TT Keyboard: Right mouse button "continuously pressed"

[stephen_usher]

My TT keyboard has developed a fault where according to the system the right mouse button is continuously pressed (well, about 60% of the time anyway) and it's been getting worse over time.

Initially I thought this was a fault that has been documented in the past which has been described as a software flaw in the system but as it's been getting worse and worse and power cycles haven't always fixed it it seemed more of a hardware issue.

So, last night I opened up the keyboard, connected it to the system without a mouse installed and powered on.

Initially pin 9 came up at the proper 5V but very rapidly the voltage dropped, bottoming out at about 2.3V.

The right-mouse/joystick-2-fire is the only connection which goes directly to the keyboard controller chip and doesn't have a dedicated pull-up resistor, meaning that it's relying upon a pull-up in the chip itself. Also, it means that the chip is unprotected from external static charge etc.

Anyway, temporarily adding a 10K resistor between +5V and the line brought the voltage up high enough for the button(s) to work correctly.

Has anyone else seen this issue previously?

I'm guessing that the controller chip in the TT keyboard is the same as that in the STFM keyboard so if the worst came to the worst I could swap it out.

[czietz]

Fwiw, my TT keyboard does have a 10k pull-up resistor on that input. I just reconfirmed this by measurement.

[stephen_usher]

Strange. Where is it on the board? I couldn't find it. None of the discrete resistors had continuity to the line.

[czietz]

One of the 10K resistor arrays, I guess.

[stephen_usher]

I can try reflowing all of them to see if that helps.

[stephen_usher]

Well, that was interesting.

Tested again and got infinite resistance between the +5V rail and the trace connected to pin 9.

Took the board out of the case... still the same.

Turned it over and traced the traces and inspected the solder... all fine and now 10K resistance between 5V & pin 9.