Introduction and Amp Problem

[jeffo]

Let me first introduce myself. I've just graduated from The University of Western Ontario with a BESc in Electrical Engineering. I enjoy poking around and fixing electronics. I hope to learn more from reading these forums and offer anything I can.

Anyway, I'm having the strangest problem with an amplifier (Pioneer SX-312R). The following equipment is connected to the amplifier:

1) Scientific Atlantic Digital Cable Box Explorer 3200
2) Pioneer CD Player PD-M426
3) Koss DVD Player (don't know the model number right now)

A few days ago I was watching TV and had the amp on. All of a sudden the speaker output was a loud low sine wave (maybe 200 Hz). Also the picture on the TV went crazy; Then the fuse in the amp blew. I opened the amp and didn't notice anything burnt or discoloured. All the non-on functions still worked such as the SR (System Remote) and you could here the relay click on. I changed the fuse and carefully plugged it back in with only the DVD player connected. The tuner worked fine so I tried playing a CD in the DVD player. The amp worked fine at high volume for a few hours. I thought that it was fixed, but now that I've hooked it back up weird things are happening again.

The tuner worked fine. Then I changed to the digital box input and turned the volume up and the picture started to shake. Thinking that maybe it has to do with that particular input (although during testing I checked every input), I played a CD in the DVD player. I turned it up a little louder and noticed that the display would dim to the music. Then pop- the fuse blows (could see the blue flash from where the fuse is). Again the SR functions work and the amp is not hot.

Now here's the really weird part. A buzzing (some harmonic of 60 Hz) comes from the stereo speakers if the 'A' speaker button is pushed in (ie if the 'A' speaker button is out, no buzzing) regardless of the 'B' speaker button. This happens even when the amp is not plugged in. Thinking that maybe the other equipment was interfering, I unplugged the power bar-still buzzing in the speakers.

The only thing really different between the hours of working is the speakers used. I used a cheap pair of speakers to test as I didn't want to ruin my good ones. But they're both 8 ohms.

I have an interesting setup right now but has been working for 6 months. Connected to my digital box's audio outputs are two piggyback cables (one to the TV and one to the receiver). The TV is connected via S-video. There is an LCD TV in another room connected to the digital box via composite and piggybacked onto the audio.

The only thing that jumps into my mind is a ground loop. But I don't really understand what a ground loop is-just have heard of them. Any help or insight? I just can't get a hold of what is going on.

The amp is 11 years old but sounds better than my much newer Pioneer 5.1 receiver. But since it is older I don't want to put that much money into it (it's only 2 channel - not even Pro Logic). If you want some pictures of the amp, let me know.

Thanks.

[jeffo]

I've fixed it; Was my fault. I was laying in bed last night thinking of what was done different lately. There are speaker wires running from the family room (attached to wall plates) to both the deck and the dining room. Only one set of wires are connected at a time (deck in summer, dining room in winter). Currently the deck speakers are connected and the dining room's wires are stuffed into the floor register.

I had disconnected the wires from the wall to clean under the wires (you know how wires seem to trap dust) and hooked them back up to the wrong plate (the plates are unlabeled).

The other end of the speaker wire was touching the metal of the duct in the dining room.

I'm a little puzzeled by this though. Could someone explain what was happening? My guess the speaker output was touching the vent and creating a ground loop. But the strange thing is that the "b" speaker button wasn't turned on. And this amp is wired such that if A+B are on, then the speakers are in series so I wasn't too worried if it was the wrong wires.

Do I feel stupid now with that being the problem - though I'm glad I bought a 5pk of fuses Though I would like to know a little more about what was going on. Thanks.

[vtech]

Hard to exactly pinpoint, but from your description it definitley sounds like the Amp was seeing some sort of a ground/short thru the metal duct. Especially if the Amp has "floating" outputs---In general Air duct/vent suppose to sit at ground potential thru the building. Lucky you did not end up blowing the output txistors----(sign of a well-designed protection circuitry)

"Ground loops" are usually meant the shortest path to ground in repair world.

[jeffo]

I agree that the amp was seeing ground through the ductwork. But I cannot figure out what exactly was happening. I am lucky that I didn't burn out the output transistors because it was the main fuse the blew. But can't understand why the amp would start into oscillations.

I guess it really doesn't matter because everything is working now.