New and Shocking!

frank kayser

Junior
Jan 11, 2011
290
1
18
Maryland suburbs of DC
Good morning, again.



Had a guitarist in the other night who notified me he was getting shocked intermittently when he was at the mic - sang with lips touching the SM58 most of the time. I'm not going to say mildly shocked because that can be so very disconcerting to the performer. Any shock is too much.



This is my local haunt, and I've never had problems with shocking or noise from the AC ever before. Once notified, my actions started with checking both ends of the mixer power cord to be sure it is seated well - no problems there.



In the next song break, I swapped out the mic cable - did not really think anything would change, and I was right. I also had the guitarist flip the (I guess the polarity?) switch on his Fender amp. Nada.



I ran home and grabbed a SM57 w/foam wind screen which ''solved'' the problem through isolation by song 3.



I looked at his amp - looked to be a vintage blonde Fender - when I asked, he said it was a re-issue with the three-wire cord - I'm suspecting the ''polarity'' switch is there to make the amp ''authentic'' but doesn't really do anything.



During the break I also moved his amp plug into the same quad box the mixer, amp, and EQ were running off to equalize any possible ground potential problem that may have been between the sound system and the guitar amp. No, I did not ask him to check further. That would have been cruel.



So the show is over, and I need to check things. Of course, his rig is gone, but I can check if it is the house rig. He's not had trouble elsewhere, and if he does, he'll get his stuff checked. That aside, I did notice a tiny shock or two between the mixer face and my fingers on a different day - the first time I thought may have been static, but I'm not so sure...



Now, we have about four outlets serving the stage - I do not know whether they are on the same circuit or not.



I'm guessing I would be looking for any stray voltage on ground. I'd meter and should get:

h-n = ~120

h-g = ~120

g-n = zero



Also if I'm checking multiple outlets, any n-n, g-n, or g-g should be zero, as well. h-h would read 0 or ~240, depending.



I would check all mic cables to ensure pin-1-chassis ground is NOT connected (I did find some older cables connected this way, and took them out of service until I can rewire them).



Check any and all instrument cables...

Check power cords to mixer, amp, and EQ...



I'm guessing a decent multimeter should do the job.



Now to the questionable stuff...

There are a number of ''cheap'' power strips on stage, plus one rack-mount ''power conditioner'' serving the entire PA.



I'm guessing that a leaking, failing MOV in any of those devices could be the problem. I'm guessing I should test them for continuity/isolation between the h-n-g. Would it be right for me to expect infinite resistance, or would a couple mega-ohms be more consistent with a MOV...



I'm also guessing that those cheapies are not really appropriate for the stage, and I should get rid of them, or at least cut the MOVs out of them. I have read previous posts about the limited life of MOVs and the potential for catastrophic failure.



Is there anything I should check on the electronics? They are all 1.5 yrs old... Amp and EQ have had 10 insertions on their connectors, max, so I doubt there would be any problem with the input/output connectors. Mixer shows no physical signs of wear or abuse.



The only thing left out are the mics - I don't have a clue as to how to test them, or what to look for - or if I should check them at all.



Would a ground-fault interrupter trip if voltage is found on ground? Would that be a reasonable ''Canary in the coal mine'' test for a problem?



So my question: Am I on the right troubleshooting track? Am I missing anything?



thanks



frank





 
Re: New and Shocking!

MOVs are a likely culprit. In addition to your good tests, I would meter from the grille of your SM58 to the ground of the outlet the amp was plugged into.



Old amps can have a couple issues - polarity being one, ''death caps'' being another. I played a vintage Hammond a while back and demanded a wireless mic to sing through once I discovered the chassis of the Hammond was at 60 volts to the mic. That's definitely enough to feel. Maybe Bob L or someone that knows more about guitar amps can elaborate.



By the way - you're probably not going to get a lot of readings that are a 0 volts. You get 0 volts inside the main service entrance between ground and neutral, because they are one and the same at this point. The farther you travel from that point, the more voltage difference there will be. As current on a circuit increases, the voltage between neutral and ground will rise, because the neutral return path has a non-zero resistance, and therefore there is a voltage drop.



A couple of volts between ground and neutral on a moderately to heavily loaded circuit isn't likely an indication of a problem. If it's more than a few volts, it's wise to try to shut power off and re-torque things.



Neutral to ground voltages are a separate issue to the shocking phenomena - or at least it better be. Absolutely NOTHING connected to either the neutral or hot wires should be exposed out of a device.
 
Re: New and Shocking!

Ground fault interrupters trip when there is an imbalance between hot and neutral.



was the ground prong intact on the guitar amp?



Could someone have modified the amp by cutting the ground internally?





 
Re: New and Shocking!

