I think I may have a blown tweeter in one of the speakers. This is for a mark.1 system (speaker part no. SS-78) with the "fake" square drive units. I tried and tried but cannot figure how to remove the grill to access the drivers. The top edge comes adrift and just uses ordinary "pegs", but the bottom part seems more firmly fixed.

phil...as far as i am aware the bottom of the grill is on pegs too.....just needs a good hand

Remove the speaker cable jack plate in the rear of the speaker. Then you'll see the screw that secures the bottom of the front bezel in place.

Unless you already gave it a good hand, then you'll probably still need to remove the speaker wire jack plate so you can retrieve the chunk that broke off so you can glue it back.

Thank you Davebush and Stormin Normin. I did try Dave's method - but to no avail. Luckily I didn't break any chunks off! I found the one cannily located screw. Thank you 'Normin. It turns out the tweeter isn't blown after all -its just "quiet" compared the other channel, which sounds more resonant and harsh, so I suspect the other one is at fault.

Ok. One thing you really need to check before swapping out any tweeters is the high-pass filter capacitors. They do go bad or degrade so I recommend that as the first order of business for you.