850 Commando Rectifier replacement

Hello gents--I have a 1974 850 Commando with stock alternator. I need to replace the old rectifier and maybe the zener diode as well. They are both working but are originals and old. Should I replace them both or just the rectifier. What do you suggest and where should I purchase the new parts. I want to get reliable stuff but not spend a fortune. Thanks for any help.

if it's NOT BROKE DONT FIX IT. serious, for the most part the rectifier and zener gave VERY little trouble it was mostly from ham fisted people working on them.there are several options, podtronics, boyer power box, typanium and sparks are a few if you feel you must replace parts

If it's not broken, MAINTAIN IT!

Clean all the connections, especially the ground connection at the base of the rectifier, and use a dab of dielectric grease on them.

If you keep them clean, and conections tight, they'll last a long time.

There is a slight improvement to be had in forward foltage drop by replacing the factory selenium plate rectifier with a more modern silicon bridge type. The replacement is a square epoxy 4 diode bridge, which should still be available at Radio Shack. You need at least 50 volt, 25 amp ratings.

I checked over at Old Brtitts for prices. Podtronics rec/reg is $35. Stock replacements are about $100 combined. It is more convenient to stay with stock because the harness and mounting locations are already taken care of. As the old saying goes, "You pays your nickle, and you takes your choice." I've got a BSA C11G that has a huge rectifier that looks like a stack of flapjacks. It's too charming to get rid of, but then it's a very light bike and easy to push

I really like the Sparx single phase "finned egg" as a replacement on Meriden Triumphs because of the "cool" factor. As it would look out of place on a norton, I typically go with thier standard "finned box" units...

I've got two old beaters that i put the cheap Radio Shack diodes on, they work fine.

maylar said:

There is a slight improvement to be had in forward foltage drop by replacing the factory selenium plate rectifier with a more modern silicon bridge type.

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The original circular finned Lucas 49072 rectifiers as fitted to Commandos (pre-MkIII) and many other types of British bike etc., are not selenium, they are SILICON diode bridge rectifiers!

And Lucas had been making and supplying that rectifier to British motorcycle manufacturers who had been fitting them as standard equipment from around 1963.

L.A.B. said:

The original circular finned Lucas 49072 rectifiers as fitted to Commandos (pre-MkIII) and many other types of British bike etc., are not selenium, they are SILICON diode bridge rectifiers!

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I'm aware that it says that in the workshop manual, but it's always been hard for me to believe. The construction is classic selenium style with the finned plates, and in my 40 years in electronics I've never seen a silicon bridge made that way. I could of course be totally wrong on this, and I'd love to have one to burn out and see if it stinks like rotten eggs (another classic selenium trait).

If you look at the fins you can see the individual silicon diode's attached to them, on the selenium ones the fins themselves were the active component's, I know what you mean by the smell though, (rotten eggs!) I was once a television engineer and we saw a lot of selenium recs until the early sixties when we started replacing them with silicon diodes,
Dave.

"and I'd love to have one to burn out and see if it stinks like rotten eggs (another classic selenium trait)."

One of my major other hobbies is restoring tube (ok...VALVE) type Ampex analog tape machines (recorders) from the 50's/early 60's and I can definitely vouch for the rotten eggs!

maylar said:

I'm aware that it says that in the workshop manual, but it's always been hard for me to believe. The construction is classic selenium style with the finned plates, and in my 40 years in electronics I've never seen a silicon bridge made that way.

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From The Lucas Service Manual (1966)



Why would they lie about it?

It does say "Types"

Danno said:

It does say "Types"

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Silicon diode types (= types of - Silicon diode rectifiers)

(As opposed to information on the Selenium types which was included in the preceding paragraphs of the Lucas Service Notes).

