Amplifier problem

[Empusa]

Hello!
First of all, i don't have much electronic knowledge, but here it goes.
I have a small stereo amplifier with two 3W/4ohm laptop speakers with usb source.
When my friend asked me to see what's wrong with it, it was completely dead, which proved to be the audio cable.
So i replaced the wires, but now one channel is buzzing increasingly louder with the volume. So if the volume is low, the buzzing speaker is kind of ok, but the more i increase the volume, the buzzing distorts the sound so badly that i can't even understand anything on that channel.
And the problem is definitely on the board since i swapped wires, speakers and anything i could to exclude other issues.
Then i started to measure the components, resistance values, voltages, they are identical on both lines and it's getting really annoying. I don't have a pump now to take out components and check them individually.
So at this stage i have only two options. Either i pull them all out from the buzzing side and measure them, either i couple the stereo signal into a mono one as i saw through a resistor bridge, which i am not sure yet of what values.
But i would really wish to fix it, regardless the fact that they are so cheap, i could sneeze and it would be more expensive.

https://imgur.com/GxUzfRf
and the two amplifiers name is: hxj8002

Thanks

 

[Les Jones]

I am assuming that the wire that you repaired is the one on the left hand end of the board. When you say a "USB source" do you mean that the sound source is a USB to audio converter or do you mean that the amplifier is just powered by the 5 volts from a USB socket ? I assume that the wire that you repaired connects to the headphone socket on the laptop or a USB to audio converter. As the volume control seems to be directly on the input of the amplifier I think it is more likely to be a fault before the volume control. Start by unplugging the wire from the sound source. Does the buzzing still occur ? If it does then unsolder the two signal wires from the board one at a time and note the results. Leave the common ground wire connected. This is the top connection in your picture that connects to pin 5 on the volume control. (Counting the pins starting with pin 1 at the left.) Report back with the results.

Les.

 

[Empusa]

I am assuming that the wire that you repaired is the one on the left hand end of the board....

Ok, sorry for not giving all details.
Yes, the USB is for the power only, because i said i fixed the audio cable. The volume control is fine, since i already bypassed it before i wrote here. If i pull the audio out, the buzzing stops. It is more like a distortion which gets amplified with volume. I said that i already did this, swapping channels and the problem is definitely on the board, on the left amplifier, as seen in the picture.
I noticed that if i measure the voltage on the C16-R20 junction and then measure on the C13-R21, it is not the same. I have less voltage 2.25 on the first one than on the second 2.55, which is not much, but the amplifiers are supposed to be identical
Anyway, I'm going to buy a soldering pump and return later, if you have any other ideas, i'll test them later

 

[audioguru]

You said you replaced the audio cable. Did you use shielded audio cable where the shield blocks hum interference pickup? Or did you wrongly swap the audio wire with the shield wire? Or does the new shield wire have a bad connection?

 

[Les Jones]

Hi AG,
I think from the description in post #3 the fault is more distortion than hum pickup in the input cable. I have found the **broken link removed**. (It is a bridge amplifier.) The fact that the voltage in the input pin of the faulty channel is a bit lower than the working channel suggest that there may be a DC offset is causing the distortion. If the supply voltage is exactly 5 volts then I would expect the DC voltage on the input to be very close to 2.5 volts.
Empusa,
Can you measure the voltage on pins 2, 3, 5, and 8. I would expect these to be very close to 2.5 volts . can you also measure the voltage at the junction of C16 and R19 with the volume control turned up high. The voltage here should be zero. This test is to see if C16 is leaky pulling the bias voltage down on the input pin. Can you post the results of these tests ?

Les

 

[Empusa]

You said you replaced the audio cable. Did you use shielded audio cable where the shield blocks hum interference pickup? Or did you wrongly swap the audio wire with the shield wire? Or does the new shield wire have a bad connection?

Oh c'mon, i said i don't have much knowledge, not zero. That is basic stuff. Thanks anyway for the intention

Hi AG,
I think from the description in post #3 the fault is more distortion .....

Sure will, after i finish cooking
So i went to buy the soldering pump or whatever it's called and i was so thrilled i found one close to me and cheap that i didn't see that the tip is made out of hard plastic, duh!, why would anyone make the tip hard since it's supposed to be rubbery to create enough suction on the pins. Cheap stuff, bleah

 

[Tony Stewart]

Melting point drops with rubber and soft plastic .. Add solder then easier to suck...
This may help.