Topic: CB750A alternator and CB750K alternator - telling the difference

[brandEn]

I have been searching around the forum and I have not found an answer.


I have a few different rotors and stators and one of them (don't know which one) I was told was from a CB750 automatic. Is there a way I can verify which one is from the automatic with a multi-meter? Are the rotors any different? Visually everything looks the same to me.


Thanks for any help.

[Prospect]

Also wondering the same thing as brandEn as I've often thought of using a cb750A coil for the increased wattage. 

[bollingball]

I think the auto put out a few more watts but don't know how to tell so here is a bump. Hey try a part number search and compare. How about the 750a section here?

[kmb69]

I have not checked the manuals but I would guess there is a difference in the measured resistence of both the field coil and the stator windings.

[Red Good]

I think the exciter coil was different not the stator . But don't hold me to it .

[Jerry Rxman Griffin aka MuthaF'er]

The wires are different gauges and the numbers on the parts are different. And yes, you can tell a difference with a meter. I've been playing in the basement catalogueing my leftovers for bike #2 and just so happens I have some of each  I'll get back down there in the afternoon and get you some readings and stamped numbers see below.

The A rotor is a big sum#$%* with a 'lip' (perhaps like the original 750 rotor?). Can't say that has anything to do with charging but perhaps extra flywheel weight for the A?? The stator has larger wires (less resistance = more current flow) and the field coil, which is different, has a different code stamped into it. ie LD113 or LD120. I have/had that info written somewhere but where??

Hoping to play with an upgrade on #2 as I'm thinking my Dyna III, which I'll reuse on #2, was one part of the reason I was losing voltage (in addition to 3ohm coils, halogen, old harness and connectors, and old ignition switch). Thought about doing it on the restore but I didn't want to jack around with a NOS harness. I have 3 used harnesses to play with now. The solid state reg/rec A unit has double output wires that occupy extra slots in the connector presumably for the 290W output vs the 210(?)W.

That's all I can come up with off the top of my head for now.

[brandEn]

Right on Jerry I look forward to hearing about your findings.

[Jerry Rxman Griffin aka MuthaF'er]

I have 3 spare field coils. I checked the resistances: #1 (code LD113) is 7.0 - 7.1 ohms ; #2 (code LD120) is 3.9 - 4.0 ohms ; #3 (code LD120) is 3.9 ohms. I also have 2 spare stators and both came out a 0.5 ohms between all pairs of wires. The stators have no identifying markings. One has black wire caps with '506' hand engraved in small numbers on the outside metal vs clear caps and no markings. The '506' means nothing pertaining to the actual part number. The A alternator has a -393 part number that does not interchange with anything but the A years so there has to be some difference?!

And from my original alternator cover with both pieces attached: code LD113 is 7.1 ohms and stator is 0.5 ohms all 3

I do not apparently have a 'known' A stator since all have the same readings and there is no ID? We really need someone with a 'known' A stator to take resistance readings. Perhaps the metal is thicker or closer to the rotor? It's the same, just came back and checked. One thing that I did notice with the one with black connector caps vs the others is the insulators on the inside between each 'pole' are taller thus shielding the windings behind it. Might I speculate this is the A part and perhaps it was made that way due to increased heat generated with increased current??

I would do an 'educated speculation' from my readings that the LD120 (with the lower resistance, which should generate more current) is the A field coil. I MIGHT speculate the stator with the black caps is from an A.     

[Rigid]

Speculation here but the K had the selenium? Rectifier and the A had the early solid state style like the early wings.  Maybe it gained some charge there with less heat and lower losses?  I just cleared out my stock of A model engines so no way the set them side by side for a visual.

[scottly]

I have 3 spare field coils. I checked the resistances: #1 (code LD113) is 7.0 - 7.1 ohms ; #2 (code LD120) is 3.9 - 4.0 ohms ; #3 (code LD120) is 3.9 ohms.

I think you have narrowed it down to the field coils. A 7 ohm coil would draw 1.828 amps, and a 4 ohm coil would draw 3.2 amps. This alone may explain the greater output of the A system. However, if you put the A field on a non-A bike, the extra current will be flowing through both the ignition switch and the regulator. If other loads, such as 3 ohm ignition coils, have been bypassed with a relay, the ignition switch shouldn't be an issue.

[brandEn]

GREAT INFO THANKS!

OK, I have three stator/field coils one = LD120-01 (presumably the A model) and two = LD113-01 (K model)

Just to be clear, the stator is the big coil with 3 wires and the field coil is the smaller one with 2 wires right?

