On my last post, I agreed to take a look at an XLS202 power amplifier brought home by my daughter. It belongs to a friend of hers, and it needs fixing.
First, we need to know what the problem is with this amplifier. So I plug the power amp, and turned it ON. It came to life, power lights on, fans running at full speed, all LEDs also turned on, including the SIGNAL, CLIP and FAULT light, and then everything turned OFF automatically.
Okay…. So I tried turning it on again, and this time it’s dead… nada. The fan didn’t even moved. Hmmmm….
I downloaded and printed the schematics off the Crown website and began studying them, trying to make sense of how everything works, signal path and any “logic” control circuits, etc.
While waiting for the printer to finish, I turned on the power switch again. This time, the amplifier came to life again, all lights on, fans on, then everything shutoff again automatically.
Okay…. there’s power coming into the system, the amplifier is trying to startup but “something” is triggering the amplifier to turn off again.
So somewhere inside this amplifier is a relay that cuts power to itself. Checking out the schematic, I see there is indeed a relay on the AC power line.
The big question now is “Why is it cutting power to itself?” Obviously, the answer is to protect itself. But what’s the condition that’s causing the amplifier to protect itself?
Reading the specs, it gives us a clue…
The amplifier hasn’t been turned on, so it possibly couldn’t be triggering thermal protection. The circuit breakers are good, since power is being delivered to the amp… it’s just shutting itself off after a second or two. There is no load/speaker connected so it couldn’t be output current limiting, so that leaves DC protection feature could be triggering the power shutdown.
Digging through the user manual, I found this bit of information too.
So now we’re getting somewhere… if DC offset voltage at the output exceeded 2 Volts, DC protection will kick in and BOTH channels will be muted.
There you go, once DC protection kicks in, the RED led lamp lits up, and the PWR and OUTPUT RELAYS are energized. The OUTPUT RELAY disconnects the speaker load from the amplifier (so if there is indeed DC voltage at the output, it doesn’t travel to your speakers and burn out your speaker coils), and the PWR RELAY effectively cuts off 110Volts AC power to the whole amp itself!
So there must be DC voltage at the output…. that’s my theory, and it’s time to open the unit and start poking around.
to be continued…