Thanks a lot for your replies, I really appreciate the help!
Allright, here we go.
Let's take a look at that PCB. There's a primary side, which has line voltage on it. Then there's a switched power supply (SMPS), that creates the DC for the Class D amp.
Commandment
Rule number one of all repairs:
THOU SHALT CHECK VOLTAGES! (Imagine this spoken by a loud voice from the sky for dramatic effect).
You have no signal, so likely there's no power at all. Could be the SMPS. Let's check.
Two difficulties:
1: You don't have an isolating transformer (and no, your variac likely isn't one), this is dangerous.
2: The heat sink is not attached, so U8, MOS1 & MOS2 are not cooled during measurement.
You want to measure the voltage over the 6 electrolytic capacitors to check if the SMPS is working. You can switch the unit on briefly to avoid overheating the now uncooled parts. Make sure not to touch any spicy parts on the primary side. Best to cover it with copious amounts of an insulating material (Kapton tape, iso tape, whatever you have at hand).
If there is no voltage across those caps, we need to work on the primary side. I didn't see any fuse at first glance. Likely broken parts are the bridge rectifier BR1 on the left, the controller U2. Not sure what U3 is. A MOSFET gate driver perhaps? Please read off the part number of U2 and U2. And lastly, the MOSFETS could be dead. In that case, that's usually caused as a consequence of another defect.
If the 6 caps have a voltage, then we need to continue on the secondary side. In that case, please clean U7, D1 and D2 and identify the parts. They might be linear voltage regulators that have failed.
Post results. Good luck and be careful with the primary side. If your house does not have an RCD (Jordfelsbrytare in Swedish), please consider leaving this to a repair shop with the proper equipment.
There is power, as the preamp output is fully functional. Also the display turns on, so I am able to adjust settings, EQ etc in the digital menu system... If the power supply was bad, wouldn't the unit be totally dead?
The preamp output and display are part of another board that attaches to the main board by way of a 26-pin connection (the black one), and are powered through this 26-pin connection.
Here are some measurements of the main board:
Bridge rectifier input: 230 VAC
Bridge rectifier output: 330 VDC
Two big caps on the primary side: 163VDC and 167VDC
(what are these for? Smoothing caps?)
6 electrolytic caps on the secondary side: 35VDC on each cap
(these are smoothing caps to get rid of ripple on the DC supply right?)
Here's what I read on the components:
U2: AVS10CB by ST electronics
This seems to be an Automatic mains selector 110/230V?
https://www.allelcoelec.com/datasheets.c3/AVS10CB.pdfU3: S27952 530P C092S_ by manufacturer "International Rectifier"
https://pdf1.alldatasheet.com/datasheet-pdf/view/530872/IRF/IRS27952.htmlU7: LM337SP GK0G2 V6 CHN S28 by ST electronics
https://www.allelcoelec.com/datasheets.86/LM337SP.pdfD1 and D2: STTH1002CB GKOSS534 by ST electronics
https://www.digikey.lu/en/products/detail/stmicroelectronics/STTH1002CB/669389What IC are U11 and U6?
They are TDA8954TH by NXP semiconductor.
https://octopart.com/datasheet/tda8954th%2Fn1-nxp+semiconductors-21124594So these seem to be the actual Class D power amps ICs. So perhaps one chip is run in mono mode for the Sub speaker and the other chip is run in stereo mode for the left and right (or mono if in mono mode setting).