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"remove the ground cap from the polarity switch" - clarification/details needed [message #19538]

Mon, 14 October 2013 14:40

revolvoer
Messages: 12
Registered: October 2013

Junior Member

Folks - I have a K300 PA head that I am working on.........in reading through some old posts, I found some information on replacing the old 2-prong "shocker" power cord on a PA head and I read this in an old post (by chicagobill):

"Clean the pots and jacks, and clean and tighten the Molex plugs that connect the different boards. Check the Power and Polarity switches to see if the plastic mounting sleeves are cracked. And install a new 3 wire grounded power cord and remove the ground cap from the polarity switch."

Can anyone please comment on how this is done? How do I locate this cap? I have installed a new/grounded 3-wire cord and did not remove anything like is described........am I possibly damaging the head running it like this?

Thanks for your input.

S. Davis
Pearl, MS

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Re: "remove the ground cap from the polarity switch" - clarification/details needed [message #19540 is a reply to message #19538]

Mon, 14 October 2013 15:39

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

In theory, the cap can stay in place with no harm being done if the cap is in good condition and the amp is plugged into a correctly wired three prong outlet.

That being said, because it does not meet modern codes, some people on the interwebs will get their shorts in a bunch if you leave it in place. So if you should want to remove it, it will be connected directly to the back of the polarity switch on the front panel. They usually are dark blue in color and the second side of the cap will connect with a screw terminal to the front panel as a ground connection.

The switches are fairly fragile, so I recommend that you clip it out instead of trying to unsolder it. Oh yes, the polarity switch is wired directly to the ac line, so be certain that the amp is unplugged if you do clip off the cap.

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Re: "remove the ground cap from the polarity switch" - clarification/details needed [message #19541 is a reply to message #19538]

Mon, 14 October 2013 18:03

revolvoer
Messages: 12
Registered: October 2013

Junior Member

chicagobill - thanks for the quick response and instructions. I was able to clip the cap off and all seems well with that aspect of the amp now.

S. Davis
Pearl, MS

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Re: "remove the ground cap from the polarity switch" - clarification/details needed [message #19545 is a reply to message #19538]

Tue, 15 October 2013 06:43

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

I leave the cap in when doing the mod as long as the cap is rated for 400 volts or better which it needs to be!
I have seen where on some amps the designer lost focus and speced a 150 or 200 volt cap and that can and will fail in time, alot of times if the amp was played on a outside gig and run off a genorator that had poor voltage regulation,or during a lighting storm hit.
This due to the fact that AC wall voltage peak is over 300 volts, what we normally speak of as the 115 to 120 volts from the wall outlet is just the RMS voltage!

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Re: "remove the ground cap from the polarity switch" - clarification/details needed [message #19548 is a reply to message #19545]

Tue, 15 October 2013 09:16

revolvoer
Messages: 12
Registered: October 2013

Junior Member

stevem wrote on Tue, 15 October 2013 06:43

I leave the cap in when doing the mod as long as the cap is rated for 400 volts or better which it needs to be!
I have seen where on some amps the designer lost focus and speced a 150 or 200 volt cap and that can and will fail in time, alot of times if the amp was played on a outside gig and run off a genorator that had poor voltage regulation,or during a lighting storm hit.
This due to the fact that AC wall voltage peak is over 300 volts, what we normally speak of as the 115 to 120 volts from the wall outlet is just the RMS voltage!

stevem - the ident info on the cap I clipped out is: PAKTRON .022 +/-20% 150vac MVL. Based on your comments, this cap was NOT rated high enough for its intended purpose.

S. Davis
Pearl, MS

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Re: "remove the ground cap from the polarity switch" - clarification/details needed [message #19549 is a reply to message #19538]

Tue, 15 October 2013 11:50

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

The cap that you removed was original to the amp and was rated correctly at 150v AC.

Before there were caps designed for this purpose, companies used caps that were normally rated for dc only. These caps were usually 400-600 volt caps. If you open up a typical vintage Fender amp, this is what you'll find.

So in or out, you are good to go.

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Re: "remove the ground cap from the polarity switch" - clarification/details needed [message #19553 is a reply to message #19538]

Wed, 16 October 2013 09:52

revolvoer
Messages: 12
Registered: October 2013

Junior Member

Excellent - all good information and peace of mind for the knowing.

Thanks!

S. Davis
Pearl, MS

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