HP-67 2104S02158 - faint display

[albertofenini]

Thanks Tony !
We'll try this suggestion
On our board the two transistors do not appear to be encapsulated but should be 2N3904 and are fairly common
for the resistors will see what we have
We may as well retrieve these from a logic board that doesn't work and for which we'll post a new thread
tae care and thanks !

(11-20-2020 03:07 AM)teenix Wrote:  

(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