Another Marshall MG50DFX with no sound...?

[acorkos]

TDA7293 chip was cooked/cracked/blackened. I replaced it, but now there is no sound...no signal at the red + speaker wire. Preamp is fine...there is normal sound at the headphone jack, and from the effects send jack. There is signal at pin2 of the TDA chip, and all the voltages on the chip are there, just no output. I'm not sure where the problem could be....i'm curious, on the TDA datasheet, pin 10 is labeled 'mute', and this thing is definitely mute. any relation to my problem? Thanks for your help.

[Steve Conner]

I could be wrong, but I seem to remember Enzo or someone saying that when a TDA7293 burns up, it can take out the circuitry connected to the mute pin. When an IC blows out, any pin can end up shorted to any other, so maybe the mute circuit got hit with the rail voltage and doesn't like that.

So check the DC level on the mute pin, compare to what the datasheet says is the right voltage for "unmute". If the voltage is wrong, then look for blown parts in whatever circuit is connected to the mute pin.

[acorkos]

the mg50dfx schematic shows the mute pin (pin10) connected to the +power rail...it's measuring 36V.

[Jazz P Bass]

TDA7293

What reading do you get at the StandBy pin (#9)?
It should ramp up to about 5Vdc at power on.

[acorkos]

interesting....0 volts from pin 9 of the TDA.

i pulled diode D19 out of the circuit...it's shorted. 1N523B 6.8V .5W zener diode. thanks for the help!