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Hi all, I've got a Backline 600 that's giving me some nasty crackling and noise, which sometimes goes away after the amp warms up, even without the preamp board connected at all - meaning it's the power amp or power supply. Scoping the power supply rails, there's a bit of ripple on the +-60v but nothing that matches the crackling at the output. Power supply seems fine otherwise; +-60v are nearly exact, +-35v and +-15v are within 10%, and no detectable noise on any of them. Bias is set correctly too.
Noise at the output is ~100mvP-P, and crackling can be up to 5vP-P, maybe higher (hard to catch on my analog scope). I can see the noise when I probe Q14/18 base and emitter (perhaps it's coming from Q7A/Q7B, hmm...). Once the amp warms up enough to stop crackling (somewhere around 40°C or ~100°f), noise at the output is around 1mv. Amp delivers plenty of power even while crackling, and fan turns on and off properly.
Here's the schematic for the power amp section:gallien-krueger-backline-600.pdf. Didn't bother with preamp, since it's completely disconnected for my testing. Haven't pulled the board out yet so I haven't checked for cold solder joints. Previous owner told me that a "resistor" had blasted apart, and indeed there was a small chunk of something rattling around in the amp, but that something looked like the plastic/rubber/whatever coating from an NTC thermistor, though the two in this amp aren't "blasted apart", and the amp wouldn't function at all without them. No signs of anything burnt or scorched at all (though again, I haven't flipped the board - will tomorrow).
Any thoughts? I'm more used to things with tubes in them.
Noise at the output is ~100mvP-P, and crackling can be up to 5vP-P, maybe higher (hard to catch on my analog scope). I can see the noise when I probe Q14/18 base and emitter (perhaps it's coming from Q7A/Q7B, hmm...). Once the amp warms up enough to stop crackling (somewhere around 40°C or ~100°f), noise at the output is around 1mv. Amp delivers plenty of power even while crackling, and fan turns on and off properly.
Here's the schematic for the power amp section:gallien-krueger-backline-600.pdf. Didn't bother with preamp, since it's completely disconnected for my testing. Haven't pulled the board out yet so I haven't checked for cold solder joints. Previous owner told me that a "resistor" had blasted apart, and indeed there was a small chunk of something rattling around in the amp, but that something looked like the plastic/rubber/whatever coating from an NTC thermistor, though the two in this amp aren't "blasted apart", and the amp wouldn't function at all without them. No signs of anything burnt or scorched at all (though again, I haven't flipped the board - will tomorrow).
Any thoughts? I'm more used to things with tubes in them.
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