Now I have been told by people who should know that older bridge rectifiers Take in all the AC they see and convert it to DC whether it is needed or not by the bike. Now this can make a very hard life for the Zenor diode that has to take unused current and convert that to heat. These same folks would have me believe that the radio shack, PODtronics, Sparx and all other newer systems only convert the AC that is demanded by the bike in the form of DC letting the Zenor live the life of Reilly. If this is true it seems like a better system. Evidently the potential to make more DC doesn’t cause all kinds of AC to be running around inside the stator.
Now I have been living under this dilution for a long time. And have because of this belief seen voltage meter readings that would seem to bear this thinking out. I realize this may not be true so please don’t burst my little bubble of ignorance. (its working for me)
I have seen AGM batteries melt under the heat of an old style rectifier and Zenor and I have seen PODtronics that have below normal voltage readings because the battery was all charged up and the headlight was off. Oh please don’t tell me I am fooling myself….Could this be a case of a little misinformation starting the formation of a useless and baseless myth by witch I have slowly formed a whole world around. Oh please someone explain… my head hurts so… bad.
Now if you’re a lead acid type of person who doesn’t mind going around making a little battery gas all is well in the world for you but if you toss in an AGM battery into this old type system you might want to consider turning on your head light. Now if you would like to see what I have seen than disconnect your battery and reconnect it using an AMP gage in the line. If you’re seeing an amp and a half positive at say 4000 RPM well, Keep in mind that more isn’t always better and that a long cruse with the headlight off might lead to you blowing an AGM up. Now since a bunch of the new systems were designed to work with lead acid batteries some of them may not be all that for our new AGM’s The technologies probably need to catch up with one another. But the people who brought you these wonderful toys may have retired and just don’t care. The string of one blaming the other could go on for years. Where’s Dyno when you need him.
:roll:

The rectifiers definatly takes all the AC they see and converts it to DC. The problem here is a voltage and current issue. You have to keep them apart.

The AC generated voltage will be higher at high RPMS then at lower. Thus you will have a higher and lower voltage on the DC side of the rectifier . However, your battery is still connected so it will pull up the voltage of the rectifer. If you disconnect your battery there is hardly any load or (current draw) so the voltage may appear all over the place.

example : rectifer is putting out 6V DC but your battery is fully charged, you will messure around 14 volts across your rectifier.


Cars work with a regulator and rectifer, rectifer does the same thing, while the regulator 'looks" at the battery voltage and will stop charging the battery by disconnecting the battery from the rectifer once the voltage is reached.

Zener diodes are the "regulators" on the Norton but instead of stopping the battery from charging they shunt the voltage above 14 volts.
Current flows through the zener it does cause heat, a waste of energy yes but it is not like you could store that energy anyway.

The only guess I could make is that newer electronics would charge battery more like a car would.

http://yarchive.net/electr/selenium_rectifiers.html

Dave
69S

The solid state regulator/rectifiers still do the same old job.

Dump excess voltage in the form of heat.

Hence, "mount in a well ventilated location".

DaveK said:

Cars work with a regulator and rectifer, rectifer does the same thing, while the regulator 'looks" at the battery voltage and will stop charging the battery by disconnecting the battery from the rectifer once the voltage is reached.

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Where does the extra juice go once the regulator has disconnected the battery?

swooshdave said:

DaveK said:

Cars work with a regulator and rectifer, rectifer does the same thing, while the regulator 'looks" at the battery voltage and will stop charging the battery by disconnecting the battery from the rectifer once the voltage is reached.

Click to expand...

Where does the extra juice go once the regulator has disconnected the battery?

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There is no "extra" juice to speak of. ....Its a more complex eletrical relationship but basically if you have no load the alternator is not generating any power...

Hello--Mark again with my original question. You guys have made some great points but I am not the engineer type. I understood most of what you said. I am just concerned about the age of my orininal rectifier and the possibility of failure on the road. I like to keep my bike up to snuff and trouble free. I only like to (blow) money on Beer and Women as it should be---- If I don't need to replace it and the original, even at it's age is reliable when maintained and cleaned with a nice dialectric contact bath, I think that is what I will do. The bike has been totally restored to like new but I will clean the original diode especially the contacts and the main mount to frame ground. I can use a VOM and check the rectifier for shorts or partial shorts. If getting the Podtronics or Radio shack rectifier is more efficient and charges the battery fine with out overcharging or overheating the epoxy Radio Shack cheapie I guess I may consider going that way. I guess that is my main question left---Should I get a new diode bride or not---Thanks for all the information. You guys have saved me hours in the past and allowed me to avoid many headaches which leaves more time for beer and women---Thanks again---Mark Cigainero