amp I bought 3 days ago not powering on - electrical engineer advice wanted

Same happened to me recently to a new-to-me preowned active subwoofer for one of my lesser systems. Everything was fine …purchased from a dealer and then suddenly …,, won’t start up.

After the preliminary checks without apparent cause, it was pro tech evaluation time.

RESULT: Poor motherboard design with the caps positioned tooclose to the power supply …. It was creating a surplus of heat . Over time, the accumulated heat exposure dried out the caps and the capacitors  were now in tipping over into failure mode and the internal amp would not start out of standby mode …..and voila …,,new capacitors  needed to be soldered in.

TAKEAWAY: it could one or more various issues discussed in this thread .. go figger. 

@parkergetdean 

Sorry to hear about your amp.

Looking at nudies online, the A308 is a true dual mono. That is, both channels are separate pretty much from the power switch onwards.

If both channels are dead when it stops working, then the fault is in the section that’s common to both channels. Which is kind of good news because that section is pretty small.

Your user manual says this:

Press the POWER button on the A308
amplifier - the blue power LED on the front
panel will light indicating that the unit is
ready for use. However, for about five seconds
no sound will be heard from your
speakers, and the red mute LED will light to
confirm initial mute action.

In other words, each time you turn it on the unit checks for DC offset on the speaker outputs. That’s protection mode. If no DC is detected, as is normal, then you should hear a relay clicking as the power LED turns from red to blue. The amp has come out of protection mode and is ready to use.

If you hear no click and the LED stays red, then like Atmasphere said the protection circuitry should be looked at first. Guess that tiny toroidal is what powers it.

For completeness’s sake, an amp that won’t come out of protection either means that there is a downstream fault and the protection circuit is doing its job, or the amp is OK and the fault is in the protection circuit itself.

It’s kind of unusual in your case that the fault is intermittent.

One last thought: the remote has a mute button. It’s a long shot, but just to check that box take the batteries out of the remote and see if that makes any difference.

Good luck!

excellent diagnosis @devinplombier !

I baked some pastry for you to come over and look at it. But you did it remotely.

I read those steps a few days ago and didn’t solve it.

One more clue:

When I turn it on, and it does not work, in about 8-10 sec, the volume dial’s light turns on and makes a popping sound, sends a signal to the speakers. When it works,  the source selector light comes on (the first of six - phono), and it takes the same time for the dial to lit up but no pop sound. 

What it does not turn on, the selector does not light up. So obviously, the power does not go to the sources, the relay cuts it?

I tried  the mute on the remote. Replaced the batteries. Does not work at all.

Most sites on line where you purchase used components will make the seller take it back if was advertised is working condition which apparently it is not. Message seller and tell them what happened. Stick with it. Over the years I've returned 10-15 components that didn't work right mostly from eBay.