Zeck A900 power amplifier - attenuated (right) channel problem?

Hi guys, I bought a prehistoric 😀 piece of final amplifier...I found that the (right) channel is much attenuated compared to the left.
The sound comes out without any noise or anything else....but the right one is about -12db attenuated compared to the left one.
Once turned on, the amplifier detects no errors, green light for both channels.
In your opinion are the potentiometers db regulators or protection relays....or something else.
What do you recommend doing...

Thank you
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...I disconnected the VUmeter circuit with attenuator potentiometers...I found the reconstruction of the tracks and a burnt resistor + capacitors, these burnt components connect the central pin of the potentiometers to the amplifier case but the track was cut... I do not understand...
At this point I thought of bypassing the potentiometers with a 22K resistor... and seeing if this is the cause... what do you think?
 

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Hi guys......I had some time to investigate this power imbalance between the two channels...I saw that the input signal is balanced, it passes through this board where there is an OPAMP TL072CP , then the signal comes out unbalanced to go to the amplifier board.. in your opinion, is it possible that this OPamp is not working well? ..Thank you
 

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