I would contact Paul Grzybek of Tube Audio Design. He's stopped making new products under the TAD brand but still repairs/restores vintage Fisher gear. I think he mostly works mostly with tuners, pre's, amps, and receivers, but if anyone would know about your unit, he would. Here's his website:
http://www.angelfire.com/biz/bizzyb/index.html
http://www.angelfire.com/biz/bizzyb/index.html