That tends to happen to string machines , keys go dead , or no sound at all . String machines are not really synthesizers , they are organs . They use the same type of super sonic oscillator with divide down circuitry that an organ uses . The best string machines are from CRUMAR and ROLAND from my experience.

The KORG DELTA is like an ARP ONMI but is more versatile , that is it's a string machine with a more versatile synthesizer section than the ARP OMNI . Plus it's WAY more reliable .
You already know about the ROLAND JUNO's .
The KORG POLY -6 is similar but it has a different sound .
The SIGMA is monophonic but is really capable of unique sounds .
The SIEL DK-600 is a dual oscillator 6 voice synthesizer ( ROLAND JUNO'S have only one ) , it has a really nice sound and it has MIDI . It's an Italian made synthesizer , make sure it works perfectly before you get one . If it has any problems , DO NOT get it , no one will fix it .
Also I forgot to mention the ROLAND SH-101 , SH-09 , and SH-2 .