[Group Buy Mic] Ouch... Group Buy mic just SHOCKED me!

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Aj
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[Group Buy Mic] Ouch... Group Buy mic just SHOCKED me!

Post by Aj » Fri Feb 05, 2010 11:58 am

Well, I was afraid something like this would happen.

I've got a pair of those TNC ACM-310 pencil tube condenser mics, and last night - during a nylon string guitar overdub session, no less - one of them started buzzing suspiciously in the monitors. It wasn't a super loud buzz, but noticable - like someone left a guitar cable unplugged from an amplifier that was left turned on.

I tried swapping out a new power cable on the mic's power supply; plugging the xlr out into a different input channel; nothing fixed it.

When I went to unplug the mic cable (one of those custom tube powered cords) from the mic, I got an electrical shock when I touched the mic body with my hand. Not cool.

Any idea what could be wrong? (and what I can do to fix this?) Obviously, I don't have any recourse to the manufacturer. And I actually like these mics - they sound terrific on classical guitar.

Aj
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Nate Dort
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Post by Nate Dort » Fri Feb 05, 2010 1:13 pm

Sounds like the b+ voltage is shorted to the chassis, which I assume is tied to the shield on the cable.
If it were my mic, here's the order I'd start checking things:

cable, inside both connectors
wiring inside the mic
tube

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Nick Sevilla
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Post by Nick Sevilla » Fri Feb 05, 2010 6:20 pm

I have a pair of these.

I'll be opening them up to check inside for dodgy wiring, I don't want to get shocked either.

If you can post pics of the inside of your bad mic, I can compare to my set, and see if there are differences which might have caused that shock.

Cheers
Howling at the neighbors. Hoping they have more mic cables.

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Post by Aj » Fri Feb 05, 2010 6:26 pm

Will do.

I'm really wondering what could have suddenly caused this problem. They worked fine the day before, and they have not been moved or jostled. Also, I'm not very comfortable working inside power supplies - inside microphones, no problem - but in power supplies, I worry about voltage. I will be very careful.

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Post by rodabod » Sat Feb 06, 2010 2:32 am

Same happened to me with an Apex mic.

Shoddy construction means that the cable can sometimes short when not inserted properly. I has my hand on the cable and my other on the mic; shock across my chest. Not cool.

Also, some of the PSU cases on these mics are not properly grounded - they have earth tags inside the box, but are screwed onto a lacquered (insulated) part of the transformer and not the case.

You get what you pay for (although you'd hope not death!)

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Post by Aj » Sat Feb 06, 2010 5:59 am

rodabod wrote:Shoddy construction means that the cable can sometimes short when not inserted properly.
Well, I didn't unplug/replug the mic cable back in between sessions, and it worked last session. So I don't think it's that.
rodabod wrote:Also, some of the PSU cases on these mics are not properly grounded - they have earth tags inside the box, but are screwed onto a lacquered (insulated) part of the transformer and not the case.
Good clue. I'll look for that when I open up the PS.

Any tips on how to prevent electrocuting myself when I open up the PS this weekend? My kids would prefer that I'm alive to drive them to school on Monday (or on second thought...).

Aj
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Post by Marc Alan Goodman » Sat Feb 06, 2010 8:58 am

If it's not a tube power supply then just make sure it's not plugged in to the wall! Caps can hold a charge but prob not bad enough to nail you.

As a side note ALWAYS keep one hand behind your back when working with electrical equipment that may hold a charge. At least that way you get shocked in the hand. If you have both on there the shock could end up traveling through your heart. Which is bad.

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Post by Nate Dort » Sat Feb 06, 2010 9:10 am

Marc Alan Goodman wrote:If it's not a tube power supply then just make sure it's not plugged in to the wall! Caps can hold a charge but prob not bad enough to nail you.
But it is a tube power supply.

If the mic was connected when you shut off the power supply, then the tube may have drained off whatever voltage was left in the filter caps. If you unplugged the mic from the power supply first, then shut of the PS, there might be some voltage stored up still.

The only way to know for sure is to check it with a multimeter. I don't work on any tube stuff unless I've checked it for residual voltage first. Every time.

I've been shocked with high voltage. It was the B+ supply in a tube mic pre, and I swear it momentarily stopped my heart, or at least threw it off for a second. There was a small burned hole in my finger where the current flowed through my right hand, across my chest, and out my left hand. Took me about an hour to feel normal again. I don't want to repeat that.

