jurb

"** But it is not a class D amp in any case, just a basic class B
design with a a pair of "helpers" to eliminate most of the heat. "

Class B or AB ?


** It just happens to be class B = zero bias current in the output devices.

The full manual and schem is available he

http://elektrotanya.com/yamaha_emx-2.../download.html

All outputs are Darlingtons, some forward bias voltage is provided set by two diodes and a resistor.



"** It was ALL the devices on the heatsink - including a pair of
1amp, 15V regulators"

Well that pretty much clinches it that it was heat.

** Yep.


"** The power amp module was easy enough to get out and then you
access the underside of the PCB by unbolting all the power devices.
A common enough assembly method these days and one Yamaha uses in
a few models."


So, after all that do you clean it and put new heatsink compound on ?


** The module has long strips of insulating material - the grey coloured silicone, re-inforced with fibreglass threads kind. I was able to re-use them with no problem, cos none of the TO3P devices dissipate really high wattages.

BTW: I think the stuff is a poor heat conductor, compared to mica and white grease.


...... Phil