Powered PA Speaker Clip Light coming on with no input

[Andysk]

Hi, I'd love some help diagnosing a problematic speaker. I have a Samson Auro D415 Speaker that is having its clip/peak light (7 in diagram) come on intermittently, even with no input plugged in and with gain/level knob (4 in diagram) set to 0.

On the healthy speaker, when I turn power on the peak light comes on for 10 seconds, then turns off and the speaker works normally. On the unhealthy speaker, sometimes the peak light stays on indefinitely. If I try resetting it without jostling the speaker, I sometimes get the light to come off, but if I leave the speaker on for a while the peak light will come on, even with no output or gain. If I set the unhealthy speaker up to with input and bring the level up a bit, behavior is the same - it will stop playing sound at some point when the gain light has come on. It does not seem related to bumping the speaker or bass vibrations.

I've taken out the preamp and power amp boards, and I can't see anything amiss - all components look good and even solder points look good as far as I can tell. I have some basic electronics knowledge and a meter, any suggestions on what to look for next, or even how to tell for sure if it's the preamp board or power amp? I've attached the schematics as well. Let me know if I can provide any more info. Thanks in advance for any pointers!

 

[Andysk]

Here are some pictures of the boards in question from my problematic unit.

 

[alec_t]

Difficult to tell from the first pic, but are the big capacitors bulging a bit at the top? The tops should be flat. Compare with the good speaker.

 

[Andysk]

Thanks for the response, Alec. I didn't notice anything looking amiss, but I'll look closer tonight and post a better photo of the caps.

 

[Andysk]

Caps look normal. :-/

 

[fourtytwo]

Hello, you have not said if the "unhealthy" unit works or not in all other respects and if it's no signal noise level is normal ? If it works in all other respects then there is no danger of damage by swapping the two preamps to determine what board is causing the problem. If this is a DSP controlled amp the fault is probably on the preamp that usually contains the DSP. It could be out of band pickup for some reason but I would expect both to do it unless there is some subtle differance in the way they are hooked up

 

[KeepItSimpleStupid]

You need to look for the presence of DC at the speaker terminals and the generation of the MUTE signal.

 

[Andysk]

I apologize for dropping the ball on this, I didn't have my notifications set up right so I didn't know I had more responses.

Anyway, fourtytwo, the other unit works in all other respects as expected with tested signals described above. I hadn't thought of swapping the preamp board, I bet it is in the preamp board too, and that would verify.

 

[Andysk]

KeepItSimpleStupid can you give me a little more background? The speaker lights up the "Peak" LED when it's not working (even with no or zero input signal as described above) so I would expect the MUTE signal is getting generated. For "DC at the speaker terminals" do you mean to measure for DC at the output of the power amp? I'm expecting there isn't any because I think the preamp is muting. I just don't know why that is getting triggered.

 

[KeepItSimpleStupid]

This topology is really wierd. LF is usually Left Front to me. here it's Low Frequency and High Frequency.

Look like if F1 or F2 blows you;ll get a constant clip.
A DC measurement from X3-Pin 4 to ground is important.
IC1 is an amp for the high frequencies.

Clip can be generated from the anodes of D10, D11 and D12. Find out which one is causing CLIP OUT to be on.

I can't find the origin of LF AP OUT on R19.