I remember that somebody else reported such an issue with 7414 (without HC) some time ago, so I think that this could cause the problem. 74HC14 is more tolerant against CMOS drivers, and your PC board is probably stuffed with such ones.
i got some 74HC14 (no HCT) but the burner doesn t work.
When i plug the lpt cable the red led lights up, is this anormal?
When i plug the lpt cable without the supply connected, the red and green led light up but not so brightly as with the supply.This doesn t happen all the times ! it happens “sometime”.