Sherwood S8900 Capacitor Troubles - diyAudio
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Old 20th January 2013, 02:01 PM   #1
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Default Sherwood S8900 Capacitor Troubles

I picked up a beautiful Sherwood S8900 for 30 bucks, and even though it works I'm going to be replacing the electrolytic caps. After researching (here and elsewhere) there are some lingering questions about replacement as well as issues with the amp I could use some help with:

1. How much higher of a capacitance can the replacement caps be? The larger values might be used simply out of not being able to find the exact ones in the amp. Im talking about power filtering caps here.

2. Can someone help me make sense of the part of the schematic attached? On the "Audio Driver Board" there are 4000uf caps that seem to be in series with the signal out from the board to the speaker terminals on the back of the receiver. On the audio driver board there are two 'voltage in' connections, a couple ground connections, but as for signal connections one from the loudness control and one to the main/remote speaker switch and then to the speaker terminals. It is this part of the circuit that has the 4000uf cap. Is this normal? Am I reading the schematic wrong? Any special considerations (bypassing?) for replacing this cap if it is in the signal path?
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Old 20th January 2013, 03:59 PM   #2
DUG is offline DUG  Canada
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The 4000uF cap keeps the 35VDC from going to the speaker.

It is needed. Going below this value will raise the low frequency 3dB point. (less bass)

They are asking for a 50V rated cap...do not go below this rating.

IMO...I would design with a cap rated for more than the supply rail: 70V

In case of drive failure.

But that is just me.
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Old 20th January 2013, 04:12 PM   #3
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Thanks for the reply. That clears up the confusion. So I assume a 4700uf cap would be safe to use then? Also, should I consider bypassing? I don't fully understand when bypassing is necessary.
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Old 20th January 2013, 04:21 PM   #4
willamp is offline willamp  United States
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Quote:
Originally Posted by Dumbledog View Post
Thanks for the reply. That clears up the confusion. So I assume a 4700uf cap would be safe to use then? Also, should I consider bypassing? I don't fully understand when bypassing is necessary.
Yes, 4700uf would be fine.

By 'bypassing' do you mean bypassing with another size of cap? If so, I would like to hear that answer too. I have sometimes seen the large electrolytic bypassed with a film cap of different capacity and I'm curious what function that serves.
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Old 20th January 2013, 04:23 PM   #5
DUG is offline DUG  Canada
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A few general rules for electronics:

Anything with area has capacitance

Anything with length has inductance

If you have capacitance and inductance (pretty well all circuits) you will have a resonance frequency.

So

All capacitors have a complex impedance vs frequency graph.

The simple model has an inductance in series with the capacitance.

At some frequency there will be a series resonance condition and a low impedance point.

Above that the "capacitor" acts like an inductance and impedance rises.

(There is a thread on here somewhere that shows what I am talking about...with pictures...I just can't find it right now.)

Bypassing the large cap with a smaller one creates a path of low impedance that the large cap is not providing at that frequency.

Hope that helps.
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Old 20th January 2013, 04:29 PM   #6
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Quote:
Originally Posted by DUG View Post
A few general rules for electronics:

Anything with area has capacitance

Anything with length has inductance

If you have capacitance and inductance (pretty well all circuits) you will have a resonance frequency.

So

All capacitors have a complex impedance vs frequency graph.

The simple model has an inductance in series with the capacitance.

At some frequency there will be a series resonance condition and a low impedance point.

Above that the "capacitor" acts like an inductance and impedance rises.

(There is a thread on here somewhere that shows what I am talking about...with pictures...I just can't find it right now.)

Bypassing the large cap with a smaller one creates a path of low impedance that the large cap is not providing at that frequency.

Hope that helps.
So it's a good idea to bypass the large cap because it would eliminate the rising impedance created by the resonant circuit of the large cap and circuit inductance?
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Old 21st January 2013, 10:32 PM   #7
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When replacing caps, which should really be replaced? I figured the power and output caps, but should all of the other electrolytics be replaced as well? There's 24 of them....
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Old 22nd January 2013, 02:20 AM   #8
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It's quite an old receiver so replacing all electrolytics is a good idea but as a newb, could I suggest that you don't try to replace everything at once or in both channels at the same time? If you do something that you later realize was wrong, you may need to retrace your steps or even repair damage, so having an untouched, working channel as a reference, may well save your neck.
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Old 22nd January 2013, 07:37 PM   #9
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All the smaller electrolytics should be replaced before the power and output caps.
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