This Is Wierd....

[acorkos]

Marshall MG100DFX head blowing fuses.
I unplug the output transistor module and the fuse holds.
I replaced the TDA7293 chip and plug in the module....there's a short somewhere.
I replace the chip again....something shorted.
I replace the entire module....something shorted.
I take off the rectifier bridge, a 1000uF cap, a resistor and 4 ceramic caps....with the output module connected, there's a short in the power supply on the main board.
With the module disconnected, there's no short.
I check the output module....no short.

What's up with this? I'm stumped. Thanks for your help.

[Twist]

First of all check the 7293 for a short between pins 13, 14 and 15. These are not very forgiving IC's. When I replace them I make double sure that all pins are soldered good and there is no solder touching from one pin the other. Also inspect closely for any broken PC traces. A short between pins 13, 14 and 15 will show up in the power supply.

[cbarrow7625]

How about trying to load the power supply with a simple resistor to see if the manifestation of your problem isn't just simply load induced (as opposed to being a problem with the actual output chip amp)?

Something in the power supply could be going bad & just doesn't show up until it is required to pull some real current. It might save you from blowing up more 7293's in the trouble-shooting process.

Just an alternative thought.

[acorkos]

How about trying to load the power supply with a simple resistor to see if the manifestation of your problem isn't just simply load induced (as opposed to being a problem with the actual output chip amp)?

Something in the power supply could be going bad & just doesn't show up until it is required to pull some real current. It might save you from blowing up more 7293's in the trouble-shooting process.

Just an alternative thought.

hoe would i go about doing that?

[Enzo]

Is the heat sink grounded? is the 7293 insulated from it?

Test this. unscrew the 7293 assembly from the heat sink and let it hang in the air still connected. DId your short go away?

[acorkos]

Is the heat sink grounded? is the 7293 insulated from it?

Test this. unscrew the 7293 assembly from the heat sink and let it hang in the air still connected. DId your short go away?

bingo

heat sink is grounded and 7293 is not isolated from ground

the short is gone when the 7293 assembly is not in contact with the heat sink