Moddeed Cambridge Audio 340A SE buzz problem - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th October 2008, 08:10 PM   #1
diyAudio Member
 
mikesnowdon's Avatar
 
Join Date: Aug 2007
Default Moddeed Cambridge Audio 340A SE buzz problem

I have just completed some mods to my CA 340A SE. I fitted a LM4562 opamp on a socket (input buffer). Replaced all signal decoupling caps with 2.2uF Mundorf MKP's. Replaced local decoupling caps on opamp and Selector chip with Nichcion Muse 100uF 25v lytics. Replaced 100uF smoothing caps after opamp Reg's with 220uF Blackgate STD's. Everything is working fine and the sound is much improved. However, there is a faint buzz on the left speaker. This shifts to both speakers as the volume knob is turned. This buzz remains even when there is nothing connected to the amp.

On a previous mod I had trouble removing the volume knob and forced it a bit in my attempts to get it off. I suspect I have damaged the pot.

Any ideas what else the problem could be?

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

As mentioned above I have modded this amp before. In my naive/inexperienced attempts to remove the original opamp I broke a couple of solder pads. These have been repaired and checked against the scematics so Im quite sure everything is fine though I dont have a meter to check properly. When I did this initail mod I had the buzz problem too. When I opened the amp I realized I had fitted a decoupling cap incorectly on the opamp (swollen cap). I removed it and replaced the original 'non-polar' decoupling caps and the buzz remained. This led me to think the opamp was dodgy. I've now fitted a brand new LM4562 and the buzz remains, ruling out the possibility of a bad opamp.

Click the image to open in full size.

To my inexperiened mind I have no idea what could cause this buzzing, apart from a damaged pot. Any ideas please?

Thanks,

Mike.
__________________
www.drugfreeworld.org
  Reply With Quote
Old 6th October 2008, 01:44 PM   #2
rabbitz is offline rabbitz  Australia
diyAudio Member
 
rabbitz's Avatar
 
Join Date: Jul 2003
Location: Newcastle, Australia
I'm not saying it's the problem but on occasions you can get increased noise from pickup when using caps like the Mundorfs that have long leads. These leads can act like an antenna. Buzz can be related to transformer noise.

Looks like your leads go down the side of the cap and along the bottom of the cap to locate in the original holes. Look at a lot of antennae and they have the same loop looking shape.

Others here will have a greater idea than me but I have experienced problems using MKP caps in amplifiers due to the above issues. When I now use MKP caps they are normally placed at the RCA socket so the leads are left straight and just replaces the signal wiring. I've have had no issues with that method. Problem is you can't do that with this amp due to the configuration.

The other thing is to check all the grounds you have soldered and make sure there is no breaks in the tracks. Check your soldering on the pot.

You also should check if the power supply, layout etc is suitable for the LM4562 as a lot of times you just can't drop in another opamp without other changes.
__________________
No longer DIY active
  Reply With Quote
Old 6th October 2008, 07:11 PM   #3
gfiandy is offline gfiandy  United Kingdom
diyAudio Member
 
gfiandy's Avatar
 
Join Date: Jul 2003
Location: Cambridge UK
The buzzing noise you have described is usually caused by 50Hz ripple from the mains getting into the circuit.

It is unlikely that this is the pot, normally when they fail they crackle. But you could check all the solder joints from it to the PCB are still good.

Most likely is that you have not connected one of the decoupling capacitors you have replaced correctly or it is faulty (probably due to being the wrong way round)

I would recomend buying a cheap meter (you can get one from Maplins for about 5) and checking all the modifications carefully.

Also if you have reduced the value of any of the decoupling capacitors this could also cause hum.

Regards,
Andrew
  Reply With Quote
Old 6th October 2008, 07:16 PM   #4
diyAudio Member
 
mikesnowdon's Avatar
 
Join Date: Aug 2007
Thanks. Its funny you mention the possibility of incorrectly orientated caps for the supply secoupling. I just posted that question on pinkfish.

Click the image to open in full size.

Click the image to open in full size.

It looks like ive got them the wrong way round to me. What do you think?

You mention that the cap could be damaged by being the wrong way round. Could there be other damage caused by this error?
__________________
www.drugfreeworld.org
  Reply With Quote
Old 6th October 2008, 07:58 PM   #5
Ted205 is offline Ted205  United Kingdom
diyAudio Member
 
Ted205's Avatar
 
Join Date: Jun 2008
where did you get the service manual from ?

