Hi group
I have a 240w single channel solid state transformer coupled power amp that isn't working. There is not a whole lot to the power amp board, but I do not have a matching schematic. I did find a schematic to a more recent version of this amp and its close enough so I was able to figure out the main power path, but not the front end, feedback, and protection circuitry. It looks like the company re-designed the front end, feedback, and protection circuitry on this later model. The power amp board had 2 open transistors, 3 shorted transistors, and showed signs of burns. The amp blew fuses when turned on.
I replaced all the bad transistors and an open resistor (burned). I reinstalled the large heat sink that connects to the two power transistors, the two pre-driver transistors, and the bridge rectifier. I verified that all transistors connected to the heat sink were electrically isolated from the heat sink and powered the amp up. It blew a fuse.
I removed the heat sink and the power transistor pair and powered things up. I found very high voltages throughout the output stages (like around 60 vdc) with a large amplitude distorted signal riding on it that was the input frequency I was feeding into the amp. The amplitude of this distorted signal followed the volume control. I assumed that this particular amp cannot be run w/o the power transistors, so I put them back in. I powered the amp back on and it worked. The only difference being I was running without a heat sink now.
There is a fan in this amp. One fan wire is connected to a ground wire that is connected to the power supply. These two wires are connected via a screw that screws into the heat sink. I found that as soon as this ground from the power supply is connected to chassis ground (via the heat sink originally) I lost all audio (but did not blow a fuse or apparently damage anything from the momentary contact). Anyway, I was running this way...no heat sink, fan wire not grounded, just audible output...thinking about what I should do next when snap, crackle, pop I had flames shooting out of the board. Both pre-driver transistors are physically cracked, I presume the power transistors are shorted, a couple of resistors have burned up, and the pc board really shows burn signs now. So, here's my question:
Could this failure be due to just running for 3-4 minutes at extreme low volume without the heat sink? I would guess not, but I don't have a lot of experience with class B power amps. If it could be, it might be worth replacing the transistors and trying again, otherwise I think the garbage is the best place for this amp.
Thanks
Dan