Re: HP 9821 - display and tape transport issues #VintageHPComputers


Hi Marius,


It’s some time ago and I don’t remember which LDR I ordered.

I looked at epay and found several sellers who sell 5mm LDR’s. So the best thing to do, is looking at the datasheet which one fits the best.

Important is R-light < 2k and R-dark is high as possible, the comparator switches at ~2.5V to get a good switching characteristic it’s important to buy a LDR with a wide Resistance range.




Van: <> Namens mariuszmleczko@...
Verzonden: dinsdag 18 februari 2020 22:23
Onderwerp: Re: [VintHPcom] HP 9821 - display and tape transport issues #VintageHPComputers it seems to be defective ? Rik can you propose some available on the market phototransistor for replacement ?
The symptoms described below are also same as mine (especially rewinding case).

A good LDR will display a resistance of several mega-ohms when fully dark and less than 200 ohms under strong direct light. The LDR feeds into a 741 op-amp running open-loop, thus functioning as a comparator. The comparator output will trip when the LDR resistance is around 18K.

Join to automatically receive all group messages.