Lights were dimmer and dimmer and alt meter started to show discharge so apparently once again problems with the charging. I started the normal procedure checking the problem and first point was to check the ground from battery to engine and to the body. Second thing was to check the ballast resistor and voltage regulator. There has been some earlier problem with the voltage regulator because it was fixed with extra wire so I decided to renew the whole volatge regulator and ballast resistor. But it didn´t help.
So, eventually I renewed the alternator and now it´s ok. Seemed that the alternator was coming to the end of life and that was the reason. But following that procedure is worth because sometimes there is only poor ground to engine/body and that´s easy and quick fix.
Alternator is Powermaster 7019 and the volatge regulator is Standard Products VR101T. Alternator is newer model than the original ´69 alternator because having two field terminals. But it can be used with the mechanical regulator as well when having a jump wire from the second field terminal to the body ground.