The dials are Electro Luminestent sheet.

So basically they glow when a electrical current is passed thru them.

Unfortunatley the sheet starts to deteriorate after many years and black "blobs" appear.

This I gather is what is happening to yours. Has your speedo, rev counter etc started doing the same?

You can easily fox the problem by replacing with new (if still availiable) or the easier option is to get a set of Briba dials.

This is a common problem, so your not alone