deskthority

Hi all,
I have a keyboard that has been driving me up the wall for a while. An old Tatung Terminal board. A great board, when it works:



However, the board had the strangest problem. It would work fine most of the time, but then certain keys would fail to register for a while, only to function again later.

It got so strange that I decided to remove all components from the PCB and replace the controller with a Teensy with QMK.

Now for the strange part: the problem persisted! Even though all that was left was a naked PCB and a Teensy.

Now, before you say "bad switches", I of course desoldered the switches in question and swapped them with some known good ones. Same problem.

I've now determined that two of the columns sometimes short. Sometimes! I.e. when I put a multimeter to the pins, mostly there's no connection, but at other times it will clearly register a connection (with a sustained beep).

Any ideas as to what could cause this kind of intermittent behaviour in a PCB?

You're on the right track, testing with an ohmmeter. Maybe attach a couple wires to the columns that are shorting and hook the ohmmeter to those so you don't have to hold the leads by hand, then flex the PC board or move things around to try to narrow down the location of the problem. It's not unusual to find a fleck of solder or a clipped off component lead, or a staple or paper clip dropped in from the top side.