  Reply With Quote
Old 6th October 2008, 08:20 PM   #6
rabbitz is offline rabbitz  Australia
diyAudio Member
 
rabbitz's Avatar
 
Join Date: Jul 2003
Location: Newcastle, Australia
Those caps are the wrong way around as if you go from pin 4 on the opamp, the track should go to the -ve side of the cap. For pin 8 it should go to the +ve side of the cap. The top 2 holes for the caps run to ground. That's assuming that the square pad is pin 1 on the opamp.

Have a look at C40, C41 on the power amp schematic and you can see how the polarity of the cap is done to suit +ve and -ve rails.

Did you replace the 47nF C7, C8 by-pass caps? You still should include those 47nF caps to by-pass the electrolytics or run them across pin 4 and 8. Someone else might be able to confirm that.
__________________
No longer DIY active
  Reply With Quote
Old 6th October 2008, 08:29 PM   #7
diyAudio Member
 
mikesnowdon's Avatar
 
Join Date: Aug 2007
Quote:
Originally posted by Ted205
where did you get the service manual from ?


I paid for it from a website. Cant remember what it was though. I could trawl back through my emails and find out but Im sitting here with an open amp infront of me about to reorientate my caps.
__________________
www.drugfreeworld.org
  Reply With Quote
Old 6th October 2008, 08:55 PM   #8
diyAudio Member
 
mikesnowdon's Avatar
 
Join Date: Aug 2007
Quote:
Originally posted by rabbitz
Those caps are the wrong way around as if you go from pin 4 on the opamp, the track should go to the -ve side of the cap. For pin 8 it should go to the +ve side of the cap. The top 2 holes for the caps run to ground. That's assuming that the square pad is pin 1 on the opamp.

Have a look at C40, C41 on the power amp schematic and you can see how the polarity of the cap is done to suit +ve and -ve rails.

Did you replace the 47nF C7, C8 by-pass caps? You still should include those 47nF caps to by-pass the electrolytics or run them across pin 4 and 8. Someone else might be able to confirm that.

Cheers, I thought so. Im about to correct my mistakes in a minuite. Yes, I replaced C7,C8 with polarized lytics. Ive heard about running a cap across the +/- ve pins. When I installed the LM4562 in my CDP I was advised by Beent not to worry about it.
__________________
www.drugfreeworld.org
  Reply With Quote
Old 7th October 2008, 12:29 AM   #9
diyAudio Member
 
mikesnowdon's Avatar
 
Join Date: Aug 2007
One thing I know was definately wrong was that the supply decoupling caps were the wrong way round so I've corrected that with new caps.

I also checked the tracks thorougly against the schematics and repaired anything I felt looked dodgy. Obviously when I get a meter I can check this properly and be certain the circuit is ok.

IMG]Click the image to open in full size.[/IMG]

I checked the solder pads for the volume pot and they are fine.

I noticed a cap just before the +15v reg had leaked so replaced that and also fitted new +/- regs to be sure it wasnt a bad regulator.

STIL BUZZING!

At least I have eliminated a few possibilitys. I used the headphones to see if the buzz was still there on my BOSE TP-1's and it was. Im prettey sure this means the problem dosent lie in the power stages. I think its got to be either the pot's or someting wrong with the input buffer circuit.

I WILL be getting a meter very soon.

Any other ideas as to what it might be[
__________________
www.drugfreeworld.org
  Reply With Quote
Old 7th October 2008, 09:55 AM   #10
gfiandy is offline gfiandy  United Kingdom
diyAudio Member
 
gfiandy's Avatar
 
Join Date: Jul 2003
Location: Cambridge UK
Hi.

If the input cap to the regulator has failed then this would be a sign that the output has been driving to low a load so you may have shorted it out (this could have happened due to the caps the wrong way round). Since you got both the caps the wrong way round I would check the -ve regulator and cap as well. The other thing that kill caps is long term over heating or eventually just time but unless this amp is very old I would think you did something to kill it.

When you have the meter check the DC and AC voltage from the opamp power pins to ground.

You should have approx 12V DC and about 50mV AC or less. If you have much more AC than this then the regulators are not working properly.

Regards,
Andrew
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cambridge Audio d500 problem seanzozo Digital Source 5 18th November 2012 07:09 PM
Cambridge Audio A1 V2 Amp Problem twits4twats Chip Amps 4 23rd February 2011 01:11 PM
Cambridge Audio 340A SE preout conversion and other mods mikesnowdon Solid State 0 31st May 2008 09:00 PM
Smart Thinking! Cambridge Azur 340A lineup Everything Else 0 15th January 2007 03:00 AM
cambridge audio a500 amp problem tim0850 Solid State 3 1st March 2005 09:13 PM


New To Site? Need Help?

All times are GMT. The time now is 04:26 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2