Topic: CB750A alternator upgrade in a 750K?

[Dunk]

I found out the CB750A alternator is rated at 290 watts, vs the 210 watts of the K and F alternators. Best I could figure from some old threads on here the differences are:
1: The field coil is lower resistance with larger diameter wire but physically the same dimensions.
2: The rotor on the A is slightly longer and about 1 lb heavier, though maybe more for additional inertial rotation for the automatic than for charging.
3: The stators are different part numbers but dimensionally the same, windings appear the same, no mention of wire gauge differences or not
4: The A used a combo reg/rect used on later model bikes, possibly with larger gauge or two wires off the rectifier

Also I figured out to visually identify an A field or stator coil from a K or F at a glance, it seems the A has black insulation pieces at the bullet plugs vs clear (yellowed with age) for the K and F.

Supposedly someone had the parts and was going to swap the field coil and report back but never did. So in theory the field should be stronger with lower resistance and larger wire for the windings, resulting in higher charging. Not sure if the rotor or stator need to be changed to get the full additional 80 watts but it should have appreciably higher output either way. One concern is wire gauge off the stator and rectifier may need to be upped as 80 watts is around 6 amps. Total output goes is about 15 amps vs 21 amps. I think the main wiring off the battery is 14ga which is fine for 15a but maybe marginally ok for 20 on short runs. Probably better with 12ga to be sure.

Has anyone done this swap of the field coil or whole alternator assembly from an A into a K or F? Results? I ordered an A field coil and intend to measure voltage at various RPM and full load amps so I can have numbers before and after.

Currently my K1 and K5 don't reach 14.5v until 3000-4000 RPM depending if headlight off/on. Charging system is adequate but marginal with both having a 55/60 H4, Dyna S, and 3 ohm coils. I'm hoping for a bit more lower RPM charging and to reach full voltage at a lower RPM. Considering resistors on the coils to bring them into the 4-5 ohm range and trying the ecovision lower power H4, as with stock 35w sealed beams I've never had a charging issue even in stop and go or traffic with the upgraded ignition. Regardless, if I can get more alternator output for cheap I'd like to do that as well. Also looking at high efficiency Schottky diode rectifiers which could increase lower RPM charging from less voltage drop/waste as heat. It seems there are solid state components off the shelf to do this, just need wired up and assembled onto a heatsink I guess.

[Jerry Rxman Griffin aka MuthaF'er]

Couple additional wires to hold the extra current so the connector plug situation is slightly different.

I have the parts but never got around to it. Replaced almost everything electrical with new during the restore and found that mythical 14.5V WITH my 3 ohm coils, and the headlight upgrade so I stopped there. There are stamped part numbers that can be used to differentiate, at least on the field coil.

No problem with the rotors. Use a later lighter one.

The stator is different. Check the resistance. Should be less. Better current flow.

[Dunk]

OK, I just ordered a 750A stator, I'll compare to a K stator I have somewhere and the one on the K1. I also have a later rotor I'll swap in, should be a bit over 1 lb lighter than the larger K1 rotor and no loss of charging like if they are lightened too much? Would be nice to get a little quicker revving out of it as a perk while I'm in there.

What RPM do you get your 14.5v with headlight on and what's your voltage at idle?

[scottly]

I swapped a late rotor onto my K1 in place of the heavier flanged rotor, and the motor did indeed rev quicker, at least in neutral.
To accurately measure the very low stator resistance, you will need something more than a typical digital volt-meter. If you are handy with a soldering iron, you can whip one of these up in short order, once you have the parts on hand:
   

[Jerry Rxman Griffin aka MuthaF'er]

My voltage dilemma was a long struggle. Replaced all switches including bars and ignition, main and sub harness, replaced all remaining bullet connectors, new electronic reg/rect, Dyna 2000 etc. When I first fired it up I broke out the meter. Revved it up and saw 14.5V then that's all the testing I did. 

You won't get any loss of charging with the late rotor just make sure those stator bullets are good and a tight fit.

I can dig my A and K/F stator and coil out of my stash later in the week if you need any info. Gonna be on the road through Wed.

[Mr Freeze]

Well you have to remember that the stator is only going to generate as much voltage as what is demanded by the system which is dictated by the field coil signal created at the regulator. I don't know how much you stand to gain by installing an A alternator in your bike if the regulator is a stock unit. Based on the 3 ohm coils and other things you've listed there may be some possible positive gains. You also should consider installing a Oregon Motorcycle Parts regulator which starts the charging at 1500 rpm instead 2000+ rpm of a stock unit.

[Dunk]

I do have a OMP regulator and their rectifier as well. With the solid state regulator I did notice charging was slightly higher at lower RPM.

[madmtnmotors]

OK, I just ordered a 750A stator

From whom?

[Dunk]

Got them from eBay. Field coil measures 4 ohms, stator measures .5 ohms on all pairs. I'll weigh them and do a visual comparison to stock K parts too.

[scottly]

The standard stator is .2 ohms; like I mentioned before, it requires more than a typical meter to accurately measure such low resistances. Check the standard stator with the same meter, just for comparison.
The standard field coil is about 7 ohms, so the auto unit has a much lower resistance, and is probably the reason for the greater output. If so, this is good news, as it's much easier to re-wind a field coil than a stator.