Electrex RR51/900SS

My first post on this forum.

I am in the middle of an on-going battle with Electrex as I purchased a new regulator/rectifier last year and cannot get it to work. Apparently this is not uncommon. Any experiences of other members would be very helpful.

Just to mention that the bike (1980 900SS bevel) DOES run with the old reg/rec fitted, but the battery does not charge. All other systems are in good working order (including electrics).

I am going to cover myself by putting this post on the Technical Help section too.

Look forward to hearing from you.


Hi again Hugh,
I’ve been running one on my old bevel for 2 or 3 years now with no problems,
I’ve also got one on my 851/916 special again I can report no problems.
Are you sure the alternator and the wires coming from it are OK, the bike should run without one fitted until the battery runs out.
I’ve not heard a bad word about them.
I did suspect a problem once and they sent me another one FOC, it turned out that it wasn’t at fault so I’ve now got a spare and this was more than a year after I’d bought it!

Steve Robins

Hi Steve - great to hear from you!

I have spoken to John Fallon who doesn’t rate them for bevels, but perhaps you could tell me how to test the alternator - I am crap at electrics!!

As I mentioned, the bike runs fine but no output to the battery.

If it is the bike, I feel egg approaching my face! :cry:



You need a multimeter set to DC Volts in the range of 10v upwards, the battery should show around 12.5 at rest.
When the the bikes running it should rise to around 14.5v, if it keeps on rising the regs not working,
If it doesn’t rise then your alternator or its wires are at fault.
The original wires were a bit undersized, also not very good quality, they can melt the insulation and short together.
If your alternator stator is still the original it’s probably in need of a rewind by now, the insulating lacquer can crack and cause the windings to short out.
The reg/rec changes the currant from AC to DC and should limit the amount to the 14.5v that I’ve mentioned.

Steve Robins

Oh dear - more dosh. Who can rewind and how much is it likely to cost?
Thanks Steve

Don’t panic and go straight for the rewind, check out everything else which Steve has described first.
My Darmah SS is about the same age and it’s fine.
My 200GT is about 20 years older and that still works OK too!

As roger says don’t panic, run the test, also use the m/meter set to AC Volts on the 2 wires from the alternator, if it’s working you should get quite a high reading, if not check the wires visually.
But a few ft of heavier gauge wire won’t cost much, so solder some new wires on, you’ll need to remove the alternator cover to do this, you can look for any signs of arcing or cracking of the laquer on the windings while your at it.
Mine had cracked a quite a few places, over the years which I’d sealed with a smear of RTV.
If all else fails a rewind costs under £100, or it did a couple of years ago.
You can buy the copper wire pre laquered and rewind it yourself, replacing the paxolin spacers while your at it.
How to do this was written up in Desmo many years ago.

Steve Robins

Thanks guys,

I think a break from building a bathroom would be a good thing to do this weekend!

I will let you know how it goes.


You will want to test the Alternator on its own as well. Disconnect it from the bike and having made sure the battery is fully charged (as you will be running it total loss) start the bike and check the outout of the alternator with the meter set on AC volts (NOT DC for this test only)

Having warmed up the old girl properly, raise the revs to a steady 4000 rpm and check the output.

As I write this I cannot remember what a good output should be, but I am sure between us we can find out! (70V comes to mind, but I would’t swear to it?) This is the test I carried out on my Bevel at the TT when the original regulator died, and it proved without doubt that the alternator was OK.

I replaced the original with a electromex as it happens and the output from it was and is far supirior in terms of a smooth supply than any standard regulator of that period I have ever come accross.

I did have a couple of electromex regulators when they first came out on a couple of specials I built and they were awfull, and both failed, and I know of other people who also had problems with these regulators around this time, however I am told they have since changed the desighn and are now OK as both myself and Steve have found!

Hello Everyone

My first post (only just found this place). I have a few Dukes including a 1980 Hailwood Rep (2 piece fairing but no side panels) that is in need of a new Regulator/Rectifier.
(its definitely a Reg/Rec problem - Ducati Elettrotecnica box)

Ive seen mixed reviews for the Electrex RR51/ESR510 - but Im guessing from what Ive read that the newer/later ones (RR51) now work as they should.

But I’d appreciate peoples ‘current thinking’ on these though before splashing out £70+ that Electrex are selling them for (EBay)

Many Thanks and Regards

Hi Andrew,
I’m sure that you’ve read what I’ve posted before on this thread?
I swear by the RR51’s and have them on 3 of my bikes, I’ve got no problems recommending them to members.
At around £70 each they’re a hell of a lot cheaper than most of the others on the market and work just fine ‘n’ dandy.
I have them on my 2 851 based bikes and my '79 900SS.
Just a litle note here, I had to file off a bit of the finning to get the RR51 to fit in the original place on my Bevel 900SS, (The same as your Hailwood) to stop it touching the battery.
I also use a battery breather catch tank (Which I got from Steve Allen’s Bevel Heaven) to avoid any problems if it gets a bit hot down there.
Ask Keef what happens when your battery boils and how much damage it can cause to your Conti’s??
But I’ve since found out that you can get a sealed PC310 Hawker Odyssey battery that will fit in the space available.
The next time that I need a new battery I’ll be getting one of them!

Steve R

Hello Steve

Thanks for the reply. Yes I read what you posted previously on the RR51’s. Ive read good and bad from a lot of folk on these but when its bad it usually it turns out to be something else thats causing a problem, or its the odd unit thats just duff. Encouraging to hear though that youve had a lot of success with these on three bikes - thats starting to become statistically significant in my book…

Ive had a few Regs/Recs replaced since I bought the MHR (a few years ago now) but always gone for the OEM part. Ive done this mainly for looks to be honest as on the MHR I have its an exposed part in full view(no side panels). I keep meaning to look into hollowing out one of the dead Elettronica ones in the shed and putting modern internals in it (does anyone offer this service do you know ?).

As to batteries - Ive long since been running a sealed one (WestCo/SVR).

The chrome on my original Contis went west in about 1982. Spilling acid on them would be an improvement. The s/s Keihans went on in '82 and still look (and sound) superb. That said its always mildly irritated me that the Conti badge isnt on there and I recently sourced some very nice repro Contis from Australia (not fitted yet and will probably stay in the wrapping paper for a good while yet)

Anyway - I’ll purchase the RR51 and let you know how I get on with it.

Regards for now