Moddeed Cambridge Audio 340A SE buzz problem

[mikesnowdon]

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.

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]

Nothing to do with the buzz, but clean off the flux and then clean with some Isoprypol alcohol. Makes it a nicer job and easier to see any faults with soldering, pads, shorts.

[mikesnowdon]

Originally posted by rabbitz
Nothing to do with the buzz, but clean off the flux and then clean with some Isoprypol alcohol. Makes it a nicer job and easier to see any faults with soldering, pads, shorts.

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.

[mikesnowdon]

Something like this:

http://www.solder.net/PCB/default.asp

[gfiandy]

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

[mikesnowdon]

Originally posted by gfiandy
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

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 -)???????

[gfiandy]

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

[gfiandy]

Oh if it doesn't bleep in either direction then it has failed open circuit and needs to be replaced. This is the most common way for diodes to fail.

Andrew

[nelsonvandal]

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.

[mikesnowdon]

Originally posted by nelsonvandal
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.

It was buzzing before with a different op-amp. Thanks for the suggestion though.