Eml synthesizers

Howdy,

 What is the going price for a "fair condition" EML 500 these days? Are 
there any mods out there to make fhe filter self oscillate at high 
resonance settings? Is there a handy site to look up for maintenance/ 
tuning on this beast? Mainly calibration stuff. I bought it 20 years 
ago for $20 and it has been handy to have around, but I would like to 
go through it and restore all the factory adjustments and any mods that 
have come along to enhance it's veratility/performance and bring it to 
it's peak peformance. Thanx in advance.
Rig