Posts: 172
Threads: 5
Joined: Jan 1970
If you feel like going into your calculator and doing a small modification, you can make your display brighter.
Open your machine. At the top of the main circuit board there are two IC's, one with 18 pins, one with 20. Pin 6 on the larger IC is connected to ground through a 3K resistor. This resistor sets the display current and hence the brightness. Replacing it with a somewhat smaller value will brighten the display.
I tried this on an HP-21. I didn't remove the existing resistor but temporarily put a 4.7K resistor in parallel with it, giving an equivalent resistance of about 1.8K. The increase in brightness was modest but noticable.
Warnings: Don't do this unless you are very comfortable about disassembling your calculator and replacing components. Don't overdo it -- too much current could damage the display or its IC's. Finally, more current will reduce battery life and increase recharge time while the calculator is on.
Have Fun!