Can anyone tell me if these are easy to service? Mine came with the Master 128 I bought from a work colleage, over 20 years ago (non eBay purchase), and they worked fine when I last used them - c. 2001!

Just wondering if anyone here has had any experience with servicing these, before I go ahead and tackle them myself, or can provide me with some hints / tips, etc. Can supply photos of it, if required.
Thanks in advance.