Vintage Synth Repair

Hi, im new to this group but have been collecting keys for some years 
and found this group looking for info on an  White ARP Odyssey 2800 I 
have just acquired. The synth has sat under a bed for at least 10 
years and is quite clean and in good condition. However on powering 
it up there was no output. Switch lights up and when I opened it up 
and checked power supply board I am getting +15v and -15v. All I get 
out of the output jacks is a hum. All wiring looks to be intact. Any 
ideas where to look now greatly appreciated. Regards, Cy.