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Help! My amp is trying to kill me!

Aarix

Aarix

May 19, 2006
491
8
Cary, NC
#1
The fan in my new Carvin BRX112NEO (essentially the B800 amp in a combo cab) makes this clicking sound. I called Carvin, and the tech said it was probably just the two fan wires smacking against it. So he suggested I open the amp up, and pull the wires out of the way. He insisted that pulling the 8 screws out would not void my warranty.

Ok, no biggie, I can handle this. So I unplug it (It's been off all day), pull the screws, slide it out of the combo and pause to admire the tidy, well assembled interior. The fan is behind a heatsink, which is behind the main circuit board, a nice 4-5 inches from the caps & transformer. I gently tap the heatsink as I'm looking for screws and BZZZT! :eek:

This thing is live! My fingers are a little tingly but fine. I figure there must be a little juice left in the caps, but I'm surprised it would bleed through the heatsink. I grab my trusty screwdriver and tap it between the case & the heatsink to finish what my fingers started ... KAPOW!

Ok, still live. My chrome screwdriver shaft now has a nice war wound. I go get dinner and come back to give it one more try ... KAPOW! :eyebrow:

What the heck is going on? Shouldn't the caps have a bleed off resistor to discharge when it's off? Why isn't the heatsink grounded? Why the heck would caps bleed through the frickin heatsink anyway?

And most importantly, what the heck do I do with this thing? I'm afraid to touch it now.

Any informed electrical advice is greatly appreciated (mostly by my wife).
 
#2
It shouldn't be doing that and I think there's something wrong. By now you should have bled off any charge in the power supply and no way should the heatsink be live. You should call Carvin and arrange for repairs.
 
#3
Having voltage at the heatsink isn't a common design feature, but not impossible either. I'd assume it's on a voltage regulator, not the output section. In any event, bleeding caps with a screwdriver isn't a good idea, try a 1k ohm resistor instead. A bleeder resistor is a good idea (though I used LEDs on amps that I built, that way you could tell by the LED going out that the supply caps were drained) but are not always there, ask anyone who ever worked on a Fender or Marshall still hot a week after its last use, and 400 volts gets your attention a lot faster than 60. Next time check the caps with a VOM to be sure they're drained before poking around.
 
#4
So the amp actually has a 12V voltage across the heatsink (which is split in half).

I tried to drain it, but it took far too long (more than 30 minutes), so I gave up and closed the amp back up. Fortunately, it still works ok.

I'll call Carvin today and ask them why they neglected to mention the risk of impending doom to me, and see what they want me to do with the amp.
 
#5
Aarix said:
So the amp actually has a 12V voltage across the heatsink (which is split in half).
Click to expand...
So I'd guess it's a 7812 regulator, and chances are instead of using a separate low voltage tap on the transformer they just pulled it from the main supply through a resistor. That would take forever to bleed through the regulator, it would go much faster draining the main caps direct.
 
#6
I was mistaken about it taking 30 minutes to discharge. Apparently the caps will keep a small harmless (~2V) potential like a battery even after being discharged.

I'll take a look at it again tonight armed with my trusty dimmer switch & multimeter. Hopefully I can do this with less drama this time.
 

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