I would check and see if the fuse updates have been done to the 2 bridge rectifiers in back . Its possible one is failing and putting stress on the transformer. Under these conditions I would not leave the game unattended or use it until these updates have been done.

You are running 230V in Belgium, make sure your transformer is wired as in this picture:

https://www.pinwiki.com/wiki/index.php/Williams_System_9_-_11#Transformer_Power_Selection_Connector

The transformers get warm, almost hot in normal operation. I've had system 11 games that I put my hand on the transformer, count to one-two-three-four-five... and I'm pulling my hand off the transformer because it's too warm!

Warm dust smell, yes, I've had that.

I'm more worried about a burning electronics smell.

Also, your coil voltages and lamp voltages should be within a certain range. I copied from this thread:

https://pinside.com/pinball/forum/topic/system-11-transformer-voltage

Quote:

Logic power
MPU IJ17 Pins 4,5,6 = +5.03 VDC @8.1 mV AC ripple
U26/U27 ROM Pin 1 = +4.91 VDC

Audio power
MPU IJ17 Pin 9 = -14.04 VDC (Unregulated)
MPU IJ17 Pin 8 = +12.02 VDC (Unregulated)
Audio board U11 (U17 on Sys11B) Pin 8 -13.9 VDC @250 mV AC ripple
Audio board U11 (U17 on Sys11B) Pin 4 +11.8 VDC @50 mV AC ripple

Controlled lamp power
PS 3J4 pins 5,6,7,8 (Sys11A) or backbox bridge 6BR1 (Sys11B)= +15VDC to +18VDC nominal.

General illumination power
PSU 3J8 (Sys11A) or Interconnect board output 2P7 (Sys11B) = 4.5 VAC to 6.3 VAC nominal

Display power
3J5 pin 3 (Sys11A) or 3J2 pin 3 (Sys11B) = +90 to +110VDC nominal
3J5 pin 4 (Sys11A) or 3J2 pin 1 (Sys11B) = -90 to -110VDC nominal

Flipper power
Game specific@flipper supply board outputs= +65 to +75 VDC nominal

Solenoid power
PS 3J3 pins 6,7,8 (Sys11A) +32 to +35 VDC
Interconnect board F1/F3 (Sys11B) +41 VDC
Interconnect board F4 (Sys11B) +75VDC

---------------

So, check that your voltages are about right.

Transformers do get warm/hot, but they also fail. One way they fail is to use too much power and get too hot.

The usual symptom is a burnt electronics smell and bubbling goo dripping out of the transformer.

Let us know if your voltages are approximately correct. If the voltages are somewhere close, and the transformer is just 'oh, that's quite warm', I'd just use the machine. Any smell of burning electronics and I'd be looking for a replacement transformer.

For the Early system 11s, including F14, the bridges were not fused and were instead tied to the line fuse which may or may not blow upon a bridge failure. The results of such a failure could lead to melted harnesses from the transformer on the minor side to a full on fire on the major side. This is why a fuse modification to directly fuse the bridges are recommended.

https://www.pinwiki.com/wiki/index.php/Williams_System_9_-_11#Bridge_Rectifier_Fuses

http://home.kpn.nl/inkochnito/bridge_board.htm

Another issue I've seen is burnt general illumination connections on the yellow wires from the transformer to the power supply. If these connections are burnt or failing it will stress the transformer . A fuse won't blow because this failure is before fuses.