smaller voltage difference on the black and white wires = smaller magnetic field in the rotor and smaller recharging voltage out of the 3 yellow stator wires.

THE BLACK AND WHITE WIRES FROM REGULATOR TO ROTOR CALL THE SHOTS IN THIS SYSTEM.
Easy to understand.

=========== THE DETAILS ====================

The seller sold me the bike thinking the rotor had 'lost its magnetic field' because nothing (metal tools, etc) was sticking to the rotor, so he said 'the rotor lost its magnetism.'  He told me it needed a new rotor and he had already replaced it once and didn't want to cough up another $150 for rotor #3.

Then Onepiece explained the rotor is an electromagnet (thanks OP, that sent me in the right direction to find the stuff below).   

Here's the way the charging system is supposed to work:


- The rotor is an electromagnet (not a permanent magnet). 

- The strength of the magnetic field that the rotor produces dictates the amount of AC voltage on the 3 yellow wires that come out of the stator -- and hence the amount of recharging voltage the battery gets.  If the 3 yellow wires show no AC voltage with the system all connected up and the bike running at 3000rpm, then the rotor must not be producing a magnetic field.  The black and white wires from the regulator send DC voltage to the rotor to produce a varying-size magnetic field -- the regulator checks the existing battery voltage and then adjusts the voltage on the black and white wires sent to the rotor to produce a variable-sized electromagnetic field. 


- if the black and white wires coming from the regulator to the rotor have the same voltage, the rotor's magnetic field is weak/non existent and the 3 yellow wires have no AC voltage on them.  And that is exactly what I see with the charging system all connected and the bike running at 3000rpm.  The way you measure the AC voltage on the 3 yellow wires is in pairs -- each pair of yellow wires will have AC voltage on them if the charging system is working.  I'm seeing zero AC voltage between the pairs of yellow wires.


- the regulator's job is to sense the amount of voltage on the battery, and then MAKE THE DC VOLTAGE ON THE BLACK AND WHITE WIRE DIFFERENT.  (Right now I see the same DC voltage on both black and white wires which might be the problem.)    The way the regulator is able to control the amount of recharging voltage coming from the 3 yellow wires is EASY:

1) black and white wire have same DC voltage?  That's the regulator saying "I've checked the battery voltage and we're good, no need to produce a big magnetic field on the rotor."

2) black and white wire have different DC voltage?  The regulator is saying "we need a magnetic field in the rotor to recharge the battery!"

The regulator apparently incrementally grounds one of the 2 wires (black and white wires sending voltage to the rotor) and that produces a voltage drop in the rotor windings and that produces an electromagnetic field which generates AC voltage on the 3 yellow wires to recharge the battery.

[onepieceatatime]

Sounds like you got it figured out. Just to avoid confusion for other readers, that is 3 Yellow wires, not 3 white wires from the stator that have AC voltage.

Did swapping in your other regulator fix the problem, or are you now waiting on one?

My guess would be that the rotor failed and killed regulator, PO replaced regulator (and fried the new one) then replaced rotor.

[Hondawggie]

Sounds like you got it figured out. Just to avoid confusion for other readers, that is 3 Yellow wires, not 3 white wires from the stator that have AC voltage.

Did swapping in your other regulator fix the problem, or are you now waiting on one?

My guess would be that the rotor failed and killed regulator, PO replaced regulator (and fried the new one) then replaced rotor.

Sheesh thanks OPiece for the correction -- I have now changed the incorrect '3 white wires from the stator' to '3 yellow wires' -- for others who read this, that is a big help -- thanks for the correction. 

OP, before I put in the regulator/rectifier that checks out okay, I'm going to test the alternator.

I have a variable DC voltage generator that I've used to test fuel pumps on fuel-injected bikes.

I'm going to disconnect the 5-wire alternator connector that has the 3 yellow stator wires and the black and white wires to isolate the alternator from the rest of the charging wire harness.

Then I'm going to hook up my DC voltage generator on the white and black wire that are connected (through the brushes) to the 2 copper rings on the outside of the rotor.  I want to put a controlled amount of DC voltage on that rotor so I can control the size of the rotor's magnetic field.

I'll then put my AC volt meter on the the yellow stator wires, looking for AC voltage across pairs of the 3 yellow stator output wires.

Then I'll increase the DC voltage applied across the black and white wires that lead to the rotor, and thus increase the rotor's electromagnetic field, slowly.

