I know this is an old thread, but here's something to be added.
I have an AVT50H that wouldn't power a cab. The TDA7293 was bad. The effects send worked fine, no headphone output.
I replaced it, checked components on the power board, put it back together and found that it would intermittently power a cab.
The headphone jack worked fine. I cleaned the contact surfaces on the
headphone jack and slightly tensioned (bent) the contacts with a jack installed in it and now it works great!
Remember, the headphone jack breaks the circuit going to the speaker
outputs and if it has bad connections/broken solder joints then no power will go to the speaker.
So if you can use headphones on these amps, then the TDA7293 is ok and the problem is in the
headphone jack or the circuit from the jack to the speaker output.
NOTE: IF REPLACING THE TDA7293, BE ABSOLUTELY SURE YOU DISCHARGE ALL CAPACITORS IN THE AMPLIFIER AND ON THE POWER BOARD BEFORE REPLACING OR THE CHIP WILL BE TOAST BEFORE YOU EVEN TURN THE AMP ON!
You can find the proper schematic at
www.schematicsunlimited.com