My '82 750 Maxim J does not seem to charge on tickover, someone told me this is a design fault and it only charges above 2000 rpm.
Can anyone confirm this and is there a modification?
Thanks.

Not sure about "tickover" I assume you mean idle. Your idle should be about 1100 RPM. What voltage are you measuring at idle vs voltage at 2000 rpm??
You should check your alternator brushes and make sure the surfaces they rub on are clean and the brushes are in spec. If they are less than 10mm you'll need to replace.

Also put what model and year of bike in your signature so we all know what you are working on.

MN