Hello wonderful people of the internet! I recently acquired a pair of not working Samson pa speakers. One had a broken fuse the other one didn't have sound.
I figured out the reason it doesn't have sound, and it's because the relay was broken. I got a new relay put it in an viola; sound. Yeah.. Then I put the thing together plug it in and the fuse blows as soon as I turn it on. Okay... Take it back apart to figure out that a damn washer fell off a screw and shorted two +- 15VDC voltage regulators. Okay I removed the washer, and replaced two output transistors. Tested it out of box it worked. Thank god I got lucky, easy fix... Yeah, right... I put it together and the damn tda7294(used for the tweeter) shorted to ground through the heat sink... Broken isolator. Okay. I got a new tda chip, isolator and a few resistors had to be replaced. Testing out of box worked, AGAIN. I put it together and who would have guessed it broke again. For unknown reasons it started blowing fuses I started speculating output transistors again, so I took them out and powered it up and sure enough the fuse didn't pop. I measured the transistors and they measured fine. Okay what the hell do I do now? I measured bias voltages of the transistors one pair had -1.5V the other had 1.5V. Super! So I put back a pair of complementary transistors, and blew them up somehow... Okay, I'm left with one pair of transistors, I put one in and measure its bias it measures 0, the other's bias without a transistor goes up to 2V. Okay odd.. Lets see if the bias goes away if I put in the other transistor while keeping the emitter pin off. The bias doesn't go away and the transistor opens to 2V. I'm speculating that that shunts the two transistors together and that's what caused the pair to blow up... I checked the driving transistors but they're fine. I measured literally every resistor on the board and everything measures fine. I don't know what to do at this point, so I'm turning to smarter people than me! I'm open to any questions, I just wanna get this thing fixed..
TL;DR: Broken relay turned into broken output transistors and now I have no clue what to do, and I'm looking for help.
Any help is appreciated! 😀
I figured out the reason it doesn't have sound, and it's because the relay was broken. I got a new relay put it in an viola; sound. Yeah.. Then I put the thing together plug it in and the fuse blows as soon as I turn it on. Okay... Take it back apart to figure out that a damn washer fell off a screw and shorted two +- 15VDC voltage regulators. Okay I removed the washer, and replaced two output transistors. Tested it out of box it worked. Thank god I got lucky, easy fix... Yeah, right... I put it together and the damn tda7294(used for the tweeter) shorted to ground through the heat sink... Broken isolator. Okay. I got a new tda chip, isolator and a few resistors had to be replaced. Testing out of box worked, AGAIN. I put it together and who would have guessed it broke again. For unknown reasons it started blowing fuses I started speculating output transistors again, so I took them out and powered it up and sure enough the fuse didn't pop. I measured the transistors and they measured fine. Okay what the hell do I do now? I measured bias voltages of the transistors one pair had -1.5V the other had 1.5V. Super! So I put back a pair of complementary transistors, and blew them up somehow... Okay, I'm left with one pair of transistors, I put one in and measure its bias it measures 0, the other's bias without a transistor goes up to 2V. Okay odd.. Lets see if the bias goes away if I put in the other transistor while keeping the emitter pin off. The bias doesn't go away and the transistor opens to 2V. I'm speculating that that shunts the two transistors together and that's what caused the pair to blow up... I checked the driving transistors but they're fine. I measured literally every resistor on the board and everything measures fine. I don't know what to do at this point, so I'm turning to smarter people than me! I'm open to any questions, I just wanna get this thing fixed..
TL;DR: Broken relay turned into broken output transistors and now I have no clue what to do, and I'm looking for help.
Any help is appreciated! 😀
With output transistors it is usually the bias voltage being too high that blows them.
Both transistors are switched on hard and short out the rails.
I would replace the bias pot to be sure it is OK. If it is intermittent contact it could blow the outputs.
I would also check the power supply, if its too high for some reason that could blow output transistors too.
A catch 22 I have seen is outputs blowing and they take out speaker.
The amp gets fixed and plugged back into blown speakers and blows again.
Both transistors are switched on hard and short out the rails.
I would replace the bias pot to be sure it is OK. If it is intermittent contact it could blow the outputs.
I would also check the power supply, if its too high for some reason that could blow output transistors too.
A catch 22 I have seen is outputs blowing and they take out speaker.
The amp gets fixed and plugged back into blown speakers and blows again.
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