humming from Sony STR AV-1020

[stingerssx]

Hi, new to the board here, but I like it so far. I have an older ('90's) Sony receiver and it has a hum. when you turn up the volume, it gets drowned out, but at lower volume it is unbearable. I've done a bunch of research with Google, and come up with either an open ground, or a bad capacitor or two. I opened it up and found one screw missing where the speaker outputs are mounted, and it was a ground. So I installed a screw. I believe that the humming decreased. I say this because I used different speakers to check it out. The first pair were 15" floor standing, and the second were smaller book shelf speakers. But the hum is still there.

So, if there are some bad caps, where would these be? Are they the two large ones on the main board, or could they be some others some where else?

Thank you in advance.

Oh, one more thing. I tried the old "bang on it" theory, and found that when banging on the right rear of the unit, the hum would either drastically reduce, or disapear entirely. But when I turned up the volume, the hum came back in full force, every time.

[HayZee518]

Almost all the time a 60 cycle hum is due to a bad ground. Banging on the unit sets up a vibration to anything that's loose. Filter capacitors are next. They are connected from the power supply [positive] to ground. Look for a metal can with a bulge somewhere in the can. This indicates a gassing condition. A short would blow a hole in it. They are a large value at a "Working Volts D.C."

[stingerssx]

When I pulled it apart, I looked for any buldging caps and couldn't find any. I checked the bottom of the board for any cold or bad solder joints, and I couldn't find any. I'll take it a part again to double check. Would pictures help any? I'll trace the two large caps and see where they are in the circut. I thought by banging on it it would rattle any loose connections, but found it odd that the hum comes back when playing with the volume only, and not banging or moving the machine itself.