If it’s old equipment of my design, that is, Alice mixers pre 1986, JoeMeek mixers, compressors, EQs and recording channels pre 2003, TFPRO rack equipment and modules, then I’m only too happy to help with repair or restoration, and I do have a stock of specialist parts from the old days.
Just drop me an email at firstname.lastname@example.org and we can look at the problems. Normally, I ask you to send the equipment to me and I fix it for a small fee. Occasionally, I can carry out modifications and even the odd improvement, but each case is unique so drop me a note first!
If you have a technical question about the old designs, please bring it up on the TFPRO forum; very often it’s information that’s useful to others too!
There is so much more to ‘design’ than just producing a workable circuit diagram; that’s about 10% of the process. The 90% is in choosing components, laying out printed circuit boards, building prototypes and endless testing to find the limitations of the ‘design’.
So, while I’m always happy to help, I avoid giving specific design advice simply because the whole development process is so complex. By all means read the advice of others, but it pays to be a sceptic! My experience is that there is usually a grain of truth behind most advice. The wise course with sound equipment is to listen to it objectively and carefully and make up your own mind.