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Old 04-10-2021, 12:51 PM   #21
craftech
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Default Re: Powered Speaker Hum When Plugged In

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Originally Posted by budwich View Post
The first post indicated that the sound can be hear at the headphones. This makes it less likely that the transformer is cause a vibration that is producing the electric hum. You need to see if you can follow the input area towards some capacitors and test those. In my limit experience with a "humming" guitar amp, there was a bad capacitor. It did not visually look bad but tested bad.
Would a capacitor cause a hum with the unit powered off, but merely plugged in? I know it gets louder when the amp is turned on and the volume of the hum is constant when it is turned on, but it is present (but low volume) with the on-off / volume switch off when you put it up to your ear as soon as you plug the unit in.

John

Last edited by craftech; 04-10-2021 at 12:53 PM..
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Old 04-10-2021, 05:08 PM   #22
budwich
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Default Re: Powered Speaker Hum When Plugged In

hmmm... don't know. But are you saying that IF you leave the unit off for a "long time", you can still here some sort of noise thereafter? also in the headphones?

the other place to check is the ground "plane" for the entire board. Locate the ground and follow it around especially towards the input area looking for cracks / cold joints.

I thought about your previous post, so I take it you are saying the transformer is humming with the unit plugged in but off?

Last edited by budwich; 04-10-2021 at 05:25 PM..
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Old 04-10-2021, 06:16 PM   #23
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Default Re: Powered Speaker Hum When Plugged In

way i read it is its not turning off .i thought the noise might be keeping it on and asked to short inputs to see if noise went away ..if it is the transformer its likely loose laminations or its drawing too much power or maybe there is dc riding on the mains power .
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Old 04-11-2021, 06:48 AM   #24
craftech
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Default Re: Powered Speaker Hum When Plugged In

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Originally Posted by budwich View Post
hmmm... don't know. But are you saying that IF you leave the unit off for a "long time", you can still here some sort of noise thereafter? also in the headphones?

the other place to check is the ground "plane" for the entire board. Locate the ground and follow it around especially towards the input area looking for cracks / cold joints.

I thought about your previous post, so I take it you are saying the transformer is humming with the unit plugged in but off?
Yes. Plug in the power cord. Put it close to your ear. It hums. Turn it on. Hum is louder and does not vary with volume knob.

John
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Old 04-11-2021, 06:54 AM   #25
craftech
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Default Re: Powered Speaker Hum When Plugged In

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Originally Posted by petehall347 View Post
way i read it is its not turning off .i thought the noise might be keeping it on and asked to short inputs to see if noise went away ..if it is the transformer its likely loose laminations or its drawing too much power or maybe there is dc riding on the mains power .
I thought about unsoldering the two 15vAC transformer secondary leads from the PCB and connecting the 16vAC output of a doorbell transformer to the PCB to see if it hums. I also have a Lionel transformer that puts out around the same amount of AC volts from fixed terminals on it.

If that works, they sell AC to AC wall wart adapters that are 1A 15vAC.

Thoughts?

John
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Old 04-11-2021, 08:47 AM   #26
stj
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Default Re: Powered Speaker Hum When Plugged In

15-0-15 transformers are common enough.
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Old 04-11-2021, 11:34 AM   #27
budwich
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Default Re: Powered Speaker Hum When Plugged In

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Originally Posted by craftech View Post
I thought about unsoldering the two 15vAC transformer secondary leads from the PCB and connecting the 16vAC output of a doorbell transformer to the PCB to see if it hums. I also have a Lionel transformer that puts out around the same amount of AC volts from fixed terminals on it.

If that works, they sell AC to AC wall wart adapters that are 1A 15vAC.

Thoughts?

John
that will work fine for a test to see if you notice any difference. Any "brick" adapter or otherwise will work also as long as its 15v AC or there about on the output and has some form of adequate power output.
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Old 04-11-2021, 02:23 PM   #28
craftech
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Default Re: Powered Speaker Hum When Plugged In

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15-0-15 transformers are common enough.
There are only two leads coming out of the secondary. Decal says 15v 1A Red - Red
John
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Old 04-15-2021, 05:23 AM   #29
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Default Re: Powered Speaker Hum When Plugged In

Tried the external transformer. Same hum.

I don't know what to try next. Replace the smaller caps around the larger one?

By pass the on-off switch / volume control (defective)?

John
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Old 04-15-2021, 06:52 AM   #30
budwich
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Default Re: Powered Speaker Hum When Plugged In

it basically indicates that it is not a transformer issue. Most times "hum" is a grounding issue... usually lacking. Look / check the "grounding / common" planes on the board for cracks or poor continuity.
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