Gallien Krueger Backline 115 amp protection circuit kicking in at low levels

[cablercdd1]

I have a Gallien Krueger Backline 115 amp that goes into protect mode at low to moderate levels. I've included the schematic. Does anyone understand what triggers the protect on these.
Thanks

[Enzo]

First off, does it do this with NO speaker or load connected? is the load you are using meet the impedance spec? ANy chance the speaker wiring has shorted across? Does it sound OK up to the point of protect?

What does it? Look at page 1 of the schematic. On the right, see Q11 samples the voltage across the output transistor emitters. That voltage gets high enough, Q11 turns on, which turns on Q10, which activates the FAULT line.

Now look to the left side, under the transformer. FAULT turns on Q13, which trips the timer chip, the 555, and that turns on the fault light and activates the MUTE.

Does the amp MUTE when it lights protect?

[cablercdd1]

With no speaker, the protect light doesn't come on. Interesting, I hadn't thought of that. I was using a resistive dummy load set at 8 ohms but with an 8 ohm speaker plugged directly it still does it. It does mute when the protect kicks in. I also noticed the bias voltage across J2 goes away when it kicks into protect. That may be a clue. I checked Q10, Q11, Q13 and Q9 and they all turn on and off when I go in and out of protect.

[Enzo]

Look back over top center, below the differential pair. The mute signal turns off the differential pair, shutting down the power amp.

HAve you adjusted the bias yet? it might simply be biased too hot. At idle, what voltage is between J2-1 and J2-2?

Q11 has timing cap C18, is it healthy and connected?

[cablercdd1]

Enzo, It looks like the bias was set way too high at 38 mv. The schematic calls for 5 mv at idle so I adjusted it and it no longer cuts out. I don't have a lot of experience biasing solid state amps and have had one or two blow up on me when I did, so I kind of shy away from touching those. I think someone must have been messing with the pot. Thanks a lot for your help.
Paul

[Enzo]

Or the parts simply drifted over time.

Glad you got it sorted out.

[cablercdd1]

Enzo, I meant to reply earlier but forgot. I checked it out the amp again the next evening and discovered it was still doing it just not as bad. Based on the info you gave me I decided to take the heatsink off and take resistance measurements of the bias resistors. One of the bias resistors was reading 10 ohms instead of .1 ohms. It must have put the whole bias circuit out of balance. I replaced it and it fixed the problem. turned it up to 11 and it didn't shut off.
Thanks again for your help
Paul

[g1]

I assume you rechecked/reset the bias after replacing the bad resistor?

[cablercdd1]

You assume correctly

[bobot62]

need help guys, my backline 115 lacks high, i have turn the treble full but the high still lacking

[J M Fahey]

need help guys, my backline 115 lacks high, i have turn the treble full but the high still lacking

Please start a new thread about your problem, this one is about something different.