This 500-C is the first Fisher I actually have tried to fix for someone else. He heard I have restored many Fisher's and coaxed me do the repair by offering to make a Bimini top for my boat. First thing I noticed was a J&J 7591 has gone nuclear, ever seen a J&J with copper side getters? This tubes shows a short on my tester, and looks like it had a melt down. The two J&J tubes were paired with two new Mullard's that normally run very hot, and there was no bias adjustment added to adjust things down. Needless to say, whoever restored this unit didn't do it much justice. They replaced the 1.2K 7 watt (R-143) with a much lower wattage resistor, this resistor burnt out and shows no resistance now, probably saved the OPT's. Hope to get the new resistor soon and get this unit back up and running, owner has agreed to have me install sampling, and stability resistors along with bias adjustment. This JJ tube can be a nice souvenir!