HP-67 2104S02158 - faint display

[teenix]

(11-19-2020 10:14 PM)albertofenini Wrote:  Hello Everyone

does anyone know if the brightness of an HP-67 display can be enhanced ?
We have cleaned this unit which works fine, but the display is by far less bright than the average HP-67 .

We have tested it with different battery packs with the same results.
We have not tested the external battery charger as we are always a little concerned in attaching a charger, although working.

Thanks for any idea you would like to share o suggest !
Take care !

If someone is interested in few pictures, even if it's not finished yet, this is the link : HP-67 2104S02158 - faint display

The display current is originally sourced by the battery, through a constant current circuit to the anode driver and finally to the LEDs.

The constant current source is created by a pair of transistors encapsulated in a round metal can on the CPU board. The current is controlled by 2 resistors on the CPU board - 464 ohm and a 2.7 ohm, and a resistor on the key/display board - 510 ohm.

The idea is to provide the LEDs with a fixed current and therefore a fixed brightness over the normal operating range of the battery.

If any of these components are faulty, high resistance etc. then the display may dim.

cheers

Tony