I acquired a Yamaha RX7 drum machine for just a few Euros a couple of days ago. The RX7 was released in 1988 as a follow up to flag ship RX5, lacking single outs and dedicated faders but with much more 12-bit sounds on board. That's why I would consider this machine "the" Yamaha drums from the eighties you'd want to have.
However, the rhythm box had several issues (hence the price), mechanically for the most: half of the buttons were stuck or moving very stiff so realtime drumming and programming was hardly possible.
The button technique is basically the same in a variety of Yamaha machines from that decade, so solutions described here may apply also to RX5, RX11 and RX15.
Opening the housing is not straight forward, I had to have a glance at the service manual to get it done. You'd have to remove 3 screws from the bottom and 3 from the back, the ones not holding ports and connectors. Then you'd have to take a screwdriver or something similar and lift up the bottom on the front edges carefully.
To remove the wiring, press down the small noses on the connectors and pull softly but firmly.
Stiff and stuck buttons
It appears that my machine has either produced too much warmth internally or it has been kept in a hot environment. All of the buttons were deformed somehow, some stronger, others just a bit.
Each button has two clips to hold it in place, supported by a pin to prevent wiggling and a cylindrical bottom holding a thin and strong spring that pushes the contact and a larger spring to move the button back up.
Deforming always happend in a way that the buttons started to fold in the middle (which you can't see if you don't disassemble the machine) so the clips on their edges weren't square anymore.
I tried to fold back with a heat gun (110°C) first, with no luck. Then I tried to bend the pin and the clips with heat. That does work, but the clips tend to twist. So finally I filed the clips to get them square again and bended just a bit.
Unfortunately the cylinder on the bottom side got deformed, too. So it was either not square any longer in one direction or it had lost it's circle shape. Only filing could help here, too.
To get the cylinder perfectly round again I used a jack plug sleeve and filed with its thread. To get it perfectly smooth you can use little Acetone that you brush on it very quick with a small brush.
One of the Accent buttons had been worked on by a former owner, sadly some of the material was missing. Since the ABS plastic is sensitive to Acetone this is not a problem, you can always add to it e.g. with pieces of broken LEGO®. I have a box with small plastic scrap for this purpose so it's just picking some pieces that might fit and do some shaping.
With Acetone you can also close and stiffen cracks and cuts. Just leave a piece of a LEGO® brick in Acetone for a while and paste the fluid plastic in/over the cut. Let it dry properly up to a few hours depending on the grade of dissolution.
This way I attached a new clip that I made of several small pieces. After building the rough shape I used a file once more to get it functional. I have repaired dozens of keys and buttons on keyboards and pianos, the acetone method is rock solid.
As a result, after reassembling all buttons were functional again.
First thing I noticed after opening up was a solder tail CR2032 to the left. So I installed a holder for easy battery replacement.
When a device is power adapter driven I always check for a wrong polarity protection diode. There is one installed here, so no worries.
Another thing I recommend is reflowing joints of jacks and power regulators, the former for mechanical reasons, the latter because heating up and cooling down frequently over a long period of time stresses the joints so they might break.