Hi Julian,
I would preform a couple of tests before deciding that the Dynastart is suspect. Get a set of Jump leads, connect up to a good battery, i.e. one fitted to a car. Then connect the negative to the negative ground lead in the engine bay. Positive needs to be touched on the big stud that is very prominent on the Dynastart (the big thick wire connecting to the Dynastart and the solenoid) - If the Dynastart than turn the engine over correctly, the solenoid is the thing at fault. It can be taken apart and the contacts cleaned if it is the fault).
There is usually a pretty good spark that happens when you touch the positive cable on to the stud, So don't get scared by it! A good firm, one movement connection is required. If you don't want to damage the crocodile clip with the big spark, then you can put something else in the jaws hat touches the stud. - Best stuff would be a piece of copper - maybe a bit of copper water pipe if you have some lying around.
If that does not work then the Dynastart brushes / commutator are at fault. This requires the Dynastart to come off the vehicle so you can take the brushes out and see if they just need replacing or the commutator is in need of cleaning.
If you don't want to do any work on the Dynastart yourselves, then you could use the place I have used. They might be a bit on the expensive side, but can do everything that the Dynastart might need doing to it! Classic Dynamo & Regulator Conversions (CDRC) Ltd (
http://www.dynamoregulatorconversions.com/) - I normally deal with a guy called Peter and send him an email detailing the issue I have (Don't forget to mention it is from a Haflinger). He usually responds quite quickly.
If you want to keep it within the club, I believe James Price, who has just setup
https://haflingerparts.com/ can do repairs on Dynastarts.
John
Haflinger 703AP LWB 1973 - (Once owned by Lady Sutherland & Sons.) Now called "Lurch" !
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