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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Oct 2019, 11:16 pm   #1
alanworland
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Default Mains capacitor job?

Been implementing a Dada upgrade kit to my Quad 44 pre amp, this all went well and it performs superbly.
A capacitor which wasn't covered was one across the mains switch in the power supply unit. I dismantled it and found the 0.047 Rifa slightly swollen and splitting! Time for a change.
I have a replacement on the way but am curious to know it's job across the mains? it appears to have a resistor involved in it's construction.

Alan
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Old 22nd Oct 2019, 11:25 pm   #2
Craig Sawyers
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Default Re: Mains capacitor job?

It is an RC snubber intended to prevent contact arcing as the switch opens.

Remember that the 44 has three switched mains outputs, and the switch could be operated when a connected power amp is delivering significant power - hence the need for something to stop the contacts from burning when a high current is interrupted.

Craig
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Old 23rd Oct 2019, 9:18 am   #3
alanworland
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Default Re: Mains capacitor job?

Thanks, I could understand that if the cap was wired across the contacts to help prevent/reduce arcing but this was wired across the load side of the double pole switch?
Do I even need it?

Alan
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Old 23rd Oct 2019, 9:33 am   #4
ronbryan
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Default Re: Mains capacitor job?

It is probably intended to prevent a loud click in the speakers when switching off, by snubbing any transient spike.

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Old 23rd Oct 2019, 11:17 am   #5
snowman_al
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Default Re: Mains capacitor job?

Quote:
Originally Posted by alanworland View Post
Thanks, I could understand that if the cap was wired across the contacts to help prevent/reduce arcing but this was wired across the load side of the double pole switch?
Do I even need it?
It was not there on early ones I believe. It was almost certainly added to stop the switch off click as Ron says.

The switch off arc is caused because the transformer is an inductor.
And an inductor 'stores' energy by magnetism. Very simplistically, if you happen to switch off just as the voltage of the mains cycle is at its highest, the transformer core will also be magnetised to its greatest, now as that magnetism fades very quickly, it causes a high voltage to be produced, enough to jump the switch gap, the arc. If you add a small capacitor or snubber across the transformer or the switch, the capacitor 'soaks up' the electricity preventing the arc.

Your 44 has a double pole mains switch, so fitting the 'snubber' across the transformer is logical. It prevents or greatly reduces the arc and is only in circuit when the unit is powered.

Where single pole mains switches are used the snubber is normally across the switch. But that can cause problems because the snubber is always at mains voltage and 'leaking' a small current into the equipment when it is switched off. Dangerous if you assume it is switched off and so isolated from the mains, worse if the snubber is faulty and you assume the same thing...

Sorry if you already understood this, Alan
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Old 23rd Oct 2019, 7:17 pm   #6
alanworland
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Default Re: Mains capacitor job?

Makes sense using the cap to absorb the energy I will be replacing it as soon as it turns up then I can put it back together, hate having stuff in bits waiting for parts!

Alan
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Old 26th Oct 2019, 6:12 pm   #7
Maarten
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Default Re: Mains capacitor job?

Last time I heard, Dada sells the exact same paper capacitor (branded Rifa, Evox or Kemet depending on manufacturing date) as a replacement part. If they still do, or if you ordered one elsewhere, don't bother fitting it (they even go wrong if stocked for long enough under non-ideal conditions) and find a polypropylene equivalent (MKP).
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