Wax and push-buttons

Hi,

Not a Picaxe related issue.

As I was visiting relatives, I came across a weird issue. They had some troubles with a 9 years old computer monitor.
The same message was often popping up like if they were periodically pressing on one auto-adjust key on the monitor. It was also very difficult to modify the setup in pressing on an other key.

I checked the normally open push-buttons and found two cases on some of them :

• 200 to 500 ohms resistance while opened
• 100 ohms while closed

In the first case, it was like a low value resistor was connected across the contact.

I thoroughly cleaned the Bakelite PCB with isopropyl alcohol. A fin white film then covered the PCB like if the board was sprayed with some wax. After the cleaning, the problem disappeared then reappeared later.

How some wax (?) could lead to this sort of issue?

Sounds like there's been some sort of corrosion within the switch causing a partial short, though resistances connected elsewhere might be causing lower than just switch reading.

If the physical button goes to a standard 6mm PCB push button it might be worth trying to replace it with something similar. Would be worth unsoldering the switch and measuring its resistance out of circuit first. If okay removing may have fixed it or it's the board, connectors, wires or what that connects to. If the monitor has the button on a PCB strip it might be worth unplugging the strip see if the problem still occurs.

I can't comment on the film / 'wax' issue.

... unsoldering the switch and measuring its resistance out of circuit...

Click to expand...

I did it although there is no other resistor on this PCB. Same result!

Dipping a switch in alcohol seems to fix the problem : measurements are then correct.
I will replace them all later.