My Zenith's F7 is haunted

[pr0ximity]

Sucks.

Anyone up for helping me diagnose what's wrong? It's an 84-key Zenith 100-1860. The basics of it are that the F7 key does not send an F7 scancode to the connected PC (both Win 7 and OSX), but it does register with the on-board buzzer that the Zenith has. So it makes a beep on the keyboard, but doesn't send a code. Every other key will beep and send a normal code to the PC, and the rest of the board works great.

Additional spooky symptoms:
- Pressing the F7 key simultaneously with any other key will often cause the other key to get logically "stuck", repeated infinitely until the key is pressed again. This seems to be the case for any key on the board, not just ones associated with that row, circuit, etc.
- Pressing the F7 key and then another key shortly after will cause that other key not to register. Pressing that key again, or waiting for a short count (a literal second) will cause that other key to register normally. Again, this is the case with any key on the board that I've tried.
- Holding the key down causes the buzzer to repeat like any other key (aside from the Sys Req key which doesn't repeat, I assume purposefully)

No other key on the keyboard exhibits this behavior, and I have tried desoldering it and swapping it with another switch twice (I've desoldered the entire board during cleaning and reassembled it, then specifically swapped the F7 and F9 switches). I've also resoldered the diodes adjacent to the switch and tested all of their continuity. They all check out with a basic multimeter.

It was not working great prior to the board's complete disassembly. I can't recall if it ever registered; it either registered once and then never again, or it never has.

I've checked the entire single-layer PCB visually many times closely with a bright flashlight to try and spot shorts or broken traces, but haven't come up with anything. I don't have a ton of knowledge of circuits and how keyboards are wired, especially older models, but since everything else on the board works without issue I'm completely stumped as to what might be wrong.

I can take some pictures of the circuity if it would help.

[pr0ximity]

For anyone who may run into this in the future, I've discovered that when used as a PS/2 keyboard all keys, including F7, work excellently. It seems to be my USB adapters that are causing issues, though I'm a little stumped as to what adapter to try next.

I have a Monoprice PS/2 -> USB that I use on my 356mini which I was originally using, doesn't seem to let F7 through.

I also have a BlueCube which I use for my Model M, but that doesn't seem to register any keypresses at all, keyboard beeper doesn't even work.

The saga continues, but at least the board is functioning!

[Darkshado]

Uneducated guess: excessive power draw?

[Melvang]

Soarer USB to USB converter? 

[Mattr567]

For anyone who may run into this in the future, I've discovered that when used as a PS/2 keyboard all keys, including F7, work excellently. It seems to be my USB adapters that are causing issues, though I'm a little stumped as to what adapter to try next.

I have a Monoprice PS/2 -> USB that I use on my 356mini which I was originally using, doesn't seem to let F7 through.

I also have a BlueCube which I use for my Model M, but that doesn't seem to register any keypresses at all, keyboard beeper doesn't even work.

The saga continues, but at least the board is functioning!

Wow, this is the exact issue I had with my 1860 before I dissembled it for its SKCL Greens! On PS/2 it was fine but when I used it on USB the F7 key flat out didn't work. It definitely isn't just you. Maybe a controller issue? My PCB was flawless when I received it, in perfect condition.

The Zenith is weird to adapt on USB. Linux had no clue it existed on PS/2, but on Windows 10 it was fine on PS/2. The bluecube didn't work, but my Belkin one did with the F7 issue.