In dire need of help with a "squealing" amp.

[istari knight]

Hi guys, I stupidly agreed to look at a friend's Cyrus III amplifier as its been acting strange of late. Well, I removed the cover & there are two boards full of SMD

The symptoms are : The amplifier turns on fine & reproduces sound on all inputs but there is a consistent "squealing" noise coming from the speakers interspersed with crackles - Sometimes it will disappear for 20 mins then the noise comes back.

I thought possible bad ground / dodgy cap / dry joint but I cant seem to find any problems

Has anyone encountered a similar fault before that could give me a clue where to focus my attention ?

Thanks :-)

[DRC]

One channel or both channels ?
Does it occur with the inputs disconnected ?

dc

[istari knight]

Hi DRC, Its on both channels with inputs disconnected & it doesn't get louder turning the volume control - Just a constant noise.

Thanks.

[istari knight]

A little update : I've left it running for 30 mins now & it sounds like a giant wasp buzzing (I think that a better description of the sound) It does this for about 5-10 minutes (changes everytime you switch on) at which point the buzzing/screeching goes intermittent for about 30 seconds then silence....As it should be....The noise can then return at any time (quicker if you turn the volume up with a source playing) If you switch it off whilst making the noise you can hear it "drain away" this is what made me think its a cap....

Oh, I've also checked for DC on the outputs & both channels read below 3mv.


Hopefully this may give more of an idea whats going on.....cheers.

[PhaseLockLoopy]

What is common to both channels?

PSU?

[istari knight]

Originally Posted by PhaseLockLoopy

What is common to both channels?

PSU?

Funny you should say that as I just detected a broken trace on the power supply board, bypassed it with a cable.

Reassembled & turned on.... Okay, The loud buzzing / screech has gone but now there is constant white noise from both channels

I have already checked for bad solder joints & reflowed any suspect looking ones....I'm really not cut out for this !

[PhaseLockLoopy]

Originally Posted by istari knight

Funny you should say that as I just detected a broken trace on the power supply board, bypassed it with a cable.

Reassembled & turned on.... Okay, The loud buzzing / screech has gone but now there is constant white noise from both channels

I have already checked for bad solder joints & reflowed any suspect looking ones....I'm really not cut out for this !

You are cut out for this -- dont crumble now -- you have probably 200 years collective experience in these forumz to tap into.

I personally dont have much experience with this particular amp - but I bet there is someone here who does...

Hmmm white noise

does this go up and down with volume control ?
or is continuous no matter what?
If you found one track break - this amp may have been dropped seek other hairline cracks - they are relatively easy to repair with patience

It could be a dual or quad op-amp common to both channels - post volume control has been hurt by the previous supply problem.

[istari knight]

Originally Posted by PhaseLockLoopy

You are cut out for this -- dont crumble now -- you have probably 200 years collective experience in these forumz to tap into.

I personally dont have much experience with this particular amp - but I bet there is someone here who does...

Hmmm white noise

does this go up and down with volume control ?
or is continuous no matter what?

Hehe thanks for the pep talk I'm normally patient but the whole design of this amp is a deliberate pain in the ***....

Yeah the white noise is constant no matter what volume although if I press the mute button it gets louder

I'll have another look with a magnifying glass, like you say maybe there's more. As for opamps lets hope not ! But I could quite believe it.....

[PhaseLockLoopy]

There is a consequence to everything

Cracks dont make themselves - overstressing PCB (as and example; in PA amps vibration and bumps during transport) as this is a domestic amp - it may have taken a fall and been surprised by the shock .

Consequence of massive raw un smoothed ripple from PSU can **** a few things up - but having found one crack another is very probable.

Hopefully you'll hear again