Odd output clipping signal JBL GTO 75.4 audio amplifier

[OiD]

Hello all,

I've been handed over a JBL GTO 75.4 4 channel 104W RMS Amplifier but im having some issues with it.

When testing the unit in the lab I noticed terrible performance and output power was only about 56W of the 280W (bridged outputs) available. I decided to hook it up to the scope and turned the input volum up and the attached image is the combined waveforms of the volume levels.

There is a moment that the signal starts to clip, but its not a "hitting the rail" kind of clipping I would expect. This happens with all channels on the amp.

According to the previous owner the unit worked great. Now the speaker smells of hot enamel from the "DC" clipping.

There is no power supply shortage, 14.4V 30A power supply and a 2-5F capacitor 15cm from the amplifier power connection. Poked it with a scope and nothing to see there.

After thinking for a bit I've been using a tone generator on my phone to test the lower frequencies so thats the first thing I will check today with a proper signal generator.

Other than the source signal being inadecuate, is there anything else to check for? I believe it's a class AB amp.

Thanks and regards!

[Armadillo]

What kind of input source did you inject in? Is it 3 wire XLR?

[OiD]

Hi Armadillo,
It was a simple 3.5mm jack to Stereo RCA jacks. The more i think about it a smartphone might not be able to provide a proper signal output... but I was testing at 50hz and I know i've listened to lower frequencies with headphones.

[Armadillo]

It must be balanced input. The signal are 180 deg apart and the ground wire to common.
See attached.



You can escape with a cheap isolating audio transformer for *repair testing purpose* as adapter. You know those cheap red, or yellow or blue type. Secondary centre leg serves as "GND". The left and the Right legs serves as 180 deg signal input.

[OiD]

Thanks, I didn't quite catch that. I will keep it in mind later and see if that is the cause. I'm still a bit confused with how the amp handles the stereo input in bridge mode. More testing will be required!

Of course if I use a signal generator with both outputs at 180º i'll get the same effect, right?

[Armadillo]

"floating". *****
And remember to turn down the volume : watch input limit.

[OiD]

OOOOOkay, it blew up. 

A signal cap blew its top  I cleaned the mess up, setup the differential probe, current clamp and went for a 60A power supply (at 14V this is the most powerfull supply).

Now the amp is pushing around 140Vp into the load and 50Ap from the supply about 46v 28A RMS. Still clips a bit, will keep checking, especially for other bad caps!