Hi Bastoune.

It sounds like your equipment is generating noise (not uncommon), raising the noise floor and breaking the squelch.
It is best to tackle these problems at the source, by limiting the noise being emitted. Noise filtering on the supply is one option. Just a capacitor is most likely not sufficient.
As Q3 says, even filters and chokes in the supply might not be enough, particularly if the radio is picking up RF noise and not supply noise.

If you cannot get to the root cause, then adjusting the FSG71M squelch threshold is possible.
See images of the internals and the schematic of the radio on my webpages:
https://sites.google.com/site/threeu.../dittel-fsg71m

In the schematic the potentiometer for the Squelch threshold is R428.
In the images, this potentiometer is just at the bottom right edge of the yellow band. There is still a speck of red paint visible on top of the white potentiometer turn plate.
There is a hole in the bottom cover over this potentiometer, but it is close to impossible to get a screwdriver exactly into the slot.
The easiest way is to remove the 4 screws of the bottom cover and remove this cover. Then you have easy access to this pot.

It is impossible/useless to make statements about other potential sources without knowing your exact system setup.
But in general, watch out for switch mode power supplies. Especially cheap small 5V USB cigarrette lighter adapters used to feed PDA's.
Also watch out for the basics: if you have a weak power bus (e.g. thin wiring that creates voltage drop or a bad ground) trouble shooting becomes very difficult.

3U