WHAT TO DO WITH THE OLD SYNTHS?What Service means
The synthesizers of the 1970s were "discrete", that is, individual components were used instead of so-called assemblies or ICs. This means they need comprehensive service from time to time.
Capacitors should be replaced as a rule after a few years, since they no longer function properly - they do not reach their values from a certain age. Power supplies also need a refresher, otherwise they produce background noise.
Often, the keyboard bushings are downplayed and drippled - the keyboard becomes rickety and often odd. In addition, there is a lot of dirt, which collects in the instrument and should be removed for a flawless use. Oxidation together with dirt is also noticeable by unstable pitches of individual keys (interrupted contacts).
The sliders have to be "re-slid" again - for this they must be completely disassembled. For all of this you need special tools, materials and a lot of experience. That is why I recommend leaving this work to the few, skilled professionals. Often these specialists are optimized to one manufacturer - this is not bad, since different synth-companies worked in very different ways. Only the specialized technicians of old synths will have the special material or parts for their favourized synths.