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2270 amp makes crackling noise after outputs installed backwards

runningdog

runningdog

AK Subscriber
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I reinstalled the 2 output transistors backwards on my 2270 left channel amplifier. Now that amp makes a crackling noise, like a transistor going bad. The noise starts 5 to 45 seconds after turning on the receiver.

It makes the noise even with the output transistors unplugged. Oh, and R786 gets smoking hot within a few seconds of the crackling noise starting.

I have pulled and tested every transistor, and all test good. I replaced the differential pair and the driver transistors to no affect. It has all its original transistors back in it now.

Should I keep fussing over the transistors or look elsewhere? I've focused on them as the most likely to be damaged by my putting the output transistors in backwards.
 
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R786 is near output transistor H758. See the board layout attached. Thanks.
 

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It's the base emitter resistor. Either 100ohm or 120ohm.

What's your dc look like at the speaker terminals?
 
DC at the speakers was a little high. I now have it at about 0.0mV and the crackling seems to have gone away.
 
Just curious how high was it?

Too high I would imagine would cause saturation but not sure if it would cause the cracking sound your getting as I've never heard it. Others may chime in in that.

There may still be an underlying issue as to what is causing r786 to be getting hot. I'm certain it should run somewhat warm but not smoking hot.
 
It was crackling and popping at 43mV. When I got it down to 'zero' it stopped.

R786 only runs hot when the crackling kicks in. It's like a switch being thrown, very audible. When it is quiet R786 runs cool.
 
I'll check both of those, thanks. It turns out that it still pops and then starts crackling. It just takes longer for it to start up, like 20 minutes. And it pops and crackles just as loud when the output transistors are not installed.
 
I'd replace the input transistors (diff pair) with an Hfe matched set of KSA992's first then see what happens.
 
Dang! On R768 getting warm, it may be that the zener it is dropping current for is intermittantly shorting out (H763). Less likely, the filter cap may be the culprit equally as much (C760). If that voltage is dancing around you'd hear it. Try clippng a voltmeter on it and observe the voltage for stability, an o-scope would be even better.

Other than that, you might try swapping H753s between channels (yes, a pain), it's electrically just after the diff pair, to see if it is the bad guy.
 
I think you called it right on the H753 (2SC845). I swapped in another 2SC845 and have not had a pop or a crackle for several hours. So I replaced it with a KSC1845 and I'm testing it again.

When the original transistor was in and I was poking around with the oscilloscope I did not pick up the crackle pop at the differential pairs, but I did at H753. Which helps confirm it is this transistor it was originating at?
 
Hours later and it is still running fine. H753 was the problem.

I thought, perhaps foolishly, that my $20 transistor tester would tell me it was bad. But it tested fine.

Thank you Randy!
 
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