Fried computer - why??

EWCguy

EWCguy

Active Member
This situation has got us boggled. For many years, we've connected a laptop 3.5mm headphone jack via Whirlwind pcDI through the built-in mic jack/cables to our analog audio console to provide audio from the laptop to the house. The connection is always the same and never uses phantom power. This week, that setup fried the laptop --- as in, it immediately lost power and never came back up. Upon examination, a visible spark was noticed (far more than static discharge) while removing the 3.5mm plug from the laptop headphone jack.

Prior to the moment of equipment failure, I had clean sound from the laptop in the house. After failure, nothing but 60Hz hum. What can I associate with the moment of failure?? I powered the auditorium seating aisle lights.

Other notable facts:
  • The laptop was connected to its power supply and connected to an outlet in the auditorium seating area. The PS remains functional.
  • The laptop SVGA output was connected to a projector via a 50' cable. The projector was not damaged.
  • The adjacent console channel had phanom power engaged. The condenser mic on that channel continued to function.
  • The two stage-mounted mic jacks in use are in the same wall plate.
  • The mic cable and the aux cable used in this connection have been checked and are without fault.
Here's a bit of block diagram showing my connections:
LaptopSound

LaptopSound

Block diagram of laptop connection for audio output to house system.
 
Sounds like you somehow got 48v into the headphone jack. I fried the headphone jack on my macbook a couple years back by connecting it directly to an XLR input channel on the board ... lesson learned -- always use a USB audio interface box to take the punishment.
 
Certainly one of the more interesting computer "deaths" I've seen.

Nothing really seems out of the norm in your description, but clearly something wasn't right (or else we wouldn't be discussing this). There is a chance that there was a static charge on the cable, which could've easily fried something small such as a transistor on the laptop. With the circuit in the laptop in the failed state, it very well could have caused the symptoms you're experiencing.

On a troubleshooting note, do you still get the hum when the laptop is isolated from AC power (running on battery)?
 
Yeah sounds like power went through the jack somehow.

Pull the battery and press n hold the power button for 10 seconds. Then press it a few more times after to make sure everything is discharged. Then plug it in and see if she spins up.

Best case scenario is it did what it’s spose to do when foreign power is introduced and just stopped.
 
The hum is a sign that AC current is flowing where it shouldn't. Since the laptop died, we can assume that the current went through the laptop. In this setup, there are two possible sources of that current, the projector and the laptop supply.

A component failure in the laptop supply could cause a voltage leak to the chassis ground of the laptop. The audio jack is also tied to chassis ground, which made the audio console the path to AC ground for the leakage current. The laptop supply could fail in terms of AC isolation while still working in terms of DC output.

A component failure in the projector could cause a voltage leak to the VGA cable. The laptop VGA connector is internally tied to chassis ground inside the laptop. Again, the audio connection made the audio console the path to AC ground for the leakage current. A ground loop between the projector and the console could also be the culprit, but less likely since it hasn't happened in the past.

In any of these scenarios, the ground path through the XLR connector on the console is usually pretty robust and hard to damage. The ground paths through the laptop are through tiny, little circuit board traces that love to act as fuses, hence the catastrophic failure.

I would take a voltmeter and measure the AC voltage from both sides of the DC plug on the laptop supply, referenced to pin 1 of the XLR cable to the console. I would also measure the voltage from the shell of the SVGA cable referenced to the XLR pin 1. More than a fraction of a volt would be interesting.

(edit #1) I can't eliminate the possibility that the DI failed and passed phantom power to the laptop. However, that would have likely just fried the audio output, not the whole laptop, because the current potential is limited by 6800 ohms. It also wouldn't cause hum. You can check the DI input for the presence of DC voltage, too.

(edit #2) The association with turning on the aisle lights is intriguing. That makes me wonder if the aisle lights are changing the ground potential at the plug the laptop was powered from. Measure the AC voltage from the plug ground to the XLR pin 1.

My bet would be on the laptop supply causing the mayhem.
 
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I'm not familiar with the PCDI but if uses a transformer, I would be asking whether it's possible for either XLR to pass any harmful voltage back to the computer.

Mac or PC laptop? If PC (especially if battery is accessible) try this, remove the 1/8" trs from audio output. Then remove laptop battery. Then power the laptop from the AC adapter.

