Sparks Fly!

[skrol]

I have a standard 1/8" phono to RCA cable that I use to connect my mp3 player to my receiver. Last night when I moved the cable it fell behind the receiver and shorted to something. There were big blue sparks and a loud POPs that came through the speakers. I immediately turned the receiver off. There is a melted spot on the ground contact on the 1/8" phono plug but I don't see where it touched on the receiver. I also do not see any place where it could have touched 110V AC. No fuses blue and the receiver seems to work ok. More importantly, my speakers are OK.

I have seen where you can get AC on the chases if the AC plug is wired wrong. This can cause problems when connecting other components.

I may have just answered the question I was about to ask as I recently added a thrift store turntable and I am not convinced that it has the correct power cable.

I took measurements between the ground contact and various places on the receiver using a multi-meter but found no issue. For now I removed the receiver from the system to check it further. Any ideas on what else to check?

Thanks
Stan

[skrol]

The more I think about it the more I doubt that it is a polarity issue. The TT power connector on the unit itself is not polarized and I would have seen sparks earlier.

I am still at a loss though.

[skrol]

The more I think about it the more I doubt that it is a polarity issue. The TT power connector on the unit itself is not polarized and I would have seen sparks earlier.

I am still at a loss though.

[Speedskater]

a] Do you have one of those 3 LED, AC power receptacle tester's?
Did you use it?
b] Is the mp3 player AC powered?
c] Is everything plugged into the same AC power strip?
d] Did you measure the turntable?

[ViperZ]

OP, is the receiver plug polarized? I've seen some older receivers with non-polarized plug do just that. You plug it in "backwards", and the chassis, which is supposed to be 'grounded' becomes live. Seen it on an old Akai receiver.

[skrol]

Speedskater
Everything is plugged into a Monster power conditioner.
I used a outlet tester after the fact and it tested ok
The mp3 player was not connected.
I have measured the turntable and found nothing.

ViperZ
I believe that the receiver plug is polarized but I will re-verify when I get home.

With the burn and pit on the phono plug, it seems that I should see a corresponding burn mark somewhere. The only things that appeared to be exposed were the back shells of my speaker banana plugs and the back shells of my RCA plugs. They are all common as verified with an ohm meter. The turntable ground was connected to the ground lug on the back of the receiver.

Thanks for the suggestions. Keep 'em comin'

[ViperZ]

Skrol, what are the 'back shells' of banana plugs? Your speaker wires are just plain wires, they are not coaxial cables. So your banana plug will have a signal voltage there, several volts. If you accidentally touch exposed positive terminal of the speaker (or metal banana plug inserted into that terminal) with something grounded, I assume you will have a nice spark, since you'll short the amplifier output to ground. Theoretically, it may have enough power to melt a rubber connector.

[skrol]

It is Just the metal cover that screws onto the banana plug. Yes, the + and - are exposed metal. There was no signal going through the system at the time, volume was set to minimum. These sparks were big and bright like if you shorted 110V. I checked for DC on the speaker outputs and it is below 2mV as measured with my Fluke 187 DMM.

[skrol]

Ok, I found where it shorted. One of the + speaker banana plugs has the corresponding burn mark and pit. Therefore, the aux-in signal ground shorted to one of the + speaker output. I am still not sure why the sparks though but ViperZ may be right. I didn't take measurements as the receiver is not presently in the system but I will at some point. I will also start putting heat-shrink insulation on my banana plugs and other metal connector shells (covers).

[TNHNDYMAN]

glad you found the issue, sorry for the hassle

[F1nut]

Another solution would be to stop playing mp3's.

[Msabot1]

Reversed polarity would cause audible hum....at least you didn't blow any of yer gear!

[skrol]

It still bugs me that it was that big of a spark. Perhaps I have underestimated the strength of the Force. More careful in the future I must be. Here is a pic of the damage.

By shorting, could I have bypassed the system (pre-amp) and had the full power of the amplifier unleashed across the short? If so, I have underestimated the current drive capability of this receiver. The speaker would have been spared as the energy would have been dissipated across the short (ie in the form of molten metal).

[ViperZ]

Ok, I found where it shorted. One of the + speaker banana plugs has the corresponding burn mark and pit. Therefore, the aux-in signal ground shorted to one of the + speaker output. I am still not sure why the sparks though but ViperZ may be right. I didn't take measurements as the receiver is not presently in the system but I will at some point. I will also start putting heat-shrink insulation on my banana plugs and other metal connector shells (covers).

That banana plug 'cover' is not electrically isolated from the signal wire, your banana connector can be taken apart, that's all. I have never shorted anything that way yet, but I have seen some speaker cables with a heatshrink over the banana plug (except for the actual part, which is inserted into a terminal, of course). Now we know why the do it :)

[skrol]

That is tonight's project.

[skrol]

I never did figure this one out. This weekend, I fully checked the receiver and went through the alignments and all looks good. On the bench, I hooked everything up, took measurements then reluctantly repeated the short. No anomalous currents, voltages or sparks. I put the receiver back in the system and repeated the short but all looks and sounds good.

What happened, I may never know.
Stan

[DracoAmericanus]

does your receiver have a BTL out put? that puts voltage on both the + and - speaker outs, so maby that's what happened, floating ground maby too?
I had an ac plug on another unit do that when it sliped , nice spark and now no HDMI on my sat box and they where both plugged in on different circuits :(
best bet when dealing with such things is to cut the power

[DracoAmericanus]

luckily nothing was cooked in your case

[DracoAmericanus]

I blew up an avr while i was working on a tape deck, the mech slipped and it's chassis hit the primary side of the power transformer and sent a burst of 120ac though the unit and out the connected rca line to the ave and blew the living snot out of, i worked for sony at the time and brought the avr in and after the other tech making fun of me they "to my suprize" honored the warranty on the avr and gave me a new one as the tech gave up on trying to fix it. I worked at that service center and they never let me off on that one lol