NAD C720BEE Tuner/Amp crackling when cold in one channel

[hedgepiganiel]

Hi all,

I've got a NAD C720BEE Tuner Amplifier that's something like 5 years old (well used though), which has developed a problem.

The sound from the right channel crackles for a few minutes after the amp has turned on. The crackling only occurs when there is a signal, and more often when there is loud bass in the signal. The problem goes away after a few minutes, so I suspect its related to temperature. The volume makes no difference in terms of whether or not I hear the crackling, but I think it goes away faster if the volume is louder.

I have tried swapping the speakers and fiddling with all the relevant cables but the problem seems to be in the amp. Recently I heard it in the left channel aswell, but its much more common in the right channel. I noticed that it only started happening when a rogue family member turned up the volume to the maximum for a long time at a party, and I'm told it turned itself off for a few minutes as a result.

I opened the case and looked at the board and noticed some areas of brown residue near the contacts of some of the heatsinks on the board. Here are some photos: https://plus.google.com/u/0/107948042005817529019/posts/Ve5ZwdhW9uV. I looked for broken solder joints and found one, but its one of the joints that holds the heatsink on rather than a component, so I doubt that is the problem.

Is anyone able to shed any light on how I might fix it? I'm comfortable fiddling and soldering if necessary but I'm no electronics expert. I'd really appreciate any help.

 

[TLS Guy]

From the way this was caused it sounds as if the power transistors have been damaged, but there are many other possibilities.

To do service work does require a circuit and test equipment generally.

You might just get lucky and find the problem with electronics cold spray. After the crackling stops spray each power transistor with the cold spray and see which one or ones bring the crackling back.

For most units power transistors have to be matched pairs, one is generally PNP and the other NPN.

If you do this yourself you need to study the techniques for replacement, heat sinking while soldering, correct application of mica spacers and how to use heat sinking compound.

Do not do any repairs unless you are certain of the cause of the problem. Shot gun repairs make matters worse.