Repairing an amp

[Spax]

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:





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

[hardwareguy]

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!

[hardwareguy]

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]

Volume pot might need some deoxit.

[hardwareguy]

Originally posted by vinnysj
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)

[Spax]

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?;



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!

[hardwareguy]

Yes, that is the capacitor I was talking about.

[Spax]

Cool, I'll ensure that one gets sorted too.

My next big challenge will be fixing my Kenwood KA3020 SE, the input selector has started to loose the right channel

One thing at a time though

[hardwareguy]

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.

[Spax]

I'm not sure what coupling caps are but I get the feeling that will be the problem

This is a random ebay pic of the 3020SE;


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