I suggest more filtering on the power output leads than just ferrite beads. Chokes with several turns of wire in both the positive and negative leads. Also, some RF bypass filtering in the mains leads to the supply. Then making certain that the entire case is providing adequate shielding. Of course, I just now looked at this thread and so these may have been suggested already.

Thank you!
I bought a bunch of chockes, different sizes, and I'll connect them in series with the leads, from the source.
I've also connected an Ampermeter + Voltmeter (LED), but they induced somehow a lot of humming.
I disconnected everything from power (scopes, soldering stations and even lights), leaving only the power supply... Nothing happened meanwhile, I mean that the same problem persists.
I'll try the chockes, and let you know...
Regards,
Tedy

 

After all, capacitors are shunt reactances while a choke is a series reactance. It may even be that both kinds are needed in this application.

 

After all, capacitors are shunt reactances while a choke is a series reactance. It may even be that both kinds are needed in this application.

.... And the results are: 100% success! Meantime, I use relative small chokes. Ferrite rings of a diameter of approx. 1 cm, ~ 2.5 mm width and 10 turns of ~ 0.6 mm wire.
I am sure that I have much more larger ones. I haven't measured them yet, even that I have an LC meter... Tomorrow I'll try to find them in my warehouse and will check what results they'll give.
So now, the configuration is: PS output -> 25.000 uF -> 2 chokes in series with each lead. One big ferrite bead near the end of the leads.
So meantime connecting the transmitter to the output, I get a drop of about 0.1 V for about one second, and it returns to the initial 3V.
It's interesting what will happen with a 5W / 6V transmitter / receiver, but firstly, I'll connect the bigger chockes, if I'll find them.

Many thanks,
Tedy