Problem with my power amplifier Oberton B1200

[HackoBoy]

Hello
I have two amplifiers, one of which had a problem with the output relay and I changed it. But I noticed that one channel hums weakly, and the other more loudly.
After swapping the terminals to the preamp, the hum moved to the other channel.
When I tested with an audio signal, the hum comes from a component mounted in the feedback of the input TL082 (opto-resistor) and the closer my finger is to the housing of this element - the hum intensifies, if I touch it even louder it hums.
It serves if the input level is too high - to limit the signal (indicated by an LED)
How to remove this buzzing (if there is no applied voltage to the optocoupler - the output shows infinity, if there is an applied voltage - the output shows 300k ohms)

 

The other phenomenon
when there is an audio source connected - and the potentiometers are at a minimum - the outputs of the amplifiers play weak music.

[HarryA]

"After swapping the terminals to the preamp, the hum moved to the other channel."

Would not this suggest the problem is in the preamp?

[HackoBoy]

I tried the following today.
It is difficult to test the speakers and I collected test cables (audio headphones connected in series through a small capacitor and resistor) and I was able to hear that the sound from the preamplifier was clear. And when I adjust the output level of the preamp - there is no noise at minimum, and no sound at all.
But when I connect headphones to the output and have reduced the amplifier to a minimum - a slight music + buzzing is heard, which intensifies when I bring my finger to an element of the feedback of the TL082 (through which the amplification is controlled)

[HarryA]

I once had an amplifier that played music with no inputs. It turned out that it was a local am radio station; the results of poor grounding. If you have a wee soldering iron try resoldering the ground on the TL082 op amp. Perhaps that will help.

[HackoBoy]

I got nervous 

initially, one channel of the speakers hummed very weakly and was almost inaudible, but directly through headphones at the output, a hum is heard

the problem turned out to be in the amplifier. I found where the input signal of the amplifier is. Even if I short the input of the amplifier it still hums.

[HackoBoy]

Hello.
Where can I buy a shielded audio cable?

The problem is that two Oberton B1200 amplifiers - at minimum amplification have Brum in the outputs + very quiet music.

When I gave up after all the measurements - I slightly moved part of the cables to a different position (now to the toroid, then to the radiator of the final amplifier) the noise started to mix in the left or right or both channels.
I test this with plugged in headphones at the output instead of running to the speakers every time

I did not expect that with serial production and many years of work - they did not understand it. They used a ribbon cable to the different boards.

1: Preamplifier to the front panel with the potentiometers.
 Ribbon 5 core cable
 for each channel 3 wires in a braid
2: Pre-amplifier to Power amplifier board:
 ribbon cable with 4 cores (the two are +15 V, -15 V stabilized by zeners from the Power amplifier board  with which the preamplifier is powered) the other two are (the signal to the Power amplifier board )
 here I will use 2 cores in a braid separated by the choke

Am I headed right?