I would be inclined to remove all the transistors on the errant channel. Then go round with a DMM and check that all the resistor are as they should be. Then assuming all is well put a brand new set of transistors in and check very very carefully, using a magnifying glass if necessary, for any shorts or broken tracks before you even consider applying power.
The best way to test power amplifiers is by using a current limiting power supply. That is what I use and it has saved my bacon many times.
As an after thought, the amp could be oscillating when switched on. If so, that will wreck the output stage in no time at all.
If you have access to a scope, you need to monitor the output at switch on and see if this is so.
If oscillation is taking place then you need to check any compensation components.
Also, it would be worth checking the out put offset voltage when you power up. It should be less than 100mV and if you can at the same time, check the quiescent current which should be less than 50mA I should imagine.
HI Whonoes ! ... Thanks again for coming back with comments.
Peter actually recommended just changing out all semi's which I'm still considering since my shopping cart for all that is still under $30 CAD. Just a lot of work to do it all but I have done crazier things for a small amount of pay.
Unfortunately I do not own a nice current limiting PS ... would just LOVE to have one !
As far as I know ... the possible cause of the initial failure was due to a speaker wire extension being needed and he maybe shorted out the speaker line and viola POP ! I replaced the output stage once ... happened immediately again. Then got the whole rig into the shop and replaced again ... 4 seconds after power up POP ! If you refer to the previous post to Martin ... I mention a possible screw up on my part installing the current sensing resistors. This has been addressed and totally eliminated as a cause for failure again.
again referring to above response to Martin ... I have spent many hrs checking and triple checking ALL semi-conductors in the amp ... none found to be bad. However, all checks were made in circuit ... your idea of removing all semi's is a great idea but a lot of work.
With all the hrs of work I have put in to date ... I think my hourly wage is about 50 cents per hr ! at this point ...
But it's become PERSONAL now ! and I rarely concede defeat to an electronics device !
I do have a scope to see if the amp is oscillating but it blows so fast I'm not sure I could make that test fast enough and if I could ... you say look for compensating components ? I'm not sure I would know which components to even start looking at ... any ideas ?
I think I will start with the light bulb power up to start with using the variac as well. Only issue I might see with using variac is the switching power supply won't start working properly until it gets to the right voltage ... then it may pop again possibly ?
I also mentioned a post or two ago that the datasheet for the mains trannies says continuious drain current rated 23 amps ! If these went over that rating I would have to think the main AC fuse should have been taken out for sure. What would your thoughts be about that ?
At any rate ... I will attempt the bulb trick and keep y'all updated ...