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jebem · (This post was last modified: 10-01-2018 09:01 PM by jebem.)

(09-26-2018 07:16 AM)Csaba Tizedes Wrote:
(12-05-2015 03:25 PM)jebem Wrote: This specimen had poor LCD contrast.
The issue was fixed after cleaning the battery holder connection pads on the PCA.

I have two workable units, one of them in perfect condition, the other is my daily used machine for calculating dawn-sunrise-noon-sunset times for my location.

Both of them has the problem with contrast: typically the contrast set is OK, but sometimes goes too poor and sometimes too dark, but the battery is new. Have you got any idea how this is fixable? If I turn off/on the unit the contrast set is fixed for a long period then starts again the changing.

Another question: the second unit is arrived this Monday and really like a new but as I can feel the bracket [(] [)] keys response is poorer than others. I will clean the unit inside, my question is between the keyboard foil and PCB is it required to clean or that will kill the contacts? Is it possible to clean the foil itself? What is the right cleaning liquid?

Thanks for the answers!
Csaba

Well, I have the same issue with these old machines.
There are several causes for the same symptom on poor LCD contrast; defective power supply is one, but other causes are a bad electrical continuity somewhere in the PCB traces, or a defective resistor or capacitor, and the LCD itself can degrade with time as well.

As for the defective key, I am sure that it can be fixed as long as the PCB conductive traces are not totally depleted or contaminated with battery leakage.

A continuity meter (ohm meter) is a required tool in these cases. You can compare a good working key against the defective one, BEFORE starting to mess around with the key foil. The normal resistance value will vary, depending on the model. In this way we can compare a good key with the suspect one. The resistance value can be as low as 10 or 20 Ohm, but I saw many working machines operating with much higher values in the hundreds of Ohm.

The keyboard foil can be removed, as long as we do it with caution to not destroy it. Try to apply some heat from a air dryer over the foil and start to lift it from a corner, slowly, preferably from the side of the defective key. There is no need to remove it completely, just lift it enough to see what the problem is.
Usually I use cotton buds with IPA to clean the dirty/oxidized/contaminated area in the key contact. Do not rub too much or the conductive material can be totally depleted, and then it will almost impossible to restore it.

Be aware to not abuse on the IPA over the PCB. In any case, let it dry completely for an hour or so, before applying energy to the machine.

God luck with your repair.

Note: IPA = Isopropyl alcohol