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Post by Aj » Sat Feb 06, 2010 12:25 pm

Shit, now I'm a little worried. I'm still going to crack this thing open and take pictures. I'll post back what I find. But I'm just not comfortable around tube power stuff - it's usually where I draw the line. I guess today I'm moving the line...
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Post by Aj » Sat Feb 06, 2010 2:51 pm

Argh... it's intermittent! I just plugged the whole thing in again; same exact signal path - and it's not doing it.

What gives... the Gear Gods are not smiling on Aj today, it seems.
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Post by Nick Sevilla » Sat Feb 06, 2010 5:33 pm

Aj wrote:Argh... it's intermittent! I just plugged the whole thing in again; same exact signal path - and it's not doing it.

What gives... the Gear Gods are not smiling on Aj today, it seems.
Sounds like a bad wire connection somewhere along the line.

You can find it without powering the unit up. You'll need an ohm meter, and some patience.

Try the mic cable that goes between the power supply and the mic itself first, make sure there is NO crosstalk between any of the wires and ground. If there is, you need to replace that cable.

If the cable is good, you need to then do the following test :

Since you have TWO of the same mics, try operating the KNOWN GOOD mic with the BAD ONE'S power supply. Remember, do NOT make or break connections unless the power is OFF.

Try the following, in order :

1.- Turn on the KNOWN GOOD ACM310 mic, and test it to ensure it is working properly.

2.- Once you know this unit works, TURN IT OFF, leave it off for about 15 minutes, while the capacitors in the power supply drain.

3.- Connect the BAD microphone to the known god power supply / cable from step 1, and listen. Is the mic good? Is it humming? If so, you have found that the issue is in the mic body itself.

4.- If the "BAD" mic WORKS with the known good power supply and cable, then the mic body and it's guts are GOOD. So now you eliminate that from the "what is bad" equation.

5.- IF, and ONLY IF the mic body worked with the known good power supply and cable, THEN replace THE CABLE from the known set with the other cable, and listen. IF the problem appears, then you have a BAD CABLE. If NOT, then you now have eliminated the MIC and the CABLE from the "what's bad" equation.

6.- This would then leave you with the power supply.

To me I believe when you unplugged the mic while it was still connected to power, started the problem. Usually these type mics run about 100-105 volts to the tube, and if you just whack and unplug it, you can either get shocked or ruin the mic, or just the tube. Hopefuly you just blew out one half of the tube in the mic.

After doing this troubleshooting, you should know what of the three parts are bad, and then contact TnC to get a replacement part.

There IS tech support, believe it or not, from the TnC guys. Try over at the PSW forum, and send a PM to either Terry Manning or Chance Patacki, owners of TnC mics. They are very helpful, although they might take a little time to get back to you. I met Chance when I picked up my order of 4 mics (2 x 1200 and 2 x 310s) and mine have worked excellently since day one. I am sure they will make it right for you.

I usually do one day recordings, and if I have to leave a setup up for more than a day, I put 1 gallon plastic ziplock baggies to cover all the mics, so dust does not get in them, and turn them off for the night. I highly recommend you do that to the mics you leave out for extended periods of time.

Cheers
Howling at the neighbors. Hoping they have more mic cables.

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Post by Aj » Sat Feb 06, 2010 5:51 pm

Ok, that post was invaluable. Thanks very kindly. Incidentally, after leaving it all on for 45 minutes, the problem has returned. So it's definitely a ghost in the machine.

I will try all these suggestions tomorrow. Hopefully, this post will be helpful for others... btw, I don't recall ever plugging/unplugging the mic into its live power supply before the problem happened (guilty as charged, however, for doing it once while trying to troubleshoot this.)

Aj
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Post by Nick Sevilla » Sat Feb 06, 2010 5:56 pm

Aj wrote:Ok, that post was invaluable. Thanks very kindly. Incidentally, after leaving it all on for 45 minutes, the problem has returned. So it's definitely a ghost in the machine.

I will try all these suggestions tomorrow. Hopefully, this post will be helpful for others... btw, I don't recall ever plugging/unplugging the mic into its live power supply before the problem happened (guilty as charged, however, for doing it once while trying to troubleshoot this.)

Aj
Let us know how it goes tomorrow, with the testing and all.

Cheers
Howling at the neighbors. Hoping they have more mic cables.

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Post by Marc Alan Goodman » Sun Feb 07, 2010 8:21 am

nate wrote: I've been shocked with high voltage. It was the B+ supply in a tube mic pre, and I swear it momentarily stopped my heart, or at least threw it off for a second. There was a small burned hole in my finger where the current flowed through my right hand, across my chest, and out my left hand. Took me about an hour to feel normal again. I don't want to repeat that.
Me too. Don't. Do. That. At least i"ve been lucky enough to take it through the hand and not the chest.

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