Adcom GDA-700 not working

A couple weeks ago my Adcom GDA-700 stopped playing. The unit powers up and emits a "thump" from the speakers when I change the input but will not produce any input sound. I am wondering if anyone has had experience working with these units particular or if this is a problem which is common to DACs. Any help would be much appreciated.
The cost of just buying another one used is about the same as the cost of fixing it.
I own a GDA 700 (and a 600 too)
I own two GDA 700s and a 600 ..if you want another 700 let me know...probably be cheaper than fixing it...I only use one and have the other for back up in case it goes bad..Actually I think there are very few moving parts and should be easy to fix IF you can find somebody willing to work on it...give me a reply if you want mine