marshall mg100dfx output IC replaced problem

[pontiacpete]

Hi, This amp came to me with the fuse blown. I replaced the fuse and powered up on a limiter and it shows it was shorted.
I unplugged the connector to the output chip and the amp powered up. Replaced the TDA7293 and mounted it to the heat sink and it still showed a short when powered up. When removing the output chip board from the heat sink, the amp powers up and works.
Perhaps someone messed with this before me because I'm wondering if this chip needs a thermal interface between it and the heat sink.
Does anyone know if it does?

[drewl]

If the heat sink is grounded, then yes, the chip needs an insulator.

Something I noticed about light bulb limiters this past weekend.
Sometimes they don't let an amp power up properly if a load is connected.
It happened on a couple large bass amps I was working on. If you power up the unit, THEN attatch the speaker/load they would power up correctly first.

Maybe because I was using a low wattage bulb, or maybe some guys already knew this.

[alexradium]

Tab is connected to -Vs so yes,you have to isolate it.

[J M Fahey]

If the heat sink is grounded, then yes, the chip needs an insulator.

Something I noticed about light bulb limiters this past weekend.
Sometimes they don't let an amp power up properly if a load is connected.
It happened on a couple large bass amps I was working on. If you power up the unit, THEN attatch the speaker/load they would power up correctly first.

Maybe because I was using a low wattage bulb, or maybe some guys already knew this.

Yes, many amps draw a large current pulse while being turned ON and if they canīt get it, they latch towards one rail and stay there.
No load (at turn on), no big current pulse.

[pontiacpete]

Come to find out, this amp uses 4 plastic or nylon shoulder washer-like spacers that are also used to hold transistors to heat sinks to attach the fan to the back of the chassis, while isolating it from ground. Seems like it would be easier to just isolate the chip from the sink. But maybe there's another reason why they decided to do it that way.
Thanks for the help guys!