Cap Across Switch on Older Amps

DreadPirate · 23rd February 2010, 12:43 PM

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?

h_a · 23rd February 2010, 12:51 PM

You worry too much about the stuff in front of the transformer

Quote:

Do these go bad over time?

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

Have fun, Hannes

sumaudioguy · 23rd February 2010, 02:38 PM

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.

Th3 uN1Qu3 · 23rd February 2010, 02:43 PM

Yes, no problem with the ceramic caps apart from their high tolerance. I think your amp just needs a soft-start circuit.

sregor · 23rd February 2010, 05:24 PM

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

nigelwright7557 · 23rd February 2010, 08:25 PM

Quote:

Originally Posted by DreadPirate

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.

abeltronics · 23rd February 2010, 08:42 PM

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

23rd February 2010, 12:43 PM	#1
DreadPirate diyAudio Member Join Date: Jan 2006 Location: Northern Virginia, USA	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?

23rd February 2010, 05:24 PM	#5
sregor diyAudio Member Join Date: Jan 2006 Location: massachusetts	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 __________________ Steve

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
schematics for older RF amps	17DoubleE	Car Audio	9	2nd September 2010 06:05 AM
MTX older RT amps?	jol50	Car Audio	4	13th April 2009 04:50 PM
Aikido output cap selector switch	cbutterworth	Tubes / Valves	2	3rd September 2007 11:43 PM
cap across mains switch?	klitgt	Power Supplies	4	11th December 2006 08:27 PM
Why older RF amps always blew up with DC in output	valterdaw	Car Audio	10	18th August 2006 02:15 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

23rd February 2010, 02:38 PM	#3
sumaudioguy diyAudio Member Join Date: Jan 2010 Location: Midwest U.S.A.	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.

23rd February 2010, 02:43 PM	#4
Th3 uN1Qu3 diyAudio Member Join Date: Feb 2010 Location: Bucharest	Yes, no problem with the ceramic caps apart from their high tolerance. I think your amp just needs a soft-start circuit.

23rd February 2010, 08:42 PM	#7
abeltronics diyAudio Member Join Date: Jul 2007	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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)