Sounds like a failing transistor, most likely in the power amp section. Get a schematic and replace the transistors in the power amp section up to and including the outputs, just shotgun 'em. Whatever you do, don't toss it to the curb, head over to audiokarma.org to off it...also, there are some very good techs there who can restore this beauty. I have a 450 (the baby of the line, 15wpc) that I got as a kid in '78. I still use it, it drives any loudspeaker I throw at it to surprising levels. I love these older receivers, and in good shape are every bit as clean as a new hi-fi unit and much more solid than any stereo receiver you'll buy at a mass market store. I've got a '72 SX-828 on my bench that I'm going to recap and replace all the small-signal transistors in, and a '75 SX-838 that I'm going to do the same to. And there's nothing like the classic glow of the analog dials at night, especially the old blue-dial machines.
Todd P, vintage scrouger and cheapskate
