Car audio 4 channel amplifier noise repair

[justin66]

Hello im desperate here. I dont find any flaws in the amplifier. But it has background noise like a pink noise sound. It doesnt change on gain adjustment. I replaced all electrolytic caps on it. And a couple of regular ceramic caps. I also bought a second one and it has that noise too. Ignore the power supply caps, noise is the same with out the caps. And their esr is also low
Amplifier ground zero gzta 4.120mk2
Video
https://youtu.be/-uS9Z5FwyL8
Pictures
http://www.part.lt/img/a523553312902c0cb3855acc401635c5886.jpg
http://www.part.lt/img/150ca75a606095b927946ad4ce53d4e1755.jpg

[Armadillo]

Try powering up with car 12v battery [maintenance free or the lead acid battery] to see if the noise disappear.

[justin66]

The noise was present in a car. Before taking it apart. Powering it with battery brings up the same result now. Prety noticeable pink noise with out any audio source connected.

[justin66]

How can i fix this?

[Armadillo]

For the schematic, I joined remont group, took 2 tests, spent 2 points to download. Maybe Russian site?

Nothing spectacular about the amplifier.

Use a oscilloscope to detect what channel the pink come from the 4 channels.

For all the R103, R203, R303, R403, temporary terminate a 2K from the said resistor to ground, one at a time to identify the channel.

its a 4125X;

http://remont-aud.net/load/car_audio/ground-zero/ground_zero_gzta_4125x/338-1-0-39197

[justin66]

Ok i joined the site, but i failed the test 4/6 ansvers i didnt know the signal shown on the osciloscope.  Anyway can you share the schematic by any chance?

Now i discovered that one of the biasing diodes was broken. I replaced it with the exact diodes from the second amp because i have 2 of them but both have the same hiss problem. I also replaced all electrolytic caps. The problem persists. I discovered that voltage drop across the biasing diodes is different. I think it is supposed to be the exact same. I attached 200 ohm resistor in paralel of one biasing diode. The hiss disappeared, but audio quality was terrible.
Hiss sound is across all the channels shorting input doesnt help. I will try grounding the suggested resistors

[Armadillo]

On a second thought, that pink noise, most likely emanated from the stepup power supply.
To confirm, temporary remove the 12v power to the transformer and apply external +v and -v power to the amplifier [and gnd].
the wink should be disappeared.
Check the zener and the all capacitors there.

[Zucca]

why not tap in with a LAB PSU to generate the V+, GND and V- and see if the noise is still there?