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?
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.
To my inexperiened mind I have no idea what could cause this buzzing, apart from a damaged pot. Any ideas please?
Thanks,
Mike.
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?
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.
To my inexperiened mind I have no idea what could cause this buzzing, apart from a damaged pot. Any ideas please?
Thanks,
Mike.
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.
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.
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
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
Thanks. Its funny you mention the possibility of incorrectly orientated caps for the supply secoupling. I just posted that question on pinkfish.
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?
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?
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.
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.
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.
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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[
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.
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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[
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
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
I changed both +/- regs and the positive rail cap. But not the neg cap as you mention. Trouble is I dont have a suitable cap to hand. I only have a pair of 4700uF Pana FC's but Im not sure fitting them in place of a 1000uF (+) and 470uF (-) is the right thing to do?
I'll be borrowing a meter from a mate tonight so I can check all the voltages and traces are ok. Never done that before so it should be interesting.
rabbitz said:
Ive thought about doing that and possibly going further with the idea and repairing the traces properly. ie: Remove all the components aroung the input buffer, clean of all he solder resist, cut out any broken pads/traces, fit new pads/traces from a PCB repair kit. Im quite certain the buzz problem lies with the damaged pcb around the opamp. Like you said its hard to see with all that flux crap all over the place.
Hi, whilst not that tidy, so long as the circuit is fully connected, I think it is very unlikely that the tracking is causing the hum. It is much more likely due to a regulator fault, incorrect capacitor placement or possibly pick up of hum from the transformer on the capacitor leads if they are on a sensitive part of the circuit. (This would not be true for the power supply which is very low impedance and unlikely to pick up hum this way.
The other possibility is that you have killed one of the diodes in the bridge rectifier after the transformer and you now have a half wave rectified power supply instead of a full wave rectified power supply. This is quite a common cause of hum in power amplifers which will keep working quite happly on a half wave recified supply.
When you have a meter we should be able to guide you through checking this. If you friend has an oscilloscope that you can borrow it would be even easier.
Regards,
Andrew
The other possibility is that you have killed one of the diodes in the bridge rectifier after the transformer and you now have a half wave rectified power supply instead of a full wave rectified power supply. This is quite a common cause of hum in power amplifers which will keep working quite happly on a half wave recified supply.
When you have a meter we should be able to guide you through checking this. If you friend has an oscilloscope that you can borrow it would be even easier.
Regards,
Andrew
gfiandy said:
Hi. The idea of repairing the PCB with 'professional' parts is mainly because I am a bit of a perfectionist at heart. I like the idea of everything looking neat and tidy. I do have a meter now but a scope is another kettle fo fish for me! I wont be able to check anything tonight as I doing a part time course.
As far as I know I need to be checking:
1: All traces go where theyre supposed to go, no shorts etc. Especially ground traces.
2, Check resistors are reading correctly. (may be a burnt one)
I dont have a very sophisticated meter so I cant meausure Capacitance. I'll have to take a punt and replace the cap before the negative regulator.
How do I check Diodes? Again maybe I'll take a punt and replace them anyway, they could do with an upgrade.
Im prettey sure the regs are fine as I changed them and it made no difference.
Anyone know why this amp has different value caps before the
+/- 15v regulators? (1000uF + and 470uF -)???????
The size of the capacitor is determined by the load on the rail. So there is probably more running from the +ve rail than the negative.
Most meters have a diode test setting on them. This normaly bleeps if the diod is conducting in that direction and reads the forward voltage of the diode (normally 0.6 to 1.0 V).
So to test a diode see if it conducts in one direction only i.e it should only bleep in one direction. If it beeps in both then check the voltage if it is close to 0v then the diode has probably failed short circuit. If it read arround 0.6v then it is possible that there is another diode conducting in the other direction and to test the diode it will have to be removed from the circuit.
Regards,
Andrew
Most meters have a diode test setting on them. This normaly bleeps if the diod is conducting in that direction and reads the forward voltage of the diode (normally 0.6 to 1.0 V).
So to test a diode see if it conducts in one direction only i.e it should only bleep in one direction. If it beeps in both then check the voltage if it is close to 0v then the diode has probably failed short circuit. If it read arround 0.6v then it is possible that there is another diode conducting in the other direction and to test the diode it will have to be removed from the circuit.
Regards,
Andrew
Hi, did you do all mods at the same time? If not, after what mod did you experience the buzz?
LM4562 is crankier than most opamps when it comes to RFI. It's the only opamp I've used that have given me such problems, and I've used very cranky opamps in very messy applications. A suitable capacitor from input to ground, good shielding and decoupling caps in close proximity to the power pins are important.
If your amp is still buzzing, maybe you should try a different opamp.
LM4562 is crankier than most opamps when it comes to RFI. It's the only opamp I've used that have given me such problems, and I've used very cranky opamps in very messy applications. A suitable capacitor from input to ground, good shielding and decoupling caps in close proximity to the power pins are important.
If your amp is still buzzing, maybe you should try a different opamp.
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