My Nady amp is zapping me.

[Brian Sansone]

Well I turned on the ole studio last weekend. I thought a felt a tingle when plugging in a patch cord to my mixer.
At first I thought I imagined it. Then my lips got zapped through the mic. That I didnt imaging.
I also realized that I got a shock when handling the main outs from my mixer to my power amp.
And the chassis of the amp itself was energized.
I know some basics, but thats it. I can read and use a meter's basic functions. I understand AC/DC basics, and resistance....Kinda
Just by the process of elimination, I determined its the amp. When I unplugged the amp from the system,
I didnt get zapped anymore.

Where do I start? I was poking around with my multi meter, and I got 30+ volts between the amp chassis, and the concrete floor.
What are some things I should check? Look for resistance or continuity between the chassis and what?
I have pushed the amp hard, a bunch of times, and its pretty old.
Also, my system in my home is old. It was slightly updated in the 80's. Some outlets have the grounds wired up, but its grounded on the neutral buss in the panel. I would imagine modern systems have a completely separate pathway back to ground, instead of sharing the neutral buss?
- Brian

 

[Boswell]

You don't give the model of your amp, but it may be one of the dangerous old US types that has no mains transformer and runs directly off rectified mains. I've never understood how those could have been sold, given that you plug guitars and mics into them, unlike the TV sets that used the same powering method.

How is the amp connected to your mixer? Do you have a transformer-based DI box you could connect between the amp and the mixer to see if that stops the whole system becoming live? Is there a proper earth connection on the mains feed to the mixer?

Do you get the problem with the amp when it's plugged into different power outlets in your home?

If the shocking behaviour is new, then the amp probably has developed a fault, and you should get it checked out by a competent repair shop. Beyond that, it sounds as though your house wiring could do with an update!

 

[paulears]

Be careful - if you get 30V on the test probe on the concrete floor - which is mostly insulator - then between chassis and a real ground - like a water pipe - you could have full mains voltage. Unless you can safely diagnose this properly, stop now!

Seriously - a concrete floor is not remotely ground, unless soaking wet. 30V could be a seriously wrong measurement. If you feel competent, then very carefully measure between the chassis and the proper ground. 30V suggests a winding fault, but more than this, a short of the live terminal to the chassis which could be pretty terminal. Certainly connecting this to other kit would be foolish till you resolve what's really happening.