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Old 23rd February 2010, 12:43 PM   #1
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Default Cap Across Switch on Older Amps

I'm working on a vintage 1970s receiver, Model 430, 25w/ch. There is a small ceramic disc capacitor (10nf) across the power switch. This receiver experiences a burst of dc on the speaker outputs when powered on (up to 2 VDC then drops to 10mv), the design does not have speaker protection relays. In fact, the instruction manual for the receiver (which no one reads) recommends having the speakers off-line during power on. Is the currently installed ceramic disc appropriate? Do these go bad over time? Will I get better results by replacing this cap with a proper Class X film cap?
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Old 23rd February 2010, 12:51 PM   #2
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You worry too much about the stuff in front of the transformer

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Do these go bad over time?
Ceramics? No, these usually last a lifetime. It absorbs the transient when switched, that's it.

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Old 23rd February 2010, 02:38 PM   #3
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That cap is there to help reduce switch arc where powering down. Has no effect as it is shorted when the amp is on and therefore out of circuit?

The DC situation is the likely the result of capacitor coupling at the output of the amp or some other capacitive coupling within the unit. Not much can be done about that so I would not concern with it.
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Old 23rd February 2010, 02:43 PM   #4
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Yes, no problem with the ceramic caps apart from their high tolerance. I think your amp just needs a soft-start circuit.
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Old 23rd February 2010, 05:24 PM   #5
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I'm guessing 430 is Harmon-Kardon 430? Early H-K recievers and amps were famous for their turn on thumps. Perhaps unrelated, but some of the mid to smaller receivers of that era (Mostly Pioneer and Kenwood) were prone to arcing, but the symptoms usually were noise or DC on the outputs and lights flickering on turn-on.
Just my .02
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Old 23rd February 2010, 08:25 PM   #6
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Quote:
Originally Posted by DreadPirate View Post
I'm working on a vintage 1970s receiver, Model 430, 25w/ch. There is a small ceramic disc capacitor (10nf) across the power switch. This receiver experiences a burst of dc on the speaker outputs when powered on (up to 2 VDC then drops to 10mv), the design does not have speaker protection relays. In fact, the instruction manual for the receiver (which no one reads) recommends having the speakers off-line during power on. Is the currently installed ceramic disc appropriate? Do these go bad over time? Will I get better results by replacing this cap with a proper Class X film cap?
The cap simply soaks up any arcing across the switch as teh switch changes state.

If you think its faulty then replace it, they are only a few pennies.
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Old 23rd February 2010, 08:42 PM   #7
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As other posters have mentioned, the cap across the mains switch has nothing to do with the amp's turn-on thump. If the turn on thump is a problem for you, i'd try this circuit: Loudspeaker Protection and Muting
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