The Jen SX1000 is one of the most affordable and straightforward analog mono synths ever made - with seperate PCBs for each part of the architecture making it a great starting point for learning how to maintain and service synths. In this video Mitch at THIS MUSEUM IS (NOT) OBSOLETE talks us through a manual-less service, using his knowledge and know-how to diagnose problems and get the synth back into ship-shape condition. Fantastic walkthrough!
The Jen is also open to some very simple modifications (including filter input, added sub-oscillator with 4-octave range and IEC power) all detailed extensively on this site: https://www.njohnson.co.uk/index.php
And in the spirit of homemade solutions, here's a very old video I made, using a guitar string to fix broken key contacts:
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