Welcome to VE, I see it's your first post. Wow, SL-7 coming out of the woodwork to play again, like the Phoenix and the whole vinyl phenomenon....
Didn't want to leave you hanging, you can try cleaning the 3 LEDs under the platter first just in case, cotton swab w/ glass cleaner.
Refer to spikkie's picture and you will see the Optical Sensor pcb. From the owner's manual, this is pcb that has the optical sensors that detect the light coming through the platter guide slits. You will see the plastic blinder hanging askew, that selectively shades the sensors and feeding the logic chip telling the tonearm to drop (if safe) and in the correct position. Clean the sensors on this pcb. Trace the connector from this board to the main mother? board and make sure the connector / contacts are clean.
Hopefully, this along w/ a new / renewed belt will get your SL-7 playing properly.
Work the switches as it seems exercising contacts work.
That's it for now, until I can review my service manual for more ideas.
Fred and your English is lots better than my French....
Thanks for your quick and friendly answer, I was afraid that Sl(7)eeping beauty stay asleep under chloroform (not secure ?)
I have to reawaken contacts, yes, so recently I ordered contact cleaner "Deoxit"and I am waiting for...is it a good product or not ? I used also a teflon grease on the tonearm guide and gears
I can see the 3 leds lightning through the platter guide slits (cover up) perhaps it's not necessary to clean them ?
I'll report back here after I finish washing up and I'm working the switches , hopefully