[SOLVED] Intermittent buttons

[ljcox]

I have 3 devices:- a printer/scanner, a multimeter and my granddaughter’s expensive calculator (that does everything except make coffee) that intermittently do not turn on when the on/off button is pressed. In the case of the printer/scanner, it is the scan button rather than the on/off button.

They work well if turned on & off frequently, but they fail if left off for some time. In the case of the calculator, it can be as little as 30 minutes.

Often it takes many presses turn them on - assuming you don’t give up in the meantime.

Has anyone had experience with repairing such devices?

Any assistance will be appreciated.

My recollection from some years ago - when I did electromechanical design – is that the rule of thumb was, in order to be reliable - switches and relay contacts needed at least 5 Volt across them when open, and at least 1 mA through them when closed. In those days the contacts were metal, so I assume the voltage was necessary to break down the oxide film that formed on them when open and the current was required prevent them from re-oxidising while closed.

But as I understand it, the modern buttons are made of conductive rubber. So the above mentioned rule of thumb may not apply. Alternatively, the rule of thumb may have been lost in the mists of time & needs to be reinvented.

 

[BradtheRad]

I have had tv remotes and a cordless telephone, where a button started needing to be pressed hard to make it work. Eventually it no longer worked. I opened up the units, and discovered oil under the rubber membrane. It hindered electrical contact.

I washed the entire membrane in a water-based cleanser that would not leave a residue. I avoided leaving lint on the rubber buttons.
I also cleaned the silvery contacts on the circuit boards.
This restored reliable operation of the buttons.

Only certain devices have developed this problem, and they have had it more than once. At first I thought the oil came from careless spills. However there was no evidence of oil outside the keypad. It must come from a breakdown of the plastic molecules, or something along that line.

 

[ljcox]

Thanks Brad,
That is very helpful.

I also dimly recall that I was once told that you can use a type of alcohol.

I think it was the type that the medical people use to clean the skin before an injection.

Len