(Why not just hook the bike's battery to the black and white wire?  The battery might push too much current through the rotor -- I'm not sure if anything bad would happen if I put the entire battery's 12 volts across the black and white wires, but hell maybe that huge battery's current will melt the brushes, insulation in the rotor windings, who knows).


As I adjust the amount of DC voltage that my DC power supply puts on the black and white rotor wires, I'm expecting to see the AC voltage across each pair of the yellow stator wires vary.  If I increase the DC volts on the black and white wire, I want to see increased AC voltage on the yellow wires.

Only tricky thing is -- I need to get the rotor spinning so I'll have to start the motor. 

[Hondawggie]

Okay here is my test rig to test the alternator (this tests all of the following: the "field coil" aka the 'rotor, and the Stator, and the brushes, and the wiring to the alternator also)

Here's the bike.  1979 cb750F.  Paid $495 for it.  May have overpaid, not sure yet.
Notice my DC voltage generator on top of the upturned white bucket.


Mileage -- you know, this has FAR too few miles -- for a Honda -- to have ANY electrical problems!


For the sake of others who will follow this thread, I found and used some of my color-coded hobbyist wires -- I've got a black test wire plugged to the black rotor white, white test wire plugged to the white rotor wire, and 2 yellow test wires plugged into 2 of the 3 yellow stator wires.  My white and black test wires come from my variable DC voltage generator.  I plan to vary that DC voltage to grow and shrink the electromagnetic field on the rotor.

When I drive the DC voltage from my variable DC generator into the white and black rotor wires, if the rotor produces a magnetic field, I'll start seeing the 2 yellow wires have AC voltage on them.  That's because the stator is affected by the rotor's magnetic field -- the stator produces AC voltage when the rotor starts making a magnetic field.

HERE I SET THE DC VOLTAGE GENERATOR TO 4.5 volts DC.
That dang magnetic field better be growing in the rotor windings

NOW I START THE BIKE'S ENGINE with the alternator unplugged to get the rotor moving so it will produce its magnetic field (the rotor needs both a DC voltage drop AND rotation to build its magnetic field that can be sensed in the stator windings).


Well okay, there we go.  The 4.5volts DC I piped into that rotor -- it MUST be making a magnetic field. Because I've got 13.5 volts AC on 2 of the yellow wires.  You can't see this, but my multimeter is set on the "50 volts AC scale" -- so find the 2nd set of numbers from the bottom that go in sequence: "0.......10........20..........30.........40.........50" and you'll see the meter's needle is at about 13.5volts AC.


I then verified the other two combinations of yellow-stator-wire pairs also produce the same 13.5volts AC -- and they do:


THE ALTERNATOR LOOKS GOOD.

I then reconnected the alternator to the bike and started the engine.
The regulator is putting out battery voltage on both the black and white wire that go to the rotor.

THAT WON'T PRODUCE ANY AC VOLTS on the yellow wires.   If the regulator puts the SAME volts DC on the black and white wire -- there is no 'voltage drop' across the rotor windings.  The black and white wire must have a voltage difference before the rotor produces magnetism. 

So either the black or the white wire coming out of the regulator is NOT DOING ITS JOB.

The battery became too weak and I now have the battery on a trickle charger.  When the battery is charged up I'm going to swap in the other regulator/rectifier I have and see if the black and white wires coming out of the regulator behave correctly (different DC volts on each wire).

[Hondawggie]

NOTE:  The Clymer manual that the seller of my cb750f gave me with the bike, on page 193 says "The rotor is permanently magnetized."

This version of the 1979-1980 cb750f, cb750c Clymer manual is the "Fourth Edition, printed in July 1981."

The way I've learned how to instantly know if the alternator has a permanent magnet or a 'field coil' rotor (electromagnet) is EASY:

- if the alternator has no brushes, it is the 'permanent magnet' type
- an alternator with brushes has an electromagnet and field coil, and *does not have* a permanent magnet.

Of course, the 1979 cb750f has brushes, hence despite what the 1979-80 cb750 DOHC Clymer manual says, these bikes don't have a permanent magnet for the alternator rotor.

I think that page 193 of the Clymer manual cost my prior owner $125 for the new rotor that's on the bike.  The reason he replaced the rotor:  "It lost its magnetism.  Nothing sticks to it."

Lazyazz cookie-cutter Clymer book editor. 

[Hondawggie]

I found the cause of the battery not recharging.

If you'll look above in this post you'll see that previously in my troubleshooting process:

- I isolated the alternator from the main wiring harness by disconnecting the 5-wire connector (3 yellow wires, 1 white wire and 1 black wire) on the right of the battery behind the right side cover. 

