FAQ - Fix for Roundcase Charging Woes

[BevHevSteve]

750 Charging

From: Brad Turner [email protected] I am presently running the ST2 alternator in my 750GT. I have it as part number 26440201A. It is a little wider than the stock 750GT alternator so requires minor fitting. Kent Jornevall originally told me about this mod and indicated he had to machined recess into rotor mounts and use 0.004-005 thousandths washers for stator (could be other way around but you get the idea). Karl at MMS did my installation and said he did minor machining to bevel gears to fit along with altered stator mounts.

I can tell you that I now ride with light on all the time and battery always comes back with a higher voltage charge than when it left, appears to be in charge mode with headlight on (sylvania halogen headlight) at anything above idle. This is a 350 watt alternator and yes it is very expensive. I am using a (I believe) Ducati 748 voltage regulator (bought on ebay for $50) which seems to work fine; it is mounted on the bottom of plastic tray under the seat. Sufficient voltage is no longer an issue!

If you would like further detail please feel free to email me directly. BTW, there is a HP drain penalty of course, and I'm sure one of the math whizzes out there can calculate it for us. I also acquired an additional 26-29 ponies during rebuild so its not an issue here.

[hulagun]

OK, I plan to make this conversion - so have been looking around for the parts. Apparently any later Ducati "rubberband" stator and rotor is usable AS LONG AS it uses an external stator (the static part with the copper wires), with an internal rotor (the rotating part). Apparently many newer Ducatis use the opposite setup so pay attention.

I picked up a used stator (cheap thrills - $30) although no rotor yet. Supposedly this part came off a crashed 916. I do not know it's part number or if this will fit a roundcase. Following description for comparison purposes only.

Description: The steel casing ring measures 112mm OD x 76mm ID, x 30mm wide. Painted sea green between the machined bare metal surfaces. It has 14 "poles" or teeth on the inner ring, which the 14 bundles of red copper wire are looped around. There are 6 crosscut grooves spaced equally around the outer steel ring. It looks like three of these channels are where the stator is clamped into the engine case. There are two yellow leads coming off it, both encased within a 45cm long black rubber sheath.

I am still shopping for the matching rotor. I assume I need one that will bolt onto the roundcase internals, and be about 75mm OD x 30mm wide.
Anyone out there have any later Ducati apart that can measure their rotor and post the info here?

I'm also wondering why the roundcase rotor can't be adapted and used, even though it is narrower. It seems to me there should still be an increase in charge just by using the bigger stator coils. The question is, if so, is it enough to negate the need for a new wider rotor? Or is it just cheaper in the long run to put the right size rotor in versus trying to make the old narrow one function...

[manitoufs]

Ivan -
From what I understand about this setup, the rare earth magnets in the ST2 rotor are the driving force behind the majority of the improvement.

Keep us informed of your progress.

[Eldert]

The regulator on a roundcase is a very crude device
it only uses 2 diodes to rectifie the charge from both coils
this means only half the power the generator generates get used
wat we need is a regulator that uses a bridge type rectifier that way
the positive wave and the negative wave of the sinus gets used
before i would swap a st2 generator i would look into the regulator first
Eldert

[manitoufs]

Yo Eldert -
Do you have any recommendations? That sounds like the first thing I would like to do.

George

[hulagun]

I have already fitted a modern solid state regulator (from Eurospares). I have yet to read definitive proof that changing regulators improves the output of a roundcase. All I have heard sofar is electrical theory and (no offense) jargon like "bridge rectum-friers" and "sine wave theory".

If what Eldert says is a viable cure or alternative to fitting a newer stator, then 'someone' needs to start a new thread here explaining exactly how it is done and what parts are required.

Of course, if there is a regulator one can buy and (pretty much) "bolt on" that will double my roundcase wattage output, as the ST2 type stator does, we would all like to know about it.
:D

[FredFrenzy]

I've put a square case 200watt rotor/stator in my 750GT when, like Brad, I had Karl at MMS doing my engine..it takes more machining then the ST2 stator and don't think they are too easy to find. I got the stator/rotor when a UK head was selling off some of his hord.

I got a Shendeng rectifier from Phil at RandR at a reasonable price. It's small with modern electornics and instructions from Phil. Mounts nicely under the seat. With a sealed AGM 14AH battery, I don't need to think about it, it's wonderful. With a dyna ignition you don't want a perennially weak battery since it won't fire below about 9v....

barry

[FredFrenzy]

I've put a square case 200watt rotor/stator in my 750GT when, like Brad, I had Karl at MMS doing my engine..it takes more machining then the ST2 stator and don't think they are too easy to find. I got the stator/rotor when a UK head was selling off some of his hord.

I got a Shendeng rectifier from Phil at RandR at a reasonable price. It's small with modern electornics and instructions from Phil. Mounts nicely under the seat. With a sealed AGM 14AH battery, I don't need to think about it, it's wonderful. With a dyna ignition you don't want a perennially weak battery since it won't fire below about 9v....

barry

[motomerle]

Where's MMS? I've had light on charging woes ever since I changed to the Hella headlight conversion in '80. NYS is a lights on state and I took to carrying a charger and lead in the tank bag when I parked it at work.

Is there a source for new/repro wiring harnesses?

[BevHevSteve]

http://www.fezone.com/mms/

The above is the URL for a website I made for Karl a few years ago, his address etc is there.

[hulagun]

I'm still waiting for someone to explain how this works and how to go about doing it.

Why spend money installing late model parts if I can just bolt on a new rectifier and "double charging output"?

Ivan

[CoreyL]

I replaced the stock alternator rotor in my 74 750GT with a rotor from a 1998 ST2. All went well.

A couple of comments on things that were not explicit in Willy Gonnason's excellent on-line article (http://willyg.homelinux.net/Alternator/ ): no puller was needed to get my old rotor off. Once the nut was un-done, the rotor just slid off. I measured carefully how far out of the case the old rotor was before I removed it. The old rotor had a thick steel slotted washer behind it which sat in a recess on the back of the original rotor. The ST2 rotor did not have a recess on the back (finned side), hence I didn't use the washer with the new rotor. Once mounted on the end of the crank, the outer surface of the ST2 was within a fraction of a millimeter of where the old rotor was. There was an important difference, however, in that the old rotor had magnets that protruded from the rotor body whereas the magnets on the ST2 rotor are flush with the rotor body, making the ST2 rotor body effectively larger in diameter. This is important since when the rotor sits in the stator, the new rotor protruded beyond the coils and can make contact with the three soldered connections to the stator. Those connections needed to be ground down a bit to be flush with the stator coil surfaces to prevent the rotor from making contact. I hope this isn't too confusing, but once you look at it, it's obvious.

Once everything was all buttoned up, I fired up the bike and measured the voltage at the battery terminals to make sure that it was charging and voltage was increasing with RPM. All was good, I was measuring over 14.5 volts (no headlight) at around 2000 RPM. Had I been thinking scientifically, I would have made before and after measurements with and without lights, but that would have required forethought and anticipation. Plus that was already done and displayed in Willy's article; I really just wanted to make sure I hadn't somehow screwed the pooch during the operation.