Classic HP LED failure diagnosis

I have a 65 that has a non-lighting minus sign. Upon dis-assembly, it can be observed that the Cathode 1 segment does seem to light very faintly when the CHS key is pressed. The minus sign does not light.

Upon additional testing, when the battery drains and all the decimal points light up, the Cathode 1 decimal point DOES LIGHT UP. This leads me to believe that it is likely the LED Module and not the Cathode driver.

does anyone concur with this diagnosis?


Hi aj4062

I agree with your diagnosis. If the Cathode 1 digit decimal point lights up at low voltage conditions, the cathode driver does his job.
A failing LED segment in the module can be the root course.
The module contains only a matrix of LED diodes and no control electronics.
The specifications can be found here.
Desolder the module and check all segments with the diode test of a multimeter.
Failing segments in classic series LED modules is a common problem.


Thanks for the confirmation Kees!

Just wanted to see if anyone else comes to this conclusion before I spend the time to desolder and replace that module.

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