I have a rack of basic audio amps that power speaker sets all over my house. Each pair of speakers is connected via an impedance matching volume control. I noticed one of my volume controls was vibrating/ thumping in line with the bass in music. I failed to react and a few days later, that amp( which powered 2 sets of speakers) failed… Would not power on. I opened the case and found a blown fuse right at the power block. Ordered an identical fuse and replaced it. When I tried powering up the amp( no music), it turned on, but quickly sparked and flames came from a large module on the circuit board.
I am far from an electronic guru, but am trying to understand what happened. The new fuse did not blow. I assume the original issue was the volume control failed( though I only had 2 sets of 8 ohm speakers running from the A and B outputs) and that caused the fuse to blow. But what would cause the sparks upon power up with a new fuse?
I think you initially blew up one or more power semiconductors (driver transistors or switching elements in the power supply) such that they failed short circuit.
The sparks an flames came when you subsequently blew up some resistors through those short circuited semiconductors (or perhaps some diodes or further semiconductors).
Happened to me with a 48V/20A power supply.
I reckon Frankenstein Monster has it. Speakers can have quite a high inductance and if the speaker connection is interrupted during operation then the magnetic field in the speaker coil (generated by the audio current) will collapse and induce a high voltage across the broken connection. This could arc over the break and blow up the amp’s output stage. Depending on the circuit architecture, you’ll need at least a new driver IC/set of output transistors, and possibly some other parts too.
If you’ve got discrete transistors for the output stage then, with power off, you can buzz them out with a multimeter, preferably set to semiconductor junction measurement (a little diode symbol). If it reads close to zero volts then the transistor has melted inside and failed short-circuit. You might see 0.5 to 0.6 volts or so (in one direction) with bipolar transistors, or an out-of-range reading with FETs. Whatever; if you’ve got a known good channel you can compare the components in the blown channel with that. That’s the joy of stereo - it’s rare for both driver channels to blow (unless it’s a stereo IC driver), so you’ve always got a good reference.
Once when I was seeing the doc about my arthritis my warped sense of humor kicked in when he was comparing the flexibility of my elbows and wrists. I asked, “What do you guys do when somebody with only one arm shows up?” He laughed.
There’s an old joke* in electronics: you can rely on a $25 power transistor to bravely give up its life to save a 15¢ fuse.
*this joke is funny, like many others, because it’s objectively true.
I agree with the others… the output stage is probably fried.
The output stage consists of either a power amp IC or discrete power transistors in a Class AB configuration. In either case, when they fail they often short the B+ supply to the B- supply, resulting in fireworks.
Assuming the output stage failed, I would remove the output module or transistors, and then perform a thorough visual inspection. If everything else appears to be O.K., fire it up and check the B+ and B- voltages, first without a load and then with an appropriate resistive load. If they’re fine, install new modules or transistors.
A schematic would certainly be handy, but not absolutely necessary.