replacing capacitors

[TonyTecson]

unfortunately I can't find a schematic for Sherwood RD 6405 on the internet... I wonder if any other model might be similar enough?

otherwise I could remove the case and take some pictures of the circuit board if that might help...... but in principle, I am going to start by replacing the two biggest electrolytic caps in there, which are located near the transformer.

seems like an ESR meter costs a bit.... if it's not crucial, and I can do without one, I would rather spend a bit less money, this time at least 🙂

just out of curiosity, how does volume indicate where the hum might come from? what connections are there between volume controls and whatever might cause hum?

thanks again guys 🙂

volume control do not indicate where the hum might come from, when you open it up it lests both signal and hum and noise in...
you can also do it by the shotgun approach, replace all ecaps big or small, but do it in stages, one board at a time, and see it that solved the problem...

 

[Enzo]

If turning the volume control to zero stops the hum, that means it is coming from before the volume control. That would imply the power amp itself is quiet, meaning the hum is not likely coming from the main filters.

Did we determine that the actual aluminum can is bulging?


If you turn up the amp to hear the hum, do the tone controls affect the TONE of the hum in any way?


service manual is at Elektrotanya
Service manual, eprom result list | ElektroTanya | Service manuals and repair tips for electronics experts

When the page comes up, there is a preview box, though there is no preview for this model. Underneath that is says "processing" for a little while. When processing changes to "get manual", then click on "get manual" to download. All the larger download buttons are NOT the manual.

 

[snakechrm]

thanks heaps for the link to the manual! I will have a look when I get home. I am at work now....

I have a picture of the two large caps on my phone (they would be the main filter?). in my opinion the top looks slightly pushed outwards/swollen, but I could be wrong. see below please, what do you think?
will check tone when I get home.

cheers heaps!

 

[indianajo]

Those caps don't lookbad but I have dozens I have replaced that made the sound better that never popped the top.
Hum can also be caused by oxidized connections, especially tin and brass. I don't expect a sherwood to have any gold plated connectors. Remove and replace to scrape oxide off (with the power off). Any ring terminals to pads on the boards, unscrew, clean, replace with stainless steel star washer in middle. No sandpaper or emory cloth, it leaves insulating oxide embedded in the trace or pin. Scratching with a pick is sometimes effective to remove oxide. Also the wiper of pots can oxidize. Moving the wiper helps some, spraying out with contact cleaner can help. No $30 a can pro non-flammable contact cleaner from factory supplies, brominated hydrocarbon dissolves PVC plastic which consumer goods are made of. Aliphatic hydrocarbon contact cleaner is flammable; no smoking, open flame, sparks within 10 m when using.

 

[Enzo]

They may be good or bad, but don't look VISIBLY bad to me in the photo.

 

[snakechrm]

so, i am pretty sure the hum is 120 hz.... kept listening to the speakers humming, then compared to youtube. now i understand this kind of hum suggests issues with full wave rectification?

i have attached a screenshot from the schematic. i see two bridge rectifiers in there, with caps around them. i have circled those in red.

i also circled the two biggest caps which i saw when i opened the receiver, in green.

was wondering if you guys could advise whether i am right to suspect the problem might be in these areas/ whether the caps involved might be the ones i should replace?

thanks again.

vlad

 

[indianajo]

Uh, no, the big ones have to do with the power amp. since the hum disappears with the volume turned down, I would suspect more C110, 111, 117, the caps after the voltage regulators for the preamp sections. 78xx are voltage regulator IC's. Those three regulator ICs can have short life themselves, do check the ripple voltage on those three caps. One regulator might be bad. Although like I said, oxidation of connectors and ground connectors may be your problem instead of e-caps.

 

[snakechrm]

Thanks �� I bought the 3 caps you suggested. Will start with them. Will also do some cleaning. Might also try changing the regulators at some stage, depending how it goes after a bit of re-capping work. Cheers heaps again, I really appreciate the advice ��

 

[turk 182]

don't replace the caps until you can confirm that the regulators are ok!
while your in there check the bridge rectifiers for shorted or open sections and measure the rail voltages.