Is it just a matter of unscrewing that big nut in the yoke that sits around the air opening? How do you get a wrench to wrap around that?
btw, I also purchased mine in the late 80's!
DISCLAIMER: I am NOT a "certified regulator repair tech"—by any stretch!
There is a special tool. It's flat and thin, with a rectangular hole cut in the center that slips over the yoke and "straddles" the yoke nut, and a handle for each hand on opposite sides.
However, I personally just use a BIG adjustable wrench. I think mine is a 12-in Sears Craftsman. Careful: Don't use one that's too small, or you'll mar the yoke nut (which, being chromed brass, is relatively soft). Ask me how I know about marring the nut! You want one that is big enough that the wrench jaws fit across the entire length of the flat sides of the nut. 12-in or, certainly, 14-in should work. Depending, I might choose to remove one or more hoses.
So, I find a vise and open it just enough so that my clean and dry 1st stage, padded with a soft towel, only just fits horizontally between the vise's jaws. (Don't clamp down!) I apply my BIG wrench to loosen. (Lefty loosey, righty tighty, correct!!??) Careful: This BIG wrench provides a LOT of leverage. I really don't have to apply much force at all. That's it!
Careful: Don't let anything fall through the now open high pressure hole into the inside of the 1st stage.
I make sure the threads and O-ring channel inside this hole are clean. I service the static O-ring that will sit between the male DIN part and the 1st stage. I screw the male DIN part finger tight into the 1st stage and then tighten with an Allen wrench (Allen key). I suppose I should use a torque wrench, but I don't. Don't forget the sintered filter!
Easy peasy. Should take your Scubapro dealer only a couple of minutes—more or less, depending on whether he/she needs to remove/reinstall hoses.
Safe Diving,
rx7diver