If it works, you need either a new laptop battery or can attempt to power the laptop without the battery installed. I've had a similar issue with a laptop I use. When the battery is very low and installed the unit does not power up.
 
Have to agree with @FMEng that the problem may well be in the laptop AC adapter. I would have it Hi-pot tested to see if there is leakage. There are a couple of ways ac power can end up on the DC output. All these devices contain a class C capacitor to bypass RF noise. These caps by their nature leak a bit of power, but usually not enough to cause problems unless the part is defective. A far more likely suspect is the transformer used in the switch-mode power supply in the adapter. They can break down and pass substantial AC power right from the line to the frame of your laptop. Do not reuse that power supply/adapter without having it checked.

ONE LAST NOTE: (Not related to this problem.) When feeding the headphone output of a laptop to a DI box, make sure you are not ending up with the ring contact tied to ground. This can occur if you use a mono cable or if the DI has the ring contact tied to the sleeve. (common) Ran into this a couple of years back, the audio output driver in the laptop popped about 45 minutes into a show. Most of these driver chips do have current limiting, but driving one channel into a dead short long enough can have bad results. Best to use a mix cable that has a resistor in series with the T and R contacts and mixes them to mono. (Stereo DI as pictured in the OP is the best way to go.)
 
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You folks are great! Yesterday, I had some time for testing before I read this post. All I did was determine confirm functionality of the mic cable system. Now, with your suggestions, I will check the laptop power chain against pin 1 of the mic cable working on the assumption that the audio system became the path to ground.
 
Update.

Testing the audio system with the DI fully connected, we were able to determine that one of the two circuit breakers that control the aisle lights is at fault. I've reported this to our physical plant.

I only wish the fault had fried my analog audio console instead of the compuers so we had justification to upgrade... :doh:

Edit: Testing the laptop power chain against pin 1 of the mic cable yielded no information.
 
EWCguy said:
Update.

Testing the audio system with the DI fully connected, we were able to determine that one of the two circuit breakers that control the aisle lights is at fault. I've reported this to our physical plant.

I only wish the fault had fried my analog audio console instead of the compuers so we had justification to upgrade... :doh:

Edit: Testing the laptop power chain against pin 1 of the mic cable yielded no information.
Click to expand...
@EWCguy Aaron; When you typed: "We were able to determine that one of the two circuit breakers that control the aisle lights is at fault." Please elaborate how you we're able to reach this determination?
For a dancer, we MAY make a reasonable technician out of you yet. Can you count past 8?
Toodleoo!
Ron Hebbard.
 
EWCguy said:
I only wish the fault had fried my analog audio console instead of the compuers so we had justification to upgrade... :doh:
Click to expand...

Well, in our case we "upgraded" from a Yorkville to a Behringer mixer! From bulletproof to cheesy! Although our power amp and speakers are still Yorkville.

If you never heard of Yorkville, assume you've at least heard of Traynor guitar amps? Same company.
 
@RonHebbard - believe me, dancing is the least of my theatrical skills (as my current show involvement displays).

To be more accurate, we determined that one of the two *circuits* that controls the aisle lights has a problem. We're starting with the circuit breaker.

With an assistant, we connected the DI, aux cable, and mic cable system to several of our mic inputs in turn. One of the tests we did was to have him touch a voltmeter probe to the tip or ring or sleve of the unconnected end of the aux cable while I had the mic channel audible through the system. In all cases, a 60Hz hum was introduced. We're figuring this was an "antennae" effect with the AC in the room. With the gain at a typical microphone level (indeed, we tested a dynamic mic earlier) and the fader at -20, mains at 0, this hum didn't even produce a visible signal on the LED meters, though we could hear it. Then we turned on the aisle lights. The hum immediately increased in loudness. This was achieved on each of the four mic inputs we tested. For me, that eliminated any specificity to the channel that was connected when the computer fried. And pointed a very direct (dare I say "middle") finger at the aisle lights, which have always been a factor in the incident.

So, I looked into the nearby electrical panel to see what was there to discover. That's when I noticed two circuit breakers labeled for aisle light control, neither looked tripped. I shut off the house lights and viewed that all aisle lights turned on when the switch was on. Then, I turned off one of the breakers - my assistant confirmed all of house right aisle lights turned off. I turned that breaker back on, the other off, and we observed the house left aisle lights now off. Now, repeating the antennae test, we got increased hum with one circuit but not the other. Ergo, one circuit has a problem.

