Vintage Synth Repair

Well hello, everyone! I have recently purchased a Arp Omni 1, but from
the first moment on I kept having trouble with one note stuck at E in
the first octave. Let me explain the symtomes: when switched to
bass(synth) I'm able to play the notes from C1 to E1 then every other
key I press after that makes the same sound. When switched to full
strings/brass no matter which or how many keys I press or how I set
the attack/sustain envelopes I always keep hearing E1 playing along
with a long sustain. Did anybody else encounter this problem and how
can I get that fixed? Help, please??