Repairing an amp

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Hope this is the best place for this, couldn't see an appropriate space in the Amp sub-forums

A friend of mine was given a Sherwood RD-6106R surround amp, I popped round to help him test it.

Things were going well untill we tested the rear left channel. Initially we thought it wasn't working but on closer listening it was but it was very quiet, it also made a pop/hiss every time the volume was increased & decreased.
As a guess I thought the amp for that channel might be duff so I popped the top off and had a peek inside hoping for something obvious (burn marks, popped capacitor, flashing sign pointing to fault).

The only thing I could see out of the ordinary was this little fellow:
An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


It looks like it was covered in ear wax, there was more but i poked a bit off to have a sniff, see if I could identify it.

Would a failed capacitor look like this, is there any wax-like stuff inside? The ones I've seen fail in the past have had the top split open and just look charred.

Does anyone have any ideas?

Cheers :)
 
Coupling cap!

That is likely the input coupling for the left surround stage.

Electrolyte can resemble dried earwax in some cases if it has slowly been seeping out of the seal and thus had sufficient time to dry out. Once the cap loses its electrolyte; it will start to dry out. Once dry, it will act like an open circuit.

Replace the cap and see if that fixes it. Its my top suspect at this moment and is definitely your "flashing sign".

Good luck!
 
Are those SamWha brand caps? If so, I'm not surprised. They are known not be very good.

Be on the lookout for corroded leads on caps. If you see corrosion (usually a green copper compound), the cap is leaking from the bottom seal.

If you see any deposit under a cap that looks different from the rest of the dirt collected on the board, the cap has likely suffered a seal failure and should be replaced.

EDIT: I just noticed the cap to the lower right of the suspected bad cap has a deposit underneath it. Its sitting next to a couple resistors. You might investigate that one as well. Replacement of all the caps in this stage might be a good idea as they are critical to the unit's operation and inexpensive to purchase.
 
vinnysj said:
Volume pot might need some deoxit.


Possibly. I use TV tuner cleaner but deoxit would probably work better.

I have to say this as a public service: NEVER use WD40 or other petroleum distillate based cleaners or lubes on a pot. Your pot will work great for a day and then melt away.

Also, check the underside of the board for cold solder joints. Lets hope its not like my friend's old Sony Pro Logic receiver.....it took 4 hours to fix all the cold joints! (it had multiple random system failures, mostly microcontroller related)
 
Thanks for the quick replies, I like to try and repair rather than replace, it's (usually) cheaper and better for the environment.

I'm not sure of the cap brand, I'll find out.

Is this the suspected leaky cap your referring to Hardwareguy?;

An externally hosted image should be here but it was not working when we last tested it.


I'll be sure to investigate the other capacitors and have a good look for any dry joints too!

I know what WD40 can do, I keep telling folks to keep it away from various vehicle parts as it usually washes away the grease that is meant to be there!
 
The input selector might just need a good cleaning. Lets hope coupling caps aren't an issue in that.

I've got a couple of wonderful Technics SU-V5 amps that have flaky channels due to selector switches. These amps are rated 60W/ch at 0.005% THD and have the cool fluorescent bar graph meters.
 
I'm not sure what coupling caps are but I get the feeling that will be the problem:bigeyes:

This is a random ebay pic of the 3020SE;
An externally hosted image should be here but it was not working when we last tested it.


It's been flawless for well over 12 years but all those times switching between CD and Aux must of taken its toll.

I love the old amps with needles or metres, as a child I was mesmerised by them:hypno1:
 
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.
 
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:

An externally hosted image should be here but it was not working when we last tested it.


Click the above pic for a super res pic:eek:

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

On closer inspection;

An externally hosted image should be here but it was not working when we last tested it.


On closer closer inspection;

An externally hosted image should be here but it was not working when we last tested it.


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

An externally hosted image should be here but it was not working when we last tested it.


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;

An externally hosted image should be here but it was not working when we last tested it.


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?

Cheers:)
 
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.
 
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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