I'm hoping that no other problem is discovered in the electrical panel. There was a remodel on our lobby over the past summer and the contractor subbed some power off that panel -- maybe they re-connected something poorly or knocked a wire loose. Luckily, these things are outside my concern. Work order submitted.
 
EWCguy said:
@RonHebbard - believe me, dancing is the least of my theatrical skills (as my current show involvement displays).

To be more accurate, we determined that one of the two *circuits* that controls the aisle lights has a problem. We're starting with the circuit breaker.

With an assistant, we connected the DI, aux cable, and mic cable system to several of our mic inputs in turn. One of the tests we did was to have him touch a voltmeter probe to the tip or ring or sleve of the unconnected end of the aux cable while I had the mic channel audible through the system. In all cases, a 60Hz hum was introduced. We're figuring this was an "antennae" effect with the AC in the room. With the gain at a typical microphone level (indeed, we tested a dynamic mic earlier) and the fader at -20, mains at 0, this hum didn't even produce a visible signal on the LED meters, though we could hear it. Then we turned on the aisle lights. The hum immediately increased in loudness. This was achieved on each of the four mic inputs we tested. For me, that eliminated any specificity to the channel that was connected when the computer fried. And pointed a very direct (dare I say "middle") finger at the aisle lights, which have always been a factor in the incident.

So, I looked into the nearby electrical panel to see what was there to discover. That's when I noticed two circuit breakers labeled for aisle light control, neither looked tripped. I shut off the house lights and viewed that all aisle lights turned on when the switch was on. Then, I turned off one of the breakers - my assistant confirmed all of house right aisle lights turned off. I turned that breaker back on, the other off, and we observed the house left aisle lights now off. Now, repeating the antennae test, we got increased hum with one circuit but not the other. Ergo, one circuit has a problem.

I'm hoping that no other problem is discovered in the electrical panel. There was a remodel on our lobby over the past summer and the contractor subbed some power off that panel -- maybe they re-connected something poorly or knocked a wire loose. Luckily, these things are outside my concern. Work order submitted.
Click to expand...
@EWCguy Aaron; What you MAY find is that since your aisle lights are spread over a comparatively large area with a comparatively larger amount of wiring, they radiate a larger, more distributed, EMI noise field for your sound system to pick up and amplify as compared to other, even higher rated, breakers whose loads are more concentrated such as one or two ellipsoidals in an FOH ceiling cove. In this sense, there MAY not be anything actually incorrect about how your aisle lights are wired. @FMEng would you mind commenting?
Toodleoo!
Ron Hebbard.
 
Induction is not enough to do the damage described. There is a failure at play here and unless weeded out, it will happen again. Each device needs to have it's ground potential measured. Visible sparks are not created my millivolts.
 
JD said:
Induction is not enough to do the damage described. There is a failure at play here and unless weeded out, it will happen again. Each device needs to have it's ground potential measured. Visible sparks are not created my millivolts.
Click to expand...
@JD @EWCguy Understood and agreed. When Aaron described his test method and explained he was basing his comments on his mic inputs picking up more induced electro-magnetic interference from his aisle lights than other circuits, I was presenting a possible reason his aisle light wiring and breakers could have this effect.
Toodleoo!
Ron Hebbard.
 
RonHebbard said:
@JD @EWCguy Understood and agreed. When Aaron described his test method and explained he was basing his comments on his mic inputs picking up more induced electro-magnetic interference from his aisle lights than other circuits, I was presenting a possible reason his aisle light wiring and breakers could have this effect.
Toodleoo!
Ron Hebbard.
Click to expand...

He mentioned some renovation work earlier so who did what to which items can be a open discussion.

Based on this troubleshooting I'd wager that there is a hot/neutral reverse somewhere, and a ground/neutral reverse somewhere else. They probably test Okay when separated but when they came together at his laptop's headphone jack it became 120-208/240 V...
 
I don't think that picking up radiated noise proves anything. If there's a wiring fault, it would be something like a neutral wire shorting to a conduit, causing a big ground loop. If that's happening, there should be a measurable voltage potential between any two of the three AC grounds involved. Make the measurements with the aisle lights on. If you can't measure it, then the electrician is going on a wild goose chase.
 
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