Apple IIgs showing vertical bars

Hello, this is my first message on this board. I'm happy to be here with other appleII fans like me.

I just bought a malfunctioning IIgs that I'm trying to repair using the schematics found online.

When turned on it shows ten vertical bars on TV (I've attached a photo to this post). No beep. Just a cycling faint click-click-click sound coming from the speaker, which is synchronized with a thin horizontal line moving up and down over the screen (this line can't be seen from the image, because it is too thin).

The keyboard reset appears to be working. When commanded, you see the outside corner of the screen getting black and then the same image shows again.

I have to clarify that the computer was in better working condition when I bought it last week. When turned on, a message showed "fatal system error -> 0911". But after many useful tests, I decided to remove the motherboard to see if there was any broken component on the back side, and when I reassembled the whole thing, the vertical bars appeared.

Since I have little knowledge of this hardware, I could really use a hint of which possible causes I should seek first.

Right now I've checked PSU and the continuity of traces from Mega2 IC. I'm going to check other traces too, but it's a hard job, and I'm not even sure that the problem could be a broken trace. So I hope that anyone here could point me in the right direction.

Thank you for your help.

_____________

Update: I made a recording of the sound from the speaker (attached)

The first click that you hear is the PSU On switch.

Then there's a first alone speaker click followed by a train of closer clicks (frequency of the clicks seems to be slightly below 8 Hz)

thanks.