Noticed my 28 Chummy dynamo wasn't charging this afternoon.On investigation found that the little finger that stops the points opening too far on the cutout is rusted to paper thin allowing a large gap that the magnetism couldn't close to charge.
Spent a couple of hours with my spare trying to work out a fix.
The 'only' possible thing I can think of is to make a 3/16" diameter post with a replica of the finger formed on the top and a thread with two nuts on the bottom. Drill a hole in the base just forward from the coil in line with the finger position through the two pieces of insulator and the steel base,fix with one nut above and one below the base plate.
I have worked out which parts need to be insulated from each other, the post will effectively be earthed by the base plate and would seem to me to be not a problem with the amateur checks I have made.
The question is what happens when the coil is magnetised, could it then short out somehow?
Any help will be much appreciated, or another suggestion if others have overcome the same problem. Cheers Ian

Location: Bristol

Hello Ian. From your description it sounds as if you are on the right lines. Electrical insulation if required will need a thin piece of plastic or tufnol at some point to form a break. If you are worried about any detrimental effect on the magnetic operation I'd suggest using brass for most of the metal parts.

Tufnol used to be made from layers of cotton cloth impregnated with phenolic resin, and is a useful rigid insulator that is suitably "period".

To clean contact points I recommend a diamond spatula (fine grade) which are available from Farnell etc. They are pricey at about £10 each but will last for years.

Location: New Forest

Many thanks John for your response. On closer inspection today found that the little finger was in fact brass not steel, very strange as I have never seen brass corrode to resemble rusty steel.Perhaps intergranular corrosion because of an impurity in the manufacturing process of the brass.
I was able to beef up the brass as it wasn't actually broken just quite bendy.
The position is now: as soon as I increase engine speed contacts close and as revs die contacts open again. Rev engine contacts close no charge no matter how high the revs. Switch lights on with engine running fast,higher than normal discharge shown.
From this I deduce ammeter working and cutout working but dynamo suspect (fuse not blown).
Question now is could it be possible for the dynamo to be burnt out when the points didn't shut because the gap was too big to be pulled in by the magnetism?
Everything has been working fine before.

Location: Bristol

Happily your CF1 is now operating correctly in the electromagnetic dept. but, are the contacts clean and making effective contact when the cut-out operates?? If not, you will get zero charge.

Ian Mc.

Location: Shropshire

I'm not an expert on electrics but I don't think the cutout contacts would close if the dynamo was not doing something.

Location: Shropshire

Thanks Jim and Ian, that's what I thought the fact that it closes must mean it's working.Ian I did clean the points, maybe they look closed but aren't, maybe riding on my repair. Too cold and dark in the garage tonight and club spares shed in the morning so will have another look tomorrow afternoon. Plan is a mirror in the car to see the ammeter and put a screwdriver across the points when they close. Cheers Ian

Location: Bristol

Had another go this afternoon, on close inspection points looked closed with the naked eye. Shone a torch upwards directly under the points and there was the problem, points not closed by a gap no thicker than a human hair. Now sorted and all back in working condition. Really pleased I didn't have to resort to my fix with the pillar idea as it meant drilling an original part, not something I would do unless there was no other way out.

Some may find this useful sorry it's a F*rd site but good all the same.
http://modelabasics.com/Cutout.htm

Thanks again for your help guys. Ian

Location: Bristol

The F**d site is excellent for the illustrations etc. Thanks Ian

Location: Near M1 Motorway Jtn 28