Thanks, Paul. I do not think it is the tube because when I change tubes the problem remains with the same channel. I also do not think it is lack of voltage in positive out, because when I measured it it was 9.9 volts. The only thing that has some positive effect is tightening up the nut that connects the socket to the chassis, but this effect disappears after few hours (this time the tube does not light up even though the leds are on). I cannot imagine why tightening this socket would be important. At any rate, do you think I should install a new socket? Thanks for any thoughts that you may have.