I've read a lot of material that says pushbutton tuning is beastly to repair or work with. Thus far, while I've only gotten my set working mechanically, it's been a pain in the rear, but I thought I'd share the techniques I've evolved to do the job thus far. I'll add more to this page when I get the thing working electronically (or at least verify that it already is).
As you can see from the picture above, the pushbutton mechanism I'm dealing with is fairly simple. The horizontal bar towards the top of the button chassis, which I will call the lock bar, is spring loaded to press tightly against the vertical bars, which I will call button bars. When one button bar is pressed, the separator pushes the lock bar to the left, releasing any other buttons latched down, then latches back against the pressed button to latch it down. The plate above the lock bar I call the retaining plate. All the button bars are threaded through it. The bracket screwed to the retaining plate is the mounting bracket.
All parts for this mechanical restoration were purchased from Home Depot, an American hardware store chain, so they're easy things to find. I needed one spring to replace the spring on the button bar, a package of self sticking felt pads, two grommets, and a little light grease, and some WD40. Tools required are wire cutters, a small file, a screwdriver, and a pair of needle nosed pliers.
Okay, down to business. I started out by taking the buttons off the button bars, so I could remove the mounting bracket. The original grommets were completely dessicated, so I replaced them with 1/4 inch grommets found in the lighting department of Home Depot. Slightly smaller grommets would have been better, but these work okay. I left the bracket off for the next steps for simplicity.
The next step is to unscrew but *not* remove the button retaining plate. I had no special desire to try and thread 5 spring loaded button bars back into the bottom of the button chassis, and it wasn't necessary in any case. By taking the screws out of the retaining plate I was able to lift all the button bars enough to allow the lock bar to slide enough to the right to get the spring out. I replaced it with a cut down spring purchased at Home Depot. Put some light grease on the lock bar at each end where it passes through the button chassis. Once this is done, thread the lock bar back through the button chassis, and screw the retaining plate back to the button chassis, making sure the button bars are properly in the notches of the lock bar.
The next step is to replace the stop pads. Originally these were some kind of rubber cut to fit the button bars exactly. Of the 5 I should have had, two remained. The stop pads are absolutely critical to proper button operation. If you look at the button bars and imagine what happens if they slide 1/8 inch further toward the retaining plate than they should, you can see that this would prevent the lock bar from engaging any button. I made mine out of self sticking felt pads, notched to go around the button bars and stuck to the back of the retaining plate. They're the brown things just inside the retaining plate. If you want to be sure they never fall off you could probably cut slots in them for the button bar, take the retaining plate off, and thread each one over the button bar, but I felt this approach was more than sufficient.
At this point, the mechanism should start to show some signs of functioning. Put light grease on each button bar's notches, and at the top and bottom where it passes through the retaining plate and the side of the button chassis respectively. A spritz of wd40 may help, but use it sparingly, as you don't want to saturate the electronics with it. With my button set, several of the buttons would bind when they should have released. What would happen was the button would be locked down, as button 1 on the left is. I would push another button and the lock bar would move left, but it would get stuck on the separator between the top and bottom notches of the button that was already down, preventing the lock bar from locking the new button down, and preventing the old button from popping back up. Apparently the button bars develop spurs or rough spots over the separators, and after much tinkering I finally filed the separators on the offending buttons very gently to smooth them out. This resolved the problem, but do be careful not to take much metal off the separator, or it will no longer be tall enough to push the lock bar far enough to the left to release its compatriots. I would use a jeweler's file if I had to do it again, although for this job I used an ordinary flat file, except for button 1, where I had to use the tang of the file because of the lack of space.
That's all there is to it. The return springs on the button bars are strong enough to do the job in this case. I imagine you could replace them with springs out of ball point pens if need be, but it wasn't necessary in my case. All the buttons now function smoothly. Now I just have to see if everything works electronically.