N_Cook wrote:

This variant , they've sorted out the bad thermal air path "design" for
the main heatsink, but introduced another.
Zobel network involving 2x 4.7R 5W in parallel litterally back to back
and also hemmed in, touching, by the caps, relay and the copious white
gunk.
A pathetic looking flat baffle plate that presumably was someone's idea
that the inlet air would then magically divert over the tops of these
Rs, unlikely.
So instead of 10W rating , presumably effectively derated to about 3W.
Etiology looks as though PbF failure of one R solder joint (burnt pcb
hole in the pcb around the pin ) leading to that R actually cracking up
and going o/c and eventually the other one going high, and along the way
the heat cooked one of the paralleled caps to short circuit . Also
cooked and melted relay case , so the contact is permanently closed,
explains the switch on and off thumps/chirps , over a few weeks, that
did not used to be there.



** Typical scenario when an amp has been made to oscillate at a supersonic frequency. The WW resistors in the Zobel overheat, destroy whatever is next to them and then detach from the PCB.

If the output devices have survived, repair is simple enough. What event caused it is not a job for a tech but requires a detective with police powers.

Had a DJ customer one who blew his power amp up twice because he did not appreciate the difference between a speaker lead and a signal lead. When he came to me the second time, trying to claim warranty on my repair work, he left very disappointed.


..... Phil