Awful buzz from Amp

[hybridauth_Facebook_100001987038189]

Hi, I've got an odd volume dependant buzz. From Azur 540a v1
Cambridge Audio online support doesnt recognize their own serial number. Sounds like 50Hz with pronounced harmonics.Tone knobs affect the tone of the humWhen the volume knob is at 10:00, the unit is silent
.Hum increases from 10:00 down to 0, hum increases from 10:00 up to max volume
.Hum exists across all inputs.Hum exists with no inputs.
With headphones plugged in, the hum appears to pan left when attenuating from 10:00, appears to pan right briefly then center when increasing volume from 10:00.
When plugging into the "pre out" hum exists as soon as the power plug is attached.
When plugging into the "pre out" hum attenuates from 1400 through to max volume, at which point the unit is nearly silent.

 I'm hoping you'll say "replace this bit, heres where you buy one, heres a photo/diagram of its location. CheersRob.

 

[Covenanter]

Hum generally arises in the power supply and nearly always from the electrolytic capacitor(s). If you know what you are doing they are easy to replace. If you are an amateur do not touch as this is the mains voltage area.

Chris

 

[hybridauth_Facebook_100001987038189]

I'm an amateur at HiFI, but I have built computers, motorbikes and repaired other electronic equipment.

I understand that caps can hold dangerous charges long after the power is disconnected and that poor soldering/work can result in shorts/catastrophic thermal runaway.

If you're satisfied with my answer, can you tell me which bit is the culprit?
Also - theres a section with 8 large 50V220uF caps. its not one of these is it?

 

[Covenanter]

Those sound like the beasts that could be the problem. Not sure why there are 8 of them. Must be some kind of balanced circuit.

I'm not advising you to do this, on your head be it, but if it were mine I would replace the lot, caps are cheap. You can get residual charge in caps but decent power supplies will have a bleed resistor and in any case caps lose their charge over time. It's a bit of an urban myth!

If you do replace them be aware that these capacitors are often "polarised" ie they have a +ve and -ve connector. This will be marked in some way. Note the way the current caps are connected and put the new ones in the same way round.

My advice is to take it to an expert but good luck if you do it yourself.

Chris

PS When I did my first degree the professor used to make sure that lots of charged capacitors were left around on the bench for the first year student to experience mild shocks. Of course this was before the days of "Health and Safety".

 

[TrevC]

I'm an amateur at HiFI, but I have built computers, motorbikes and repaired other electronic equipment.

I understand that caps can hold dangerous charges long after the power is disconnected and that poor soldering/work can result in shorts/catastrophic thermal runaway.

If you're satisfied with my answer, can you tell me which bit is the culprit? Also - theres a section with 8 large 50V220uF caps. its not one of these is it?

220uF 50V is a small cap. Are you sure you don't mean 2200uf or 22000uF? Why the picture of the fuses?

 

[hybridauth_Facebook_100001987038189]

Heya,

Yes I misstyped a "0" 2200uF
The fuse picture was a misunderstanding of someones comment that the problem is likely to be in the power supply.

I've had a quick check of those caps with a multimeter and they look good. Maybe the multimeter is too crude a device? how confident are we that replacing the 8 2200uF caps will fix the problem?

Is there anywhere else I shoudl check first?

 

[eggontoast]

Highly unlikely to be the main smoothing caps (which is what the 8 x 2200uF caps are) as the hum wouldn't be volume dependant.

 

[ellisdj]

I'm an amateur at HiFI, but I have built computers, motorbikes and repaired other electronic equipment.

I understand that caps can hold dangerous charges long after the power is disconnected and that poor soldering/work can result in shorts/catastrophic thermal runaway.

If you're satisfied with my answer, can you tell me which bit is the culprit? Also - theres a section with 8 large 50V220uF caps. its not one of these is it?

Thats a lot of fuses - why so many fuses in use in that product??

Also noticed quite a few for sale on ebay for starting at £15 - it might be cheaper and quicker to buy another one but I know its fun taking on a project like this so fair play to you if you do it !!

 

[Covenanter]

Highly unlikely to be the main smoothing caps (which is what the 8 x 2200uF caps are) as the hum wouldn't be volume dependant.

The volume dependency is a strange part of the story but hum has to have a source and if it is mains hum, which is distinctive if you know it, then that will be the power supply. It is difficult to diagnose these things at a distance but changing the caps is easy and cheap and it would at least eliminate them from the picture if it didn't work.

Chris

 

[Gazzip]

Before you start removing caps from the PCB have you checked for other possible culprits that could be more easily solved?

Poor earth; ground loop; high or low incoming mains voltage; replaced the power lead etc. etc.

Probs none of the above but worth looking at first if you have not already.

 

[hybridauth_Facebook_100001987038189]

re: Poor earth; ground loop; high or low incoming mains voltage; replaced the power lead etc. etc.Well.. I'm not sure. There are a couple of places where the internal boards connect to the chassis and then on to the earth pin on the IEC connector.These have 0 resistance. looks good. but I cant say for sure that one or other signal path that is supposed to connect to one of these earthing points is making a bad connection. cos I dont know where to look.
High or low incoming mains / power lead - well its actually my mates amp, he's moved house and now I have it at my house, so we can rule out any common element there (so three different power sources). also the power lead has changed a couple of times.
Ground loop. - If I understand ground loop correctly - we can rule this out as I've tested the amp in isolation from other sources. So I think these things are ruled out but I'm not overly confident. There are smaller caps scattered across the boards, also a large yellow safety cap "Factory Outlet Safety X2 capacitor DAIN DAIN 0.68ufK".
The 8 large caps have not yet been replaced, waiting for feedback from my mate (who owns the amp)

 

[TrevC]

re: Poor earth; ground loop; high or low incoming mains voltage; replaced the power lead etc. etc. Well.. I'm not sure. There are a couple of places where the internal boards connect to the chassis and then on to the earth pin on the IEC connector. These have 0 resistance. looks good. but I cant say for sure that one or other signal path that is supposed to connect to one of these earthing points is making a bad connection. cos I dont know where to look. High or low incoming mains / power lead - well its actually my mates amp, he's moved house and now I have it at my house, so we can rule out any common element there (so three different power sources). also the power lead has changed a couple of times. Ground loop. - If I understand ground loop correctly - we can rule this out as I've tested the amp in isolation from other sources. So I think these things are ruled out but I'm not overly confident. There are smaller caps scattered across the boards, also a large yellow safety cap "Factory Outlet Safety X2 capacitor DAIN DAIN 0.68ufK".The 8 large caps have not yet been replaced, waiting for feedback from my mate (who owns the amp)

I think you need to take it to an expert.

 

[hybridauth_Facebook_100001987038189]

yeh

 

[Vladimir]

IMO it's a bad cap in the preamp section. If this was in the PSU or power amp section, it would not change with turning of the volume control. The caps in that CA are most likely generic bottom of the bin Capxon and such.

Maybe it's even a cold solder joint or a loose connector, but still in the preamp section IMO. Best to take it to a service tech since it involves troubleshooting with measuring and dissasembly + soldering.

 

[Vladimir]

You can try pluging unplugging the plastic connectors on the board at least. That is easy to do by yourself if the amp is out of warranty. But do it gently.

Clicky