I do have the schematics (circuit diagrams) for this set. (c. 1985/7). The great thing about it is that it was still made essentially in 'discreet' components and is technically serviceable by a competent person. Even the 'private channels' are simplicity to program (with diodes!). It was one of the first synthesised boat radios, as compared with ones with individual crystals for each chanel.
Mine is still going strong.
Sorry, I'm not offering to fix it! ... but any specific questions I'll try and answer.
As a start, have you checked the actual hi/lo power switch?
If you wan't a (third generation) copy of the schematic, I'll be happy to oblige.