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Old 24th April 2009, 11:26 PM   #11
diyAudio Member
Join Date: Apr 2009
Looks like a nice amp! The coupling cap isolates two stages in an amplifier to prevent the passage of DC. (audio signals are AC)

They are usually small, like the two I suspect are bad in the Sherwood amp. When they go bad without leaking and fail intermittently, that's when I get a little mad. Finding the bad one in a big multichannel amp can be a nightmare.
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Old 29th April 2009, 04:48 AM   #12
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Join Date: Jul 2003
Location: Dona paula, Goa
To find a bad coupling capacitor,

Just connect a good one across it.
OR use crystal earphone to hear the audio.

Gajanan Phadte
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Old 4th May 2009, 01:35 AM   #13
Spax is offline Spax  United Kingdom
diyAudio Member
Join Date: Oct 2008
I finally got the chance to have a look at the Amp tonight with some interesting findings....

After blowing all the dust out of it and getting settled down I figured out the best way to remove the board from the chassis, this was thankfully an easier task that I had first imagined.

With the board out I flipped it over to see if there were any of the cold solder joints that hardwareguy had cautioned about.

I must say I was expecting the worse but I was pleasantly surprised, I'm no soldering expert but everything looked quite nicely done:

Click the image to open in full size.

Click the above pic for a super res pic

The Eagle-eyed may have already noticed something amiss down at the bottom right.

On closer inspection;

Click the image to open in full size.

On closer closer inspection;

Click the image to open in full size.

I flipped the board over to see what awaited me on the other side;

Click the image to open in full size.

There we have it, R902SL seems to be the culprit, this would make sense as its marked as 'SL' which I would speculate to be 'Surround Left', the faulty channel.
The others around it being labeled as;
'SR' (Surround Right)
'FL' (Front Left)
'FR' (Front Right)
'FC' (Front Centre)
Which seems to support the labeling abbreviations.

I also got another picture of the mucky capacitor that was the original suspect;

Click the image to open in full size.

I'll be replacing this and its suspected leaky partner in crime to be safe.

After this dissection I would propose that at some point the Amp was pushed too hard, possibly with some lower impedance speakers (the Amps rated for 8 Ohms), which resulted in the resistors untimely demise, would this be along the right lines for a cause of death?

My biggest question is can the burnt track be repaired? I've never undertaken a retrack (is that a valid word?) before so any pointers would be great.

It didn't occur to me to check the resistace of the dead res in comparison to its counterparts so I will do that first thing tomorrow.

Other questions are;
What are the orange lozenge shaped components, I would guess at capacitors due to the 'C' infront of their assignments. Would any of these need replacing?

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Old 4th May 2009, 07:00 PM   #14
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Join Date: Apr 2009
Most likely the resistor just ran warm from day one and the solder joint there was less than perfect. The thermal cycles tend to make less than perfect joints go bad. (even good joints don't like wild temp swings)

It looks like the joint has been subject to arcing. To fix it, use solder wick to remove all the old solder then thoroughly clean the joint with alcohol before resoldering the joint.

You may want to replace the resistor if you find it is difficult to solder to.

Those orange caps are probably good. As they are not electrolytic, they are not subject to leakage. They can last more than 50 years under the right conditions.... I have some in some old tube gear and they are still in spec.
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Old 4th May 2009, 07:52 PM   #15
Spax is offline Spax  United Kingdom
diyAudio Member
Join Date: Oct 2008
Ah cool so I may be able to re-use the resistor too? I'll get right on that
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Old 5th May 2009, 03:02 AM   #16
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Join Date: Apr 2009
Reuse the resistor if the body is not charred or cracked and it measures within the specified value.

It looks like it may be just a joint gone bad, in these cases usually only the lead gets oxidized. A bit of sand paper can renew the surface of the lead so it can be retinned.
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Old 5th May 2009, 11:48 PM   #17
Spax is offline Spax  United Kingdom
diyAudio Member
Join Date: Oct 2008
Unfortunately the resistor was showing KOhms when its counterparts were showing 10 Ohms so I've got replacements on the way.

The only thing now is the track, despite a wipe over there is still some blackening, I had thought of soldering on a length of copper cable between the leg of the resistor and the next component, would this be suitable or is there a different method I should use to renovate the track?
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