Disrupting hum in channels 2, 4 and 6 when Mains and/or Subs set enabled

[MikaX]

Hello everybody,

Here another day I bought a used piece of MODEL 24. The seller had bought it originally in May 2020. I started testing it for couple of days ago and noticed that channels 2, 4 and 6 make a humming noice when the Mains and/or Subs are enabled. Other channels behave as expected, although there is clearly noticeable, disturbing background hum (like from power supply) even when all the Mains and Subs are set off. I need to know more about this issue before I turn back to the seller with this issue. If it turns out to be fixablewith reasonable cost then I am not willing to make it an issue and keep the device. I am not skilled in electronics at all, but I have couple of friends who are.

How to make the MODEL 24 hum:
Pre-conditions:
- Main and Sub buttons of every channel disabled
- Main volume at its maximum position

Hum occurs now when I
- Switch the Main on in one of the channels 2, 4 or 6.

Hum becomes even louder when I
- enable rest of the Mains of the channels 2, 4 or 6
- move the Sub slider to the its maximum and enable Sub of the 2, 4 and 6.

The noise is very disrupting and is visible even in the output level meter. It makes the device unusable.

And my questions are:
1) What is wrong with my device? What is broken?
2) If there is suggestions concerning what is wrong with the device then I would be grateful if I knew how to fix it or have it fixed, what can I do about it?
3) Is there reserve parts available somewhere for MODEL 24?
4) Is the circuit board schematics for the MODEL 24 available somewhere? Or at least the part of the schematics that show the problematic area in this issue?

I recorded a video clip to demonstrate the issue and made it available here:

I appreciate any help that you can give me in this issue!

 

[-mjk-]

Make the seller take it back and return your money. Nobody can tell you what is wrong with it from a description and a video. It's going to take an electronic technician time to go through it and find the problems. Then you have to order the parts if you can find them. We are in the middle of a global supply chain breakdown. Like I said, make the seller take it back and return your money.

 

[MikaX]

-mjk-, thanks for your opinion. I guess that we will get this sorted out with the seller.

 

[-mjk-]

The thing is @MikaX, your issue sure seems to point to the seller not being forthright about why s/he is selling the unit. The problem could literally be anywhere in the signal chain, so unless the seller is willing to adjust the price based on what it cost you to get it fixed, the return is the only option. Bench time is going up, not down. Techs are backed up too, because these shortages are preventing people from buying new gear (because they can't get it), and being forced to fix older gear but can't get the parts. Lose/lose.

 

[MikaX]

I just wanted to tell you how this case was closed...

The device was sent to Thomann for service, that ended up being a warranty repair. It just took so long time that seller and I agreed that I get my money back and ordered new one from Thomann. Everything is not broken in this world.