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Strangest problem I've ever had

Ok.
Bought a house.
Paid guy to run wires through the wall for a 3 point system.(too many obstacles & $$ for surround)

Have walls patched and painted. Moved in.
unpacked My Pride and joy B&K 7 channel.

system includes Onkyo sr705, HDMI from cable box to onkyo, connected polk rti12 speakers to B&K with bananas.

turned it all on, incredibly loud buzz snap crackle pop, onkyo shuts off and burnt wire smell fills the air.

Trouble shoot. all wires were hooked up right.
connect speaker wires directly to onkyo one at a time. the farthest one away in the wall shuts off onkyo. I run a new wire on the floor to that speaker. It works.

So it's the wire in the wall, it got compromised some how when the patched wall..?
not so fast.
I have an old kenwood preamp and adcom 2 channel. I hook up "bad wires" in wall directly to kenwood and the speaker works fine. how can bad wire short one amp and work on another???

So then I hook up an RCA pre out channel from the onkyo to the adcom and the same very loud buzz comes on the good speakers.

So the Pre outs on the Onkyo are definitely sending the buzz to both amps... can we assume the Onkyo pre outs are shot.
Million dollar question. Why does the line in the wall short out the onkyo but work on my old kenwood.

So then I test a good speaker line on each output on the onkyo. They all work... I'm doing this with it on, even though I know better... Then I went to connect the bad wires to the onkyo again and I got a spark when I connected the red wire...I've never seen a speaker wire spark off a preamp????!!!!
i've never seen that. I'm going to get a meter and read the line resistance and see if there is a difference with the bad wire.
My concern... If I go get a new preamp, can the bad speaker wire blow it again.
Makes no sense.
That wire was hooked up through the B&K. B&K was connected via RCAs to Onkyo. How could it short the onkyo going back through the B&K.
do we think the onkyo was toasted in the move. If so, how is it that just that 1 line shorts the onkyo and none of the others... too strange
riasillo

Showing 4 responses by almarg

almarg's avatar
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almarg
9,670 posts
 
Metro gets all the credit. All I did was to describe how to use the meter.

Glad the mystery is solved!

Regards,
-- Al
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almarg
9,670 posts
 
Yes, that is a head-scratcher indeed. Some questions and comments:

1)In my experience as an electrical engineer, often when inconsistent and puzzling symptoms are present it turns out either that the problem is intermittent, or there are two different problems that are present at the same time. Not sure what that may specifically suggest in this case, but those are possibilities that should be kept in mind.

2)What kind of speaker cables are you using? Are they by any chance cables like Goertz or Polk Cobra, that have extremely high capacitance which can cause some amplifiers to oscillate?

3)Have you determined where the burning smell was coming from? Was it coming from the speaker wiring, the speaker, the Onkyo, or elsewhere?

Regards,
-- Al
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almarg
9,670 posts
 
To check for a short in the wiring you have to disconnect both the speakers and the amplifier, so that the wires are connected to nothing at either end.

Then set the meter to R x 1, and touch the two leads of the meter to each other. There should be a knob on the meter, perhaps on the side, that is called "ohms adjust" or something like that. While the two leads of the meter are touching each other, adjust the knob so that the meter reads 0 ohms.

Then touch one lead of the meter to either the + or the - conductor of one of the speaker wire pairs, and touch the other lead of the meter to the other conductor in that pair. It doesn't matter which end of the cable you do that on. The meter pointer should not move at all, and should indicate approximately infinity ohms, or whatever it indicates when the meter is not connected to anything. (There is probably a separate screw-type adjustment on the meter to set the reading to infinity when it is not connected to anything).

Making a meaningful reading while the speakers are connected can be tricky, and is probably impossible using the R x 1K or R x 10 scales, because on those scales you won't be able to distinguish between the speaker's resistance and a short. Using one of the higher scales, such as R x 1K, also creates the possibility that you are reading your body's resistance, if your fingers are in contact with the metal part of the test leads or the wires when you make the measurement.

Regards,
-- Al
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almarg
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Damage a separate connected preamp? Maybe Al could expand on that.....
Hi Jim,

Assuming that both components have 3-prong power plugs, as you realize the short would result in a direct connection from the "hot" output terminal of the amp to the chassis of both components. What happens from there is obviously hard to predict without knowing the details of the grounding configurations of the components, and other aspects of their design. But depending on the impedances between the chassis of each component and their circuit grounds, some fraction of the current flowing from the amp’s “hot” output terminal will find its way back to the amp’s circuit ground via the Onkyo receiver (being used as a preamp) and the return conductors of the cables connecting the two components. One conceivable scenario would therefore seem to be that a feedback loop is created from the amplifier's output to some upstream circuit point or points, via the Onkyo's grounds, resulting in a large amplitude oscillation, resulting in damage to circuitry in the Onkyo that is designed to handle low level signals.

Or something like that :-)

Best regards,
-- Al
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