SX-1080 - Protection at high volume

Try separating the pre from the main amp and try one of the SX units as a pre.
Or just pull the jumpers and wait.
A wonky pre can cause weird issues.
At the very least, narrow it down to a section or channel.
The wise advice in post# 14 is to be considered.
 
Barkingshins said:
The Power Amp boards in the SX-1080 are AWH-071... and I'm not seeing a Q14 on them.
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My bad, I misread the thread title as a 1280. Correction, has Q8 been replaced on both power amp boards? They serve as the overcurrent sensors for the protection circuit and are known to get flaky and cause the exact problem you are having with the intermittent protection.
 
mattsd said:
My bad, I misread the thread title as a 1280. Correction, has Q8 been replaced on both power amp boards? They serve as the overcurrent sensors for the protection circuit and are known to get flaky and cause the exact problem you are having with the intermittent protection.
Click to expand...
I actually didn't replace any of the transistors on the power amp boards. Do you suggest KSC1845 as a suitable replacement?... Cuz I actually have a few of those laying around.
 
zebulon1 said:
Try separating the pre from the main amp and try one of the SX units as a pre.
Or just pull the jumpers and wait.
A wonky pre can cause weird issues.
At the very least, narrow it down to a section or channel.
The wise advice in post# 14 is to be considered.
Click to expand...
That's a fantastic idea. I should have thought of that. I have a rock-solid SX-950 sitting about three feet away from my workbench.
 
Well I spent about four hours at my workbench tonight and I'm cautiously optimistic that the issue is resolved.
After replacing Q2, Q5 and Q8 (because I just happened to have appropriate replacements for those) on both power amp boards and getting bias and offset back to specs, I was unable to replicate the problem. I was able to get a 1kHz signal at 150mV to output through the dummy loads at max volume producing over 140W before noticeable distortion appeared on the scope.
Not to shabby!
My wife and I played records through it at a decent volume for a couple hours and it didn't miss a beat so, as I said, I'm cautiously optimistic.
Many thanks to all of you who offered help. I can't even begin to tell you how much I've learned from AK and its members in the last 8 months or so.
Have a great weekend!
 

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zebulon1 said:
I the receiver and I like the bench.
Got Plans?
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Thanks! I originally built that bench right after I got my ham license and designed it for my ham shack. That was a few years ago now and I still haven't gotten my shack off the ground. It does function nicely for this kind of work though. I don't have proper plans for it, but I do still have my SketchUp file if you're interested.
 
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