Yesterday, I noticed a problem with the No-Si digital clock. The hours figure show something unreadable. On closer inspection, it looked like the '5' was always burning regardless the actual hour, through the intended figure. I thought a possible cause might be the triode tube, so I exchanged the responsible E90CC with another one in the clock. Bingo, now the '3' was constantly burning. So yes, the tube was the cause. Luckily, I have some spares laying around, so I put in another E90CC and the problem was solved.