Question Low buzzing sound - advice on possible cause

malc c

Novice Member
Hi Guys,

Hopefully this is in the right section, if not then could the mods move the post where required.

I have a Sherwood AV amplifier that is circa 20 years old. It's no longer used for surround sound (long story) - so its connected to the PC so we can listen to streamed music. It's been fine up until the other day when I noticed a very low buzzing sound from the speakers (Floor standing Gale units) whilst I was waiting for the PC to boot. The PC's line out is connected to the analogue input (CD) and isn't noticeable when the music is played, unless there is a really quiet section. The digits on the display for the volume run from 0 to 70, and the buzzing only starts to get louder after 40. At full volume with nothing being played there is a constant underlying hum with the buzzing.

I first thought that it may be the output from the PC, so installed a dedicated sound card, but the buzzing is still present. If I disconnect the two phono plugs at the rear of the amp, the buzzing is still there too, which would suggest the problem is with the amp rather than the PC or any ground loop issue

I've also tried connecting via the coax digital output from the new card and when nothing is playing there is no buzzing even at full volume. However as soon as the PC outputs audio the buzzing starts, and then stop when the audio stops. I'm assuming that with digital output, the signals will go through some digital to analogue conversion before being fed into the pre-amp section of the amplifier?

I've attached an image from the scope of the output from the headphone socket (max volume) just in case it helps someone advise me what may be the cause. My thoughts are either power supply, or possibly needing a re-cap of the pre-amp ??

Thanks in advance

Malcolm
 

Attachments

dannnielll

Well-known Member
Remove all inputs from the amplifier and increase the volume up towards maximum. .. speakers attached. Switch through the various input options. There should be a slight hiss, much louder hiss when you go to a phono input,and no buzz. If that is what you get,then the amplifier is still ok.
If the buzz is still there it is the amplifier,and likely the power supply. So thinking about the caps is a good idea.
If the buzz is gone, i would be thinking earth loop ,when lines are attached. ,Has the PC been connected to a different socket?.
 

malc c

Novice Member
Thanks for the reply.

With all inputs removed and the volume turned up I get the following when switching through inputs:

Optical - No buzzing
Coax 1 - No buzzing
Coax 2 - No buzzing
Analogue - Buzzing
Aux - buzzing
FM - buzzing under the hiss - but the buzzing isn't as "sharp"
Video 1 - buzzing
Video 2 - buzzing
Video 3 - buzzing
Tape monitor - buzzing
6 channel direct - No buzzing

Yes the PC and Amp are on the same power block - but always have been for years, and there was no problem.

So I removed the cover, and whilst all the smaller capacitors looked fine, there are two large 6800uF 63wv caps near a cable that comes from a huge transformer which look to have bulging tops. But they may well be designed like that as they are plastic tops rather than the normal three way venting tops, but one does look slightly (and I mean slightly) to be bulging more than the other.

So it looks like the two large smoothing capacitors may be the cause. Time to have a look on RS Components for suitable replacements and have a go at repairing this old amp.

Malcolm
 

dannnielll

Well-known Member
By dimensions, look to total volume not just electrical values. The devices you have suggested have a ripple current rating of 4.5"amps. What is the model of your amplifier... I have access to hifi engine a free documentation source for older service manuals .
 

malc c

Novice Member
Thanks. Whilst I'm confident with the practical aspects of changing components, I lack the knowledge with the theory and technicalities such as ripple current you mentioned.

The amp is a Sherwood RD-6106R audio / video receiver.



I've googled the model, but can only find a user manual - hopefully you will have more luck in finding a service manual or circuit diagram

Malcolm
 

dannnielll

Well-known Member
Thanks. Whilst I'm confident with the practical aspects of changing components, I lack the knowledge with the theory and technicalities such as ripple current you mentioned.

The amp is a Sherwood RD-6106R audio / video receiver.



I've googled the model, but can only find a user manual - hopefully you will have more luck in finding a service manual or circuit diagram

Malcolm
I have found the appropriate manual on the website hiengine.org it has a free enrollment process. As the pdf file is 24 mb ,I will not try the patience of AVForms . The capacitors are GE c85 series.
The RS component should suit, but getting higher current rating and higher temperature rating is always a good idea,even at a higher price.
 

malc c

Novice Member
I have found the appropriate manual on the website hiengine.org it has a free enrollment process. As the pdf file is 24 mb ,I will not try the patience of AVForms . The capacitors are GE c85 series.
The RS component should suit, but getting higher current rating and higher temperature rating is always a good idea,even at a higher price.
Thanks for the info. I'll spend some time sourcing a pair of capacitors with the same footprint, but with higher values as suggested.
 

malc c

Novice Member

malc c

Novice Member
Also noticed a slight bulge in C112 (2200uf 25v) which looking at the diagram in the manual is one of a pair just before the two +/- 15v regulators - I will also look at replacing these at the same time, and might as well replace the 4700uf (C117) cap on the +5 line whilst I'm at it
 

malc c

Novice Member
One question, if someone could explain why the buzzing is only happening when an analogue input is selected, or when sound is played through the digital ports. If it was leaking / blown caps, would the buzzing not be present all the time ?
 

dannnielll

Well-known Member

malc c

Novice Member
Well the order has been placed, parts should be here Tuesday. The fun part will be dismantling the receiver, which appears to needs removal of all the boards. If the unit had a removable base it would be simple !
 

dannnielll

Well-known Member
One question, if someone could explain why the buzzing is only happening when an analogue input is selected, or when sound is played through the digital ports. If it was leaking / blown caps, would the buzzing not be present all the time ?
No I cannot answer that one,except there might be a mute circuit engaged in the absence of a recognised data stream. This was why I had suggested the hiss test. Live amplifier inputs will create hiss on the output, proportional to the circuit gain.
 

malc c

Novice Member
Well the five caps arrived today and this evening was spend stripping the unit and replacing them.

I took my time, and was careful when removing the 20 year old connectors, eventually getting the board out and able to swap out the caps. Once complete I bit the bullet and powered up the amp. No smoke !! - and the protection relay clicked in so that was a good sign.

I connected the speakers and set the input to analogue. No buzzing... turning the volume up fully and all you can hear is a faint hiss, and it is really faint. You have to put you ear to around 6" away from the speakers to hear it. - plugged in the digital input and played a song through the PC - clear sound with no buzzing !!

I've attached a couple of images. The old supply caps, and the new caps installed

Thanks for your help and advice. Might get another 20 years out of the thing now :)
 

Attachments

malc c

Novice Member
Using a cheap tester that ran from a 9v batery this was the results - hopefully they mean something to someone :)

First 6800 uf cap tested 6499uF, 0.03 ohm ESR, 1.2% Vloss
Second 6800uf cap tested 6520uF, 0.04 ohm ESR, 1.3 Vloss

First 2200uf cap tested 1225nF, 0.14K ohm ESR, 76% Vloss
Second 2200uF cap tested 1472uf, 0.54 ohm ESR, 3.8% Vloss

4700uf cap tested 1826pf, but then retested to 3946uF, 0.17 ohm ESR and 2.9% Vloss

Obviously the 2200uf that tested 1225 nF was way out and had the bulge at the top.
 

dannnielll

Well-known Member
Any electrolytic which measures between 50% and 200% of the rated value is still ok!. So it looks as if it was only one capacitor which had actually failed. Which is of course exactly what happens in real life. The good news is of course it failed open circuit , so did not blow diodes or the trafo.
I d say your marantz is good for another 20 years.
 

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