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HP Forum Archive 17

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Brightness display HP65
Message #1 Posted by Ronald P on 18 June 2007, 3:33 p.m.


I have a HP65 in repair this moment. There was one element in the Led burned out so I had to replace that led unit. It works 100% now for all digits. But now the brightness is different between the units.

I know HP did match the brightness at production for the 3 units, but unfortunately I had only on spare unit, so no option to match brightness.

Is there a way to make a unit less bright, can I put a (small) resister somewhere, or lower the value of the inductors affecting only one unit ?



Edited: 18 June 2007, 3:34 p.m.

Re: Brightness display HP65
Message #2 Posted by Dan W on 18 June 2007, 7:20 p.m.,
in response to message #1 by Ronald P

The cathodes of each digit are all tied together, so there are 5 cathode lines. You would need 5 resistors. Theoretically it could be done if you can find room to attach them.

The anodes for each segment are tied together too, but they are also tied to the anodes of the segments in the other chips. A resister in the anode line would dim the segments in all of the chips.

Good luck! if you get it done, let us know. Also I've heard replacing the LED chip is quite difficult. If you have some tips from your experience, please post them!

-- Dan

Re: Brightness display HP65
Message #3 Posted by Ronald on 19 June 2007, 4:01 p.m.,
in response to message #2 by Dan W

Hello Dan,

Thank you for the the tip.

Removing the led unit was actually not that difficult. I used desolder litze (is that the correct word, a lot of twisted thin copper wires). This literally sucked out 95% of the solder for each joint. After that I used an L shaped tweezer which I put between the board and Led unit and applied a little bit of careful force to pry it ot. After that you touch with iron the joints that did not came clean completely and its done. 5 minutes max.

I will let you know on the resistors.



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