Replacing Power Transistors

[lowell]

So is it REALLY necessary to replace all paralleled transistors if just one is shorted? Is it necessary to then replace the other side set of transistors as well? So if one NPN is bad, do all paralleled NPN need replacing as well? And what about the PNP side? Basically if one output is shorted do they all need replacing? What is the best and most necessary approach? Is it different with FET vs BJT? I hate to waste parts that don't need replacing, but then again I am aware it's best to have paralleled transistors all from the same batch for matching purposes.

I have some ideas all jumbled from years gone by and am looking to sort this out once and for all. I'm sure it's been answered before but a search in the forum revealed a whole lot of threads.

[Jazz P Bass]

Basically, I replace the bad one.
(Always a good idea to check the drivers)

I see no need to mess with the other side.

In theory, I would have to guess that the surviving parallel transistors probably 'took a hit'.

[olddawg]

So is it REALLY necessary to replace all paralleled transistors if just one is shorted? Is it necessary to then replace the other side set of transistors as well? So if one NPN is bad, do all paralleled NPN need replacing as well? And what about the PNP side? Basically if one output is shorted do they all need replacing? What is the best and most necessary approach? Is it different with FET vs BJT? I hate to waste parts that don't need replacing, but then again I am aware it's best to have paralleled transistors all from the same batch for matching purposes.

I have some ideas all jumbled from years gone by and am looking to sort this out once and for all. I'm sure it's been answered before but a search in the forum revealed a whole lot of threads.

It's better if you have an exact replacement. That said I've seen seriously abused units run for decades with a single substitute as a repair. It also depends on the design. Back in the day I would never do it on a Yamaha... a Pioneer wouldn't care. In any event, always cook it good on a load bank. If it doesn't smoke under your own torture you are probably good. Look at the output on a scope. Check the bias. Look at the crossover notch.

[Jazz P Bass]

And if there are separate ballast resistors ( as it should be) you can check each individual transistor for current draw.

[Enzo]

Depends on the fault. If a transistor shorts E-C, then it shorts across its parallel buddies, taking all the current away from them. In other words it doesn;t stress one to have a neighbor short E-C. If one shorts B-C, in my experience that usually takes out the whole row of parallels, so you have to replace them all, they are all shorted now.