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