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Louie I have a speaker which is humming. I have checked the…

For Benimur only. Louie I...
For Benimur only. Louie I have a speaker which is humming. I have checked the power supply section and everything checks out ok. The output looks to be good. Please advise.
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Customer reply replied 3 months ago
Quick update. When I play with the level knob on the amp I hear a little noise and something that sounds like a faint sound of someone scratching the cone. If the level knob is bad would it cause humming issues?
Answered in 7 hours by:
3/26/2018
Louie
Louie, Technician
Category: Home Theater-Stereo
Satisfied Customers: 13,438
Experience: Have been in the business 25+ yrs., down to component level.
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Hi and welcome to JustAnswer; an independent site with members providing product information, repair tips and technical assistance as a form of paid Q & A service.
My name is ***** ***** I'll be assisting you with your question regarding your "...speaker which is humming..."
I apologize for the extended wait time you experienced, I've only just come online now and would like to assist you with your question.

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Initially, appears to be noise piggy-riding the input.

Let's start with cleaning the level controls using compressed air or better yet, plastic safe / non-staining CRC 2-26, DeoxIT DN5 or LPS 03116 Contact Cleaner or equivalent to clean the insides of these controls but allow sufficient drying time.

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Customer reply replied 3 months ago
I changed the pot with a new one and that fixed the little noise when moving the level knob back a d forth without an input signal. The hum still remains. When I put the level knob all the way up the hum. Does get approximately 5% louder. Please advise.

That the 'level knob all the way up = hum...5% louder' would indicate that the potential source of the hum is before this stage in the pre-amplification.

Let's try check and review the basics. Proposed preliminary tries (if have not been done) and even if just for test purposes:

  • • determine the resistance of the pot's center pin to common ground;
  • • what happens to the hum when the this pot's left pin is shorted to common ground?
  • • what happens to the hum when the this pot's right pin is shorted to common ground?
  • • consider cleaning the board itself particularly the solder points and the gap/s between these points and the trace/s. (old toothbrush + contact cleaner + sufficient drying time)
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Customer reply replied 3 months ago
I used W3 ground on output transistor board and measured 8.13K ohms on the center pinof the level knob.
Customer reply replied 3 months ago
If I use W3 ground on the output transistor board and do the left and right pin ground test will it mess anything up? Do you recommend a better ground?

Use pin 14 of IC U15 as signal ground.

And as posted sometime ago, the amp must not be plugged in when doing resistance checks hence "...mess anything up..." would be most unlikely.

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Then measure the resistance of this pin 14 of IC U15 to chassis / input ground.

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Customer reply replied 3 months ago
34;Use pin 14 of IC U15 as signal ground." I get .1 ohms"Then measure the resistance of this pin 14 of IC U15 to chassis / input ground." I get .1ohms

Please send scope display pictures of pins 4 and 8 of ICs U4, U5 and U7.

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Customer reply replied 3 months ago
I'm assuming with power on and no signal correct?

Yes please; power on and no signal.

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Customer reply replied 3 months ago
It will be hard to take a picture and hold the probes. What kind of wave are we looking for? Can you send me a picture of what it should look like? Also, is it ac or dc?

Normally, DC; both pins should show flat clean lines above and below the 0 horizontal line.

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Customer reply replied 3 months ago
Just so I'm clear I need to measure in DC and I ground to input ground and put the probe to pins 4 & 8 of u4,u5 and u7. Correct?Also, if some other component is shorted to ground the way I check that is to have the black probe on ground and the red probe on the component like a resistor and check each side and if one side is close to 0 ohms that means that its bad. Correct? Is this the best way to find a short to ground? Any other ways?

Yes; scope's GND probe to chassis ground. Input probe to pin 4 and then to pin 8 of the ICs U4, U5 and U7.

The above would check how "clean" is the DC supply to these ICs.

Ideally, a flat line.

This scope test would show if AC component is piggy-riding the DC flatline and is the source of the humming sound.

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Customer reply replied 3 months ago
They are all straight lines Louie.

This then brings us back to the potential ground issues of the input and pre-amp sections and/or motorboating; i.e. low-frequency noise due to a leaky filter capacitor.

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Customer reply replied 3 months ago
Where should I start?

Where should I start?:

  • all the ground connections of the input and pre-amp sections; i.e. all the parts tied to a ground symbol (▼);
  • all the capacitors of the input and pre-amp sections starting with the electrolytic C9, C3, C4, C18, C21...
Louie
Louie, Technician
Category: Home Theater-Stereo
Satisfied Customers: 13,438
Experience: Have been in the business 25+ yrs., down to component level.
Verified
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