Fixing a broken D&R700 mixer

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Hello, this is my first post here and first of all i would like to express how glad i am to have found this forum.

A few months ago i purchased an old D&R700 mixer. It was dirt cheap and i figured that fixing it up would be a good learning experience.

Now iam at my wits end with it. I opened it up, checked the wires, measured the meters, checked the circuitbords and looked at every little part looking for signs of damage. I even tried connecting it ungrounded.

So what is the problem? Well the main left channel is constantly peaking even when no audio is plugged in. I tried individual channels and none of them seem to be responsible except the main left.
The left gives a combination of the typical electric buzzing sound, hissing and crackling.

Does anyone here have an idea what i should be looking for?

I am only a beginner and learning everything on the go so any input is helpful even if it doesnt pan out.
You have to trace the signal and find were it gets bad. Feed signal in some channels and check signal were it enters the main audio pots. If You have a good signal there then the problem is on the output section. Have You cleaned pots / faders already ? Perhaps it's just a faulty output op-amp that was zapped probably by some phantom adapter. Trust me semiconductors look OK even when faulty, unless there is catastrophic failure and they burn / explode.
I just repaired a $35 PV8 mixer off ebay. Master volume slide pot was worn out. Duh. $3 from newark. $2 in 2mm x10mm screws to screw pot to front panel. Bypassed Peavey's custom mount for pot from the board.
Left channel dead after input clip LED's on 8 inputs. No left volume indication, no left headphone out, no left master out, no left monitor out, no left tape out.
Culprit was a broken via through the board. $0. Good as new. Ran a 28 ga wire from one component pad to a capacitor lead.
New PV10 are $379.
Traced music from FM radio earphone through the circuit with a Simpson 266XLPM meter on 2.5 VAC scale. Used a component lead in an alligator clip lead as a probe. Used the ring of a RCA plug plugged in output as analog ground for meter. Use a .47 uf cap in series with negative clip lead else analog meters respond to DC on AC scales. AC is music.
Didn't have a schematic. Datasheet on op amp IC's showed where in, out, feedback(minus in) are. Place the music stopped was the problem. Used 3 strengths reading glasses & mechanic's light to get good look at it.
Didn't have to learn how to solder surface mount IC's resistors & capacitors to board. Very scary technique to a 69 year old. Wasn't the problem.
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