Atari 1040STF not producing any image on power-up

[StealthGunner]

Hello, everyone, I've been a long-time Commodore 8-bit user and have never owned a 16-bit machine up to this point - or anything non-Commodore.
A week ago I was offered a gifted 1040STF (german keyboard layout) and an accompanying SM124 monochrome monitor that a coworker claimed worked up to about two years ago.
Since I haven't even seen one in the wild up to this point, I took the offer, naturally.

 

After bringing it home and letting it acclimatise for about half an hour I connected them up (after checking that the 13-pin monitor cable had no kinks or broken pins) and powered them on.
The monitor's power LED lit up when the dial was turned beyond the switch detent.
After powering on the monitor, I powered on the 1040STF itself. The bottom-right power LED turned on, but there was no FDD activity, no head seeking, and no image on the monitor. The screen remained black throughout and no amount of messing with the brightness and contrast did anything.
I had left the device powered on for about a minute to see if anything would change, but to no effect.

 

I opened the case up just to take a look at the power, keyboard and floppy cables to see if there was anything obviously disconnected, but I couldn't spot anything out of the ordinary. I gave it a preventative blast of compressed air to knock dust out of the machine and reconnected the power and monitor just to make sure the situation's the same, and it was - power LED on, no FDD activity, no image on monitor.

So, I'm not sure which is at fault - the STF or the SM. Considering the proprietary monitor pinout is a bit of a hassle, is there something I can do to test the monitor itself, or test the Atari itself without a video cable of any sort (i.e. where to expect certain voltages ad so on)?

[ParanoidLittleMan]

There should be keyboard clock too, if machine is alive. But it's not that easy to connect STF to some speaker, earphone. You have connector pinouts online.

Although, it's 90% computer failure. If's very old, so could say that it is normal - by me it happened at least 10 times so far with my Ataris. If you can not make it work by removing socketed chips, cleaning and seating them back, then better leave job for some expert, service. Otherwise could just make it worse - not recommended to keep it on power longer than 1 min, and not much times.

If have voltmeter need to check presence and goodness of main power voltages. Instead asking where to check, get schematic and service manual at devdocs.org ... Use search online. It will give you instant answers.