Failures on most of these are due to capacitors going bad. Look for swollen and/or leaking cans. Burn spots may sometimes also be seen. This failing, there is a guy that specializes in repairing these. Mac Caps
http://www.maccaps.com/MacCaps/Repair_Service.html
I’ve used him and been satisfied with the results. He does quality replacements and is a careful repairman. But, like what he works on, not the speediest of shops, but worth the wait.