HP 9830A always turns on with blank display

his itself Are you enjoying yourself ? As far as I can see you're right about the 18k2 resistor, but there is more. The 100k is changed from a feedback resistor to a parallel load of the inv. input and the Au is changed from 10 to infinitive. I've to look into it, maybe just removing the 18k2 resistor does the trick also. If you read the whole article, you'll see it's about the difference in behavior between normal LDR's and CDSe LDR's The last ones are almost unobtainable and sensitive to IR, standard LDR's are sensitive to visible light. The problem is they react different, you can't just replace a CDSe with a standard LDR that doesn't work. Also there is a big difference in resistance between light and dark between the two types, that's why you have alter the EOT-circuit. It needs to be more sensitive for smaller changes, and as it seems you can just remove the 18k2 resistor and not replace it. I wrote it because it's a working (tested) solution for an actual problem. AND you can't just replace the CDSe type with a normal off the shelf LDR that doesn't work, I tried.. So be careful before just shouting something without testing it! And about Tony, he did a lot of work making the diagrams available for everybody .. It's ok to look at his diagram and if you find an error, he'll be the first to admit it and correct it.. And don't throw his work away, it's a first class job, if you don't like the presentation that's on you. But reverse engineering so much HP hardware is hell of a job and he helped, by making them public, a lot of people. He reversed engineered more the 100 items, how much did you do? Everybody makes an error ones in a while I do, Tony does, and you?? -Rik