HP-65 1333A02242 display missing all the decimal dots

[teenix]

(11-10-2020 06:49 AM)albertofenini Wrote:  Hi Tony
Thank you very much for the interesting explanation.

Let me ask you, are RCD and STEP the same ?

At this point what’s your diagnosis ?

The cpu board proved itself working when put in another unit so I
would assume something is wrong on this keyboard pcb

Is the wave for the h segment (the decimal point) correct ?

Could the anode driver or the cathode driver possibly defective ?

By the way We have also another unit , which we posted about this summer, with all the g segment missing, would it possibly be a similar problem ?

Take care and thanks again!!!

Hi Alberto,

The step and RCD are different signals. The step advances the cathode driver to the next digit. The RCD resets the cathode driver to start the digit scan again.

I would say the RCD is working if you see digits on the display.

If you look at the signal on the step pin of the cathode driver (pin 9) you should see 15 pulses that repeat every 280uS. 14 of these will be spaced 20uS apart and one of these pulses will be spaced about 5uS. This one is the decimal point digit.

If the above waveform is not correct I would suspect the anode driver. The attached image is the 5 data lines and RCD pin with only a decimal point on the display. This happens when after CLx you press the decimal point button.

If you see correct waveforms then I would suspect the keyboard PCB. Corroded open circuit traces for the DP output of the anode driver (pin 17) and soldering, to the LED displays. (bottom of display chips nearest switches, 3rd from right)

The DP coil might be open circuit as well.

For the G segment, I would suspect the anode chip soldering, keyboard traces and G segment coil.

cheers

Tony