My readings were like this on the smaller field coils
LD120-01 = 4.0 - 4.5 ohms
LD113-01 = 7.5 ohms

I didn't measure the stator coils yet assuming they are the same as Jerry's reading.

[scottly]

The standard 750 stators (the big coils with the 3 wires  ) have such a low resistance that it can't be measured accurately with a typical meter. I think the main difference is in the field coil. If you want to compare the magnetic strength of a 4 ohm field to a 7 ohm field, connect the white lead to the + of a 12v battery, and the green lead to - (This is exactly what the regulator does when calling for full power.) and see how much each one sticks to a flat piece of steel. KEEP YOUR FINGERS OUT OF THE WAY WHEN YOU ENERGIZE THE COILS! 

[brandEn]

The standard 750 stators (the big coils with the 3 wires  ) have such a low resistance that it can't be measured accurately with a typical meter. I think the main difference is in the field coil. If you want to compare the magnetic strength of a 4 ohm field to a 7 ohm field, connect the white lead to the + of a 12v battery, and the green lead to - (This is exactly what the regulator does when calling for full power.) and see how much each one sticks to a flat piece of steel. KEEP YOUR FINGERS OUT OF THE WAY WHEN YOU ENERGIZE THE COILS! 

haha, I won't be trying this. Good info though.

[brandEn]

So now that I have determined that I do in fact have a 750A coil/stator I need to repair the wires. I need to add some length and new connectors. What kind of wire should be used? The stock stuff seems to wrapped in fiberglass or something like it. 

[Jerry Rxman Griffin aka MuthaF'er]

I've asked Jon Weeks to get us a measurement from an A stator and see what resistance it has. If my suspicions are accurate ( ) it will have the same measurements. I have also asked him to take a look at the bullet connector sleeves for color and to take a look at those insulators on the inside. It would be handy to be able to ID an A stator.

My next questions will pertain to wiring/wiring harness to determine what's different on the A harness to accomodate the double power and ground coming from the A reg/rect unit.

Scottly, couple thoughts: I'm thinking the A has the same ignition switch the F's have so that may be a non-issue with the exception they may have a shorter life span. Secondly, my thinking about the ohms and less resistance relate back to my days playing with slot cars before I got my license. There were a few things we did to the electric motors to make them go faster. We would decrease the resistance in the windings and shim the magnets with a little tape (stronger field). A - we would rewind the 3 poles with larger wire, less resistance for the current and the current traveled quicker and allowed more power to run through them making higher output/power (stronger field)  or B- we would dewind some original turns making for less resistance/lower ohms also (stronger field) (more stealthy in a stock motor class and less likely to get caught because the only thing obvious would be the little blobs of solder)

So... I'm not thinking amp draws, I'm thinking output due to increased current flow. Is that where you're explanation is coming from also by referring to amp draws (the power going in to elicit more power out)? I am no electrical engineer and my brain does it's own thing 

[Jerry Rxman Griffin aka MuthaF'er]

So now that I have determined that I do in fact have a 750A coil/stator I need to repair the wires. I need to add some length and new connectors. What kind of wire should be used? The stock stuff seems to wrapped in fiberglass or something like it.

Damn good question! However, is there insulation after the bullets inside the larger sheath? I'm thinking it's mainly for heat protection as the neutral switch wire (low draw) has the wrap also.

[scottly]

The A field coil should create a stronger magnetic field in the rotor, maybe due to more turns of larger wire? Try the test I suggested to Branden; the greater the magnetic force, the greater the output will be, with the rotor and stator remaining the same. (Watch your fingers! )

[scottly]

OK, I dug into one of my old reference books:
ampere-turns; A measure of the magnetomotive force generated by a coil, equal to the current through the coil (in amperes) times the number of turns in the coil.

[Jerry Rxman Griffin aka MuthaF'er]

OK cool, we're on the same page (without the test-tomorrow). Larger wire = less turns in the same space = can carry more current faster (without burning up) = stronger magnetic field.

Mikey er Branden can eat it er pinch his fingers and report in 

[brandEn]

So now that I have determined that I do in fact have a 750A coil/stator I need to repair the wires. I need to add some length and new connectors. What kind of wire should be used? The stock stuff seems to wrapped in fiberglass or something like it.

Damn good question! However, is there insulation after the bullets inside the larger sheath? I'm thinking it's mainly for heat protection as the neutral switch wire (low draw) has the wrap also.

I don't think it does. So I wonder if regular 16 gauge automotive wire will be good enough? I want to solder splice it together and wrap it with heat shrink.