The perennial charging issues.

[Rex]

The latest-  it motors slowly at 6V, nothing at all at 4V.
Pressing the brushes to ensure good contact was done as a first step..it made no difference.
I'm doubtful the brushes are at fault in any way as they came from a reputable and well-known dynamo/mag rebuilder.
So, what's the consensus, new armature first or new field coil?

[Swarfcut]

Rex It hinges on your ultimate aim. Cheapest option, halfway house or top quality replacement.  It would be good to know what is wrong, but in the grand scheme of things another serviceable unit may not be cheapest, but is far easier.

 Replacing the field coil is tricky to get right, making sure of good physical all over contact of the pole piece and body, hence the screwjack Lucas Tool. Maybe you can pick up a body with a good coil, failed armature.

 If you decide to change both armature and field coil, the cost of an upgrade to 12V is not much more, but needs the 12v regulator cost to be factored. Assessment by a specialist is your other alternative.

 The armatures you have are an enigma, both failed the same, one good, one bad, both good? Substitution in a good working body would be a way forward if you can lay your hands on one.

 Swarfy.

[KiwiGF]

It also might be worth checking the rpm you are testing the dynamo at, they run faster than you think, I’ve never worked it out but you need a relatively fast drill (1500rpm plus?) to spin them fast enough to get to “start up” rpm (eg only using residual magnetism) especially if the brushes etc aren’t 100%.

Someone no doubts knows but I guess the dynamos run well above engine rpm?

Around 1.25 x engine speed ideally.
It may often take somewhere around 1800rpm to get a dyn to kick in. Sometimes, they need a really good blast to encourage them to behave - after which they will often stay behaving. When it's got into the groove, it might play from a little over 1000. Cordless drills and slow lathes aren't much good for doing some of this.
A bit of light extra pressure on the brushes can often get 'em going. (But if there's anything wrong with the armature, it's all in vain.)
A cold bulb hooked up won't light in a hurry, especially a 6v one - it will need pre-warming using a battery, or to be hooked up after the dynamo has started to deliver.

This thread IS confusing but I think we were talking about a test that involves independently powering the field coil with 6v, then spinning the dynamo up and powering a high wattage bulb. In this scenario the dynamo should easily light the bulb.

[KiwiGF]

This thread is long but has quite a bit of info including test results.

https://www.a7a10.net/forum/index.php?topic=13999.0

I found fitting a new field coil pretty easy, I just used a piece of 3/4 inch pipe through the middle of the case and a bench vice one end and vice grips the other to clamp the core against the case, then an impact driver on the screws (with  Loctite). It was pretty obvious if the core was held hard against the case or not as the retaining screws would tighten within half a turn once the core was hard against the case.

Having said that, I doubt a coil with 3 ohms resistance could be faulty unless it is shorting to the core.

I’ve not experienced an armature failure, do they short out between coils? Or short to the armature “core”?

[groily]

This thread IS confusing but I think we were talking about a test that involves independently powering the field coil with 6v, then spinning the dynamo up and powering a high wattage bulb. In this scenario the dynamo should easily light the bulb.
[/quote]

You're right Kiwi - wasn't following properly, sorry.  So yes, it should light.

[trevinoz]

As I have said all along, armature.
Can you borrow a known good one?
It's fairly simple to set up a drop test rig. I screwed a couple of pieces of aluminium channel onto a piece of timber and drilled a hole through each at comm centre height and fitted a round head brass screw through each with a spring for tension and a nut on the outside.
Apply a low voltage to same and read voltage drop across each pair of segments.
They all should be fairly close in value to each other, they will vary slightly due to the different resistance in each coil.

[trevinoz]

Rex,
I set up a generator with a known open circuited coil armature.
I excited the field from another source and drove it up to around 2500 rpm.
The output was in the order of 12V, however when I reverted back to the bridged terminals test, I could only get 0.7V.

[Rex]

Righto, here's an update.
As the tests etc didn't definitively show a faulty armature or field coil, and as 12V items are the same price as 6V ones, I decided to bite the bullet and buy both, on the basis that I didn't want to be doing the job twice.
An impact driver made short work of removing the field coil, and the new one installed by using two deep-throat G clamps on the pole piece and again the impact driver.
The dynamo was then assembled and tested by motoring...nothing. Getting the trusty AVO out showed that the field coil was O/C straight from the pack. Unbelievable, so this was returned to the seller (another week wasted) and a replacement sent.
The new one was assembled into the dynamo, and during testing (on the lathe) all output ceased. Bizarrely the AVO had chosen that moment to give up the ghost..
Now the dynamo looked to be a goer, but due to the man-handling it needed a respray..easy enough, but the spray gun just happened to choose this moment to block one of the side air bleeds and the paint spattered badly, necessitating a clean and a re-respray.
Now it was looking good so reassembled into the bike, the terminals bridged and the meter showed a healthy output.
Reconnected it to the DVR2 ( a known worker from another bike fitted at the start of this saga), and...nothing. Regulator appears to have given up the ghost, and yes I did reconfigure it for 12V operation.
 I swear this charging system is cursed in some way, but sod it, today was a lovely day for a ride, so I did just that. Replacement can wait for another day as I've already lost enough riding days this year.