Thanks, TJ for the clarifications - so some of those measures can be non-zero, but I'm guessing in the single digits? The SM58 grille to ground sounds like a winner, too. I'm assuming that should be true zero.



The amp is new - maybe I missed the decimal point - one 1/2 years old. Shouldn't be a polarity or death-cap issue (I hope!)





Tim - I checked the amp plug when I moved the plug - he also had some typr of special musicians AC powerstrip - must have been quite expensive - extruded aluminum, quite heavy for its size. Cord in snakeskin. Not to say it is an better or worse than anything else - the ground connector on the fancy power strip and on the amp were intact. Whether the amp's chassis ground had been tampered with - knowing ''the kid'', his parents, and the folks he works with, and the fact that nothing he has is hotrodded, I'm going to guess everything is copacetic.



frank
 
Re: New and Shocking!

MOVs are a likely culprit.



Definitely worth investigating. I recently ran into a situation where a 'helper' had daisy chained power for 2 of my processing racks together, both having Furman PL series ''power conditioners'' in them, so one rack's PL was plugged into the others, then that one into a quad box which was home run to the distro. I had my laptop with it's grounded PSU plugged into the daisy chained rack, but the lappy was sitting on my drive rack (3rd rack) which was plugged directly into the quad. I kept getting a tingle when i was typing on the laptop if my wrists rested on the drive rack's metal edge. Broke out the DMM and found some voltage present between the racks. After some investigation, I discovered the daisy chained racks and plugged the cascaded one directly into the quad. Problem gone. But that would indicate to me that the MOVs in one of more of the 10+ year old Furmans are leaking. Replacements will be by SurgeX.



As the years have gone by, I have become pretty disenchanted with Furman. I've seen 1st hand several of their AR series products burn up (literally) without adverse conditions present, bad experience with their ACD-100 'distro' and their customer service. This was just another confirmation that they make an inferior product.



Greg
 
Re: New and Shocking!

Are you sure he isn't just getting a shock from static like when you rub your shoes on carpet and touch a doorknob? Ask him what the shock feels like, if it's a little *snap* when he touches the grill it's probably static, if it's more of a tingle or buzz then you may have an AC problem. This weather we have been having really seems to make static more of an issue.
 
Re: New and Shocking!

Are you sure he isn't just getting a shock from static like when you rub your shoes on carpet and touch a doorknob? Ask him what the shock feels like, if it's a little *snap* when he touches the grill it's probably static, if it's more of a tingle or buzz then you may have an AC problem. This weather we have been having really seems to make static more of an issue.

Tom, I haven't ruled out static yet. I would like to rule everything else out before I fall back on static. Yes, the stage is carpeted, and it has been cold and dry. The fact that it was an intermittent - not every time he touched the mic - could point to static. The zaps were not audible as in a ZAP!



Still, a good reason to go through the stage and purge the junk and make sure it is safe.



frank
 
Re: New and Shocking!

The mic is probably providing a path to ground for some rouge current coming from the guitar rig,,,



Of course your good ground can still kill the guy, but I would suspect his guitar amp chassis for voltage wrt ground.



If mains wiring is flaky enough the stage outlet ground voltage differences could be enough to feel on a sweaty lip...



In these cases very rarely is the mic or mixer the source of the voltage, but it does happen occasionally. It should be quick work with a VOM to chase this down.



Obvious first measurement if between mic and his guitar... Then if you see steady AC voltage figure out where it is coming from.





JR
 
Re: New and Shocking!

Obvious first measurement if between mic and his guitar... Then if you see steady AC voltage figure out where it is coming from.





JR

Back in the 'ol days
icon_rolleyes.gif
that was a standard procedure I would use. Set everything up and then have the guitar players walk near the mic and I would stick a VOM between the strings and the mic body to see if there was any potential. In a number of cases there was
icon_eek.gif




This was back in my pre distro era and I used tube and solid state amps for the PA.



But at least we caught it ahead of time so nobody got hurt.
icon_biggrin.gif
 
Re: New and Shocking!

Obvious first measurement if between mic and his guitar... Then if you see steady AC voltage figure out where it is coming from.





JR

Back in the 'ol days
icon_rolleyes.gif
that was a standard procedure I would use. Set everything up and then have the guitar players walk near the mic and I would stick a VOM between the strings and the mic body to see if there was any potential. In a number of cases there was
icon_eek.gif




This was back in my pre distro era and I used tube and solid state amps for the PA.



But at least we caught it ahead of time so nobody got hurt.
icon_biggrin.gif



You can spot the very experienced guitar players.

They are the ones touching the tuning pegs to the mic windscreen before they sing. Any noise through the amp means it's time to investigate.

Not a replacement for a meter, but a simple trick to check.