leigh

Caps from power hot/cold to ground - should always remove?
« on: January 05, 2006, 04:44:35 PM »
I searched for this but don't know what they're called.... it was practice for a time to build gear with small caps (like 0.01 uF) running from the power lines to ground. I assume this is to shunt RF that might have gotten on the lines? But in instructions I've read for adding a grounded power cord to old, ungrounded gear, it says to remove these caps.

True or false - this should universally be done?

I'd only seen this on gear with 2 prong, ungrounded cords before, but today I saw it on a solid state Altec mixer, which came stock with a grounded plug. In this case, there were small caps running from both hot and cold leads (blk/white) to ground.

Thanks,
Leigh


NewYorkDave

Caps from power hot/cold to ground - should always remove?
« Reply #1 on: January 05, 2006, 04:46:26 PM »
I always take 'em out, just for safety's sake. You can make a technical argument for them being there; but for me, safety trumps everything.

leigh

Caps from power hot/cold to ground - should always remove?
« Reply #2 on: January 05, 2006, 06:01:07 PM »
Quote from: "NewYorkDave"
I always take 'em out, just for safety's sake. You can make a technical argument for them being there; but for me, safety trumps everything.


The danger being that the caps could fail and short out?

Leigh

Brian Roth

Caps from power hot/cold to ground - should always remove?
« Reply #3 on: January 06, 2006, 02:30:27 AM »
Quote from: "leigh"
Quote from: "NewYorkDave"
I always take 'em out, just for safety's sake. You can make a technical argument for them being there; but for me, safety trumps everything.


The danger being that the caps could fail and short out?

Leigh


Yes!  In this day and age, there are caps specifically intended for connection across the AC mains, or from mains to chassis.  These have specific UL listing requirements.

In Ye Olde Daze <g>, the manufacturers used "any old cap" they had access to.

Bri
Brian Roth Technical Services
Salina Kansas, home of the best vinyl on the planet!

http://www.BrianRoth.com
recordingservicesandsupply.com/
www.qualityrecordpressings.com/
store.acousticsounds.com

leigh

Caps from power hot/cold to ground - should always remove?
« Reply #4 on: January 06, 2006, 02:40:00 PM »
Thank you Dave and Brian, it's good to have a clear answer on this!

Leigh

CJ

Caps from power hot/cold to ground - should always remove?
« Reply #5 on: January 06, 2006, 03:51:47 PM »
Whats wrong with a 0.047 @ 600 VDC across the line?
The pwr trans pri has inductance that blocks a lot of stuff, but there is still capacitance in the coil. A cap on the line will help catch stuff that the coil does not.

I do not understand why this is unsafe?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Brian Roth

Caps from power hot/cold to ground - should always remove?
« Reply #6 on: January 07, 2006, 03:38:39 AM »
There are now specific UL and IEC tests for caps that are attached across the mains, or from mains to chassis.

I found this via a quick Google search:

http://www.evox-rifa.com/technote_pdf/rfi_fact.pdf

There are many aspects tested, like high surge voltages and flammability.

Better safe than sorry when playing with anything on the AC mains...

Bri
Brian Roth Technical Services
Salina Kansas, home of the best vinyl on the planet!

http://www.BrianRoth.com
recordingservicesandsupply.com/
www.qualityrecordpressings.com/
store.acousticsounds.com

NewYorkDave

Caps from power hot/cold to ground - should always remove?
« Reply #7 on: January 07, 2006, 03:40:34 PM »
CJ, read more carefully. We're talking about caps from line to chassis.

I'm against the whole idea of caps from line to chassis, because you can't always assume that your chassis is properly grounded. If the chassis were floating... well, you know the rest.

CJ

Caps from power hot/cold to ground - should always remove?
« Reply #8 on: January 07, 2006, 04:03:20 PM »
Yes, I understand, but every piece of equipment I have opened up in tha last zillion years has that cap.
Chassis is neutral if it's grounded, so in a way, the cap does go across the line, if everything is wired right, wall socket not excluded.

120 is actually low voltage, if you think about it.

Yes, I hate the shock from the Fender, but I'm used to it. Thats why I flinch every time I walk past one. Pavlov's Dog!  :razz:
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

 

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