Yesterday, I moved my amp system back to my workbench so I could use it while sorting through the 6 boxes of junk circuits we have lying around. I had put a Molex connector on the cord coming out of the amp so I couldn't plug the thing in backwards. Well, you can probably see where this is going... Anyways, I had to re-connect the loose wires on the connector to the banana plugs on the power supply. I looked back inside the amp at the power switch. I hooked up the wires accordingly. I should have double checked.
All electronic devices have magic smoke inside them. Let this smoke out and the device will never work again. I let the smoke out. I hooked up the power supply backwards, turned the amp on, heard a pop and felt my heart sink as the smoke rose. The amp had a thick fog inside it which was coming out through little vents inside the case. Man, that stuff smells bad! I got out the screwdriver and started to take apart the case. After I took the whole thing apart, I had to locate the burnt out part inside the mess of wires. I found a blown transistor on each board. It was transistor #5 on each of the boards, a BC547. After taking them off, I had to find a suitable replacement for them. Thankfully, my Dad has accumulated a ton of little parts and has a fair amount of them sorted in little drawers. Since I didn't know anything about how to find an equivalent transistor to the ones I blew, I had to ask Dad. He did a bit of Googling, went to the parts drawers and pulled out a bag of 2n2222 transistors. I replaced the blown ones with the new ones and went to test it. I got music but lots of heat from the driver transistors. After we checked the other transistors for shorts, Dad consulted the schematic and concluded that the power supply was being crowbarred.
Crowbarring is where the two driver transistors in an AB amplifier turn on at the same time which shorts the negative and positive rails of the power supply together. since the rails are shorted, lots of current can flow through the driver transistors which makes a lot of heat. Concluding that that 2N2222 must have been a bad choice for a replacement, Dad went back to the computer to look at the data sheets. After about a minute, he called me over and said, "Josh, what do you notice about the packages on these two transistors?" I looked at the data sheets and saw that the collector and emitter pins on the 2N2222 were in the opposite positions as the ones on the BC547! Oops! once I replaced the two transistors with new ones (in case they got damaged from being put in backwards) and remembered to stick the in the right way, the amp worked perfectly again.
While I had the whole thing apart, I added a blue LED just for fun.
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