- I then applied varying amounts of DC voltage (using my DC voltage generator test equipment from Frys Electronics) from zero to 4.5 volts DC - onto the black and white alternator wires.  I kept the white wire at ground and applied 1 volt, 2 volts, 3 volts etc. on the black wire. The black and white wires expect DC voltage and pass it through the 2 brushes and then into the rotor
('field coil') part of the alternator -- the larger the DC voltage drop applied across the black and white alternator wires, the larger the electromagnetic field is produced by the spinning rotor.
The white wire stays at ground and the black wire gets an increasing amount of DC voltage that in turn grows the electromagnetic field produced by the rotor.


- I used a multimeter to measure each pair of yellow (stator output) wires as I varied the DC volts being applied as a voltage drop across the black and white rotor wires (white wire at ground, black wire gets the increasing DC volts) -- sure enough, the AC voltage output of the stator rose and fell as I increased then decreased the DC voltage on the black and white wire.
THAT SHOWED MY ALTERNATOR (wires, brushes, rotor, stator) WAS WORKING.

THAT ALSO SHOWED ME HOW THE ALTERNATOR WORKS, AND HOW THE VOLTAGE REGULATOR IS SUPPOSED TO WORK.  The white white is low (ground) and the black wire has DC voltage on it -- ie. there is a "voltage drop" across the black and white wires and that voltage drop goes across the rotor to grow a magnetic field which is used by the stator to produce AC volts to recharge the battery (after being rectified to DC by the rectifier-part of the R/R).

In order to get the AC voltage output from the stator to recharge the battery -- the R/R (regulator/rectifier) needs to do the same job my DC voltage generator was doing -- apply a DC voltage drop across the black and white wires leading to the alternator.  The regulator needed to keep the white wire at ground and the black wire needs to have DC volts on it.

So with the alternator still disconnected, I then moved on to see if the Regulator was producing its voltage drop across the white and black wires.


I started the bike; regulator is all connected as normal; alternator is unplugged; measure the voltage drop across the white and black regulator wires -- NOTHING.  NO VOLTAGE DROP.
Recall that above I found the alternator only producing AC recharging volts on the yellow wires when the black and white wire had a voltage drop.

SO THE REGULATOR WAS BAD PERHAPS.

But I had my doubts about that because the bike's seller had just bought a new R/R and it was the one in the bike.  He handed me the old R/R in a small box when I bought the '79 cb750f.  I knew it was the original one in the bike -- it was all crusty and dirty.

Well no harm in trying -- I removed the new R/R from the bike and put in the original one that came with the bike new in 1979. 

Alternator still disconnected, bike on and running, measure the battery voltage on the black and white regulator wires

And the old regulator WORKED, not a damn thing wrong with it.
I measured 12.1 volts DC on the black regulator output wire and ground or near zero volts on the white wire from the regulator.  So unlike the new R/R which had zero voltage drop across the white and black wire -- now I had a 12.1 volts DC voltage reading across the white and black wire.

I reconnected the alternator.


I revved the engine and measured the battery voltage -- 14.5 VOLTS.


So what I can't figure out is what led the prior owner to throw out the old R/R -- and how the new one got burned up.


And then I found that with the battery removed from the bike and the ignition switched 'ON' I found the + and - battery leads are nearly shorted together - I measure only 1 ohm resistance between the + and - battery wires with the battery removed and the key switch on.

MAYBE THAT'S THE REAL PROBLEM that fried the original rotor or something hale I don't know.

The bike runs fine now but I will not ride it nor even put the battery in until I find out why only 1 ohm resistance on the black and red battery wires.

SORRY for the long post.  But not much on the board here that discusses the '79 cb750f charging system troubleshooting in the right amount of detail for a hambone like me -- so I tried here to explain how I diagnosed the problem (bad regulator) step-by-step for anyone else trying to fix a 1979-82 cb750f charging system.

[James T Kirk]

check this out for DOHC charging system troubleshooting:
http://www.cb750c.com/publicdocs/charging_system/genesound_charging_system.html

kirk out

[Hondawggie]

check this out for DOHC charging system troubleshooting:
http://www.cb750c.com/publicdocs/charging_system/genesound_charging_system.html

kirk out

Good call Jim, dam good call.  I found that today.  *AFTER* I spent 3 days of hell doing trial-and-error learning it the hard way and fixing it. THEN I found gene's charging system troubleshooting.

I don't understand why that didn't come up in all the google searches I did. 
Self-reliance = slow, annoying progress then you find you wasted 3 frigging days.