On 2019.04.08 05:29, Martin wrote:
since a few days, my qq(´) and qq(´)¹ don't work with a single press. I have to press twice.
The problem most likely is oxidation of the electrical contacts of the key switch. The silver or gold plating of the contact surfaces may be compromised by mechanical wear.
Back in the 1970's, keyswitches were rated in terms of tens of millions or hundreds of millions of keystrokes; keyswitches of today are rated in terms of tens of thousands or hundreds of thousands of keystrokes. Search and read the manufacturer specification sheets for keyswitches.
A factor which exacerbates the situation is the ongoing effort to reduce energy consumption; nowadays voltages across keyswitch contacts can be too low to break through oxidation and the currents flowing through contacts may be too low to burn off the oxidation.
Research years ago by Honeywell Corporation revealed that gold-plated contacts are not always a good approach. Though gold does not oxidize, the presence of oil vapour in the atmosphere can result in formation of an insulating polymer on gold contact surfaces. Silver contacts appear to be the proper approach; silver oxide is conductive.
Mechanic design of the contacts also is a factor; when the contacts are bars or rods which cross at an angle, the contact pressure is higher pressure than if the contacts are in the shape of buttons. The pressure helps break through oxidation.
Some manufacturers offer "lifetime" keyboards; but the lifetime in view appears to be that of the keyboard and not of the user. Keyboards today can have a useful lifetime measured in months. Much depends upon the environment in which the keyboard is used, particularly the atmospheric humidity.
The proper approach to the problem of low voltage and low current would be to design keyboards in which the keyswitch contact voltage and current are an order-of-magnitude higher than the voltage and current of the logic circuitry.
RLH