Here is an example of a SVTIIIpro I recently had in the shop. The unit had been worked on before and inferior service landed it in my shop. This amp came to my shop with a blown output section, which isn't too uncommon, but this could have been avoided. Let's begin with one of the root causes. Here you can see the output FET's that I removed from the unit. Notice anything?

You got it an Irfp9240 has been substituted with an Irfp9140. This is NEVER the right thing to do. Below you will see the other possible cause of this failure.

I wish this picture came out better but I think you can see that the emitter resistors on the right are not mounted properly. You see how the resistors are standing up off the heat sink? What's worse is so you see the big blobs of solder on each leg? This is because these resistors were stripped from something else and the leads were not long enough to make it through the heat sink on the SVTIII. What's worse is that the holes in the heat sink are only so big and these big blobs of solder remove the needed clearance. Two of these resistors could actually touch the heat sink and short out the amp! it's pretty clear that when this amp failed before the irfp240's failed and took out the emitter resistors. Along with the base resistors and a few other components. Here's what it should look like and what it DID look like when it left my shop. Notice the slight arch in the leads and the fresh heat sink compound on the new FETs I can't believe how many guys don't apply new heat sink compound.

 

But wait....there's more! Here is a solder joint on a critical transistor. This should not have been overlooked.

The red arrow points out the bad solder joint that was overlooked. This is on a leg of one of the pre amp transistors adding to the instability of the amp.

 

Below you can see that the amp is ALMOST ready to go but is still suffering from some instability.

 

After replacing two more resistors and a leaky transistor we are ready for biasing. That's right biasing. Solid state amps are more sensitive to bias than tube amps and if it's not set right they will go into thermal runaway and blow up. I mean to say catastrophic failure, you know the expensive kind of failure. If it is biased too low it will sound distorted. I make sure that all amps that leave my shop are biased, then burned in for a few hours and rebiased to ensure that there is no chance that you amp can go into thermal runaway.

 

Above you see a sine wave as measured on the output of the SVTIII with a load. I am playing with the bias trim pot while this is being filmed. Do you see the flat spot that appears in the middle of the wave? This is known as crossover distortion. This is a result of under biasing and will make you amp sound distorted and cut out at low volumes. Although an oscilloscope is not the only test equipment required to properly bias an amp it does make for a nice visual illustration of the importance of proper bias.