Burnt power transistor

[htgeek]

I have a 200w power amp that wasn't working so I opened it up and found that a transistor, 2N4403 was cracked and two power transistors, 2SC3298's were black and burnt.

I have attached a picture showing T13 with a red circle around it that had cracked and T11 and T14 with the orange circle are the two burnt power transistors. Could T13 being cracked cause a short somewhere that would cause T11 and T14 to burn out, everything else on that board looks fine.

[Conrad Hoffman]

In a DC coupled amp problems usually travel from output towards the input. It's difficult to say what-all might have gone wrong and what parts are now bad. That's why test equipment exists! You need to check each and every active device, and I'd check all the passives too. The chances of fixing this by visual examination alone are low.

[djk]

"Could T13 being cracked cause a short somewhere that would cause T11 and T14 to burn out, everything else on that board looks fine. "

No, T13 is a protection transistor.

Most likely T7, 11, 10, and everything to the right should be replaced, check all the emitter resistors too.

Fire the amp back up with a 75W lightbulb in series with it.

C48 may also be damaged.

Parts are cheap, time is not, and some stuff may test OK and still be damaged.

[jaycee]

Phone jacks for speaker outputs - almost certainly speaker plugged/unplugged while the amp was on, shorting the output. Do yourself a favour and replace the phone jack with a Speakon connector!

T11 is the bias control device, so that blowing up has probably turned on full bias and killed the drivers (T14/T15) and outputs (T16-T19). T13 is a VI limiter transistor and has probably killed itself trying to shut down the output stage. T7/T10 might also be toast.