Another Marshall avt thread

[Milcman]

Greetings. So I’ve read pages and pages of avt issues but none seem to match mine. I had a pop then the front leds went out. I replaced the 2 2.2ohm resistors and yay fans and lights back on BUT. I got an awful hum through my speakers. With it without a guitar plugged in and no matter where my vol settings are this hum is the same very loud. Upon investigation I have 45/50v at loudspeaker left Jack. Headphones and emulated line out next to headphones. The hum is present through the headphones socket and the fx lines make no difference, I’ve put a brand new jj tube in too. I’m completely lost, why do I have this voltage on the outputs I can’t see any visible issues with the pcb or components. Any help would be fab guys. Ta

[Milcman]

Avt 275 btw

[nosaj]

Most likely lost your output transistors. schematic attached
nosaj

[Enzo]

Most bad parts don't look any different. DC on the output is a very common failure mode of solid state amps, particularly the one-chip type.

When an amp goes to DC, the result will almost always be a loud hum. TURN IT OFF, that "loud hum" will destroy your speaker.

Most likely a bad TDA7293

[Milcman]

Most bad parts don't look any different. DC on the output is a very common failure mode of solid state amps, particularly the one-chip type.

When an amp goes to DC, the result will almost always be a loud hum. TURN IT OFF, that "loud hum" will destroy your speaker.

Most likely a bad TDA7293

ah ok. I’ll look at replacing them. I’ve read I need to make sure the caps are discharged before plugging back in?

[Milcman]

Most likely lost your output transistors. schematic attached
nosaj

Thanks I’ll look there then

[Jazz P Bass]

Ive read I need to make sure the caps are discharged before plugging back in?

Most definitley a Yes.

[Milcman]

Most bad parts don't look any different. DC on the output is a very common failure mode of solid state amps, particularly the one-chip type.

When an amp goes to DC, the result will almost always be a loud hum. TURN IT OFF, that "loud hum" will destroy your speaker.

Most likely a bad TDA7293

so both are replaced with all the caps reading 0 before I plugged anything back in. Still the same no change. I’m at a bit of a loss where to go next to be honest

[nosaj]

Are you still getting the loud hum? Did you already have the tda7293 on hand?
nosaj

[Helmholtz]

When an amp goes to DC, the result will almost always be a loud hum. TURN IT OFF, that "loud hum" will destroy your speaker.

Just to clarify, it's not so much the loud hum but rather the (inaudible) DC current which destroys the speaker.
45VDC across an 8 Ohm speaker will heat the voice coil(s) by around 250W without a chance of convection cooling as with AC.
The audible hum is just increased ripple caused by the excessive DC current the power supply is delivering to the speaker.

[Milcman]

Are you still getting the loud hum? Did you already have the tda7293 on hand?
nosaj

I installed two stmicroelectronics 7293. The hum is actually now present on both loudspeaker inputs. I can’t see anything else that could be the issue.

[nosaj]

Disconnect your speakers and see what dc voltage you have at the speaker terminals.
nosaj