After serious thinking AK400 wrote :
Obviously a deal of care is to be taken in releasing the tension, but
does anyone know of a correct setting up method?

Trial and error worked for me for the retensioning - not a Marley BTW.

Fit it and make a guess at the tension, check the balance, then tension
a bit more or less as required.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk