Amplifier repair & parts substitutions

[HooRide]

So I was able to diagnose some problems with a car amplifier I've got on my bench and found (among other things) a pair of MOSFETs that had gone bad (gate and drain were essentially 0 ohms).

The parts, IRF540A and IRF9540, are marked as obsolete but I found IRF540NPBF & IRF9540NPBF as possible replacements.

My question is, because their specs are not identical, should I replace all parts on both stereo channels to match, replace all parts on one channel to match or just replace what is blown?

[peter.mitchell]

If one side is blown the other side might be on its way out, and at the price of the FETs i'd just replace them both.

[Tube_Dude]

As the sound quality perception in a car is not so important than in a residential system, added to the noisy environment , I doubt you will find any difference in sound quality between the two channels, if you replace just one pair of Mosfets...

[Simon]

The parts, IRF540A and IRF9540, are marked as obsolete but I found IRF540NPBF & IRF9540NPBF as possible replacements.

My question is, because their specs are not identical, should I replace all parts on both stereo channels to match, replace all parts on one channel to match or just replace what is blown?

The newer parts should work, and yes change them all while you are there.

[HooRide]

Replace them all I will, they're only $2 and some change each and there are only 8 of them.
There are 2 npn/pnp pairs per channel which is why I was worried about mixing as there would be one new and one old pair sharing the speaker load.

I'll never know for sure what happened to this amp but it looks as if it tried to ground itself through the RCA inputs as two the two 1n5401 diodes that protect the RCA ground were vaporized as well as the traces and a good part of the PCB is burned.

[T4P]

Hold on a sec with FETs, npn and pnp do not apply, it's the wrong logical explanatory, it's N-channel and P-channel
I won't go further but the modern IRF540 and IRF9540 should be a lot better, i have some IRF520s (Old and New, this is the IRF540s brother) lying around and the new ones perform marginally better

[HooRide]

Hold on a sec with FETs, npn and pnp do not apply, it's the wrong logical explanatory, it's N-channel and P-channel

Yes, my mistake.

I won't go further but the modern IRF540 and IRF9540 should be a lot better, i have some IRF520s (Old and New, this is the IRF540s brother) lying around and the new ones perform marginally better

We'll see how it goes, I was given the amp for free so at the very least it'll be a learning experience.