Diagnosing a key not registering - I made a mistake and don't even know how c:

[Ludo_Lorenz]

Howdy!

I am fixing up an old keyboard and am going to upgrade its switches. I've just desoldered it annnnnnd three keys aren't working.

I used switch hitter to check this. I have access to basic soldering tools and a multimeter at hand, so how would you, my friend,  recommend I go about diagnosing the problem?

The following keys are not registering when I short the pads:
Right Fn key
Q
T

And that's it! The rest works like a charm! The issues only happened after I desoldered the switches, and I am not an experienced solderer in any way shape or form, and my first desolder job was the letter A of this plate! I'm quite certain I am to blame here, but I'm so inexperienced I can't even know what I did wrong.

Can you? If you have any idea, please let me know and thank you for your time and attention! Love ya!

[_rubik]

Nothing immediately jumps out to me from those pictures. Some good places to start are

- Use some solder wick and rubbing alcohol to clean up any left-over solder and flux. It's hard to tell what's wrong without a clean working surface.
- Check that the traces are intact all the way to the IC. Some boards are pretty brittle, so one slipped screw driver might sever something.
- Test the resistors. I doubt it's the case (usually pretty obvious), but sometimes you can break them.

Also, what temp were you soldering at?

[Ludo_Lorenz]

Nothing immediately jumps out to me from those pictures. Some good places to start are

- Use some solder wick and rubbing alcohol to clean up any left-over solder and flux. It's hard to tell what's wrong without a clean working surface.
- Check that the traces are intact all the way to the IC. Some boards are pretty brittle, so one slipped screw driver might sever something.
- Test the resistors. I doubt it's the case (usually pretty obvious), but sometimes you can break them.

Also, what temp were you soldering at?

Thanks for the feedback! 

I'll get on cleaning up the holes and all of the gunk left on the surface!
I believe one of the traces might be damaged, yes! I noticed the PCB seemed extremely brittle. I slipped the iron on the board a few times and thought I might've trashed something. If so, should I attempt to jump the connection somehow?
I'll also test the resistors. Each switch has one resistor associated to it, right? So if there is any damage, it should be on that key's resistor, I assume.

As for the temp, I am not sure. I was using a pretty tame 40w iron, but for some of the pins I had to leave the iron on for a looooong time, I'm a beginner, but know that something could have been damaged then and there. Honestly I'm surprised the board even turns on given how badly I did, but it was a great learning experience and I am saving it from geting in the bin so I'm happy  .

Thank you very much for the reply! I'll get on working on the board ass soon as I get home. You're the best!

[_rubik]

I believe one of the traces might be damaged, yes! I noticed the PCB seemed extremely brittle. I slipped the iron on the board a few times and thought I might've trashed something. If so, should I attempt to jump the connection somehow?

That's probably the culprit (if I had to guess). If you can hunt down the damaged trace you can either 1) bridge the two exposed ends which might require a longer wire 2) gently scrape away the mask to expose the trace with a craft knife and bridge a smaller section over the damaged spot.

The latter is trickier but sometimes preferred because it means a smaller bridge. If you don't feel comfortable doing surgery or worry that the pcb is too brittle, you could just bridge the entire trace and tape down the wire with electrical tape.

[suicidal_orange]

Looks like a single sided PCB so all the traces are visible - at least the ones you need are.  One pad of each switch is connected to the end of a diode, the other is connected directly to other switches and in the first two pic also to jumpers soldered near the switch you can use.  Just follow the pale green traces to see where they go.

Here's a pic of the third one to explain how to find and fix the breaks:



Touch orange to blue - if it works solder a wire from pink to blue.  If not touch pink to cyan - if that works solder orange to cyan.  If neither work touch blue to cyan and it has to work, if not you're in the wrong place.  If it works solder orange to cyan and pink to blue as you've broken both sides (not very likely!)

[Ludo_Lorenz]

Looks like a single sided PCB so all the traces are visible - at least the ones you need are.  One pad of each switch is connected to the end of a diode, the other is connected directly to other switches and in the first two pic also to jumpers soldered near the switch you can use.  Just follow the pale green traces to see where they go.

Here's a pic of the third one to explain how to find and fix the breaks:

(Attachment Link)

Touch orange to blue - if it works solder a wire from pink to blue.  If not touch pink to cyan - if that works solder orange to cyan.  If neither work touch blue to cyan and it has to work, if not you're in the wrong place.  If it works solder orange to cyan and pink to blue as you've broken both sides (not very likely!)

Thank you for the instructions! Just managed to get some time on the board and test the traces. It's great that the board is single sided since I was able to test the current along the traces and boom! Found a gnarly nick on the board straight across the traces for the Q and T keys! YEEEEEEEEEEE



Extremely satisfying to find out what was causing the problem, thank you so very much for helping me find this! You're awesome 

As for the Fn key, I didn't find any problems at all, so I got the suspicion that maybe switch hitter just wasn't identifying the key. I remembered the Fn key could be used to brighten or dim the LEDs on the board so I tested to see if that still worked and yep! It did! So the Fn key is working just fine 

[Ludo_Lorenz]

That's probably the culprit (if I had to guess). If you can hunt down the damaged trace you can either 1) bridge the two exposed ends which might require a longer wire 2) gently scrape away the mask to expose the trace with a craft knife and bridge a smaller section over the damaged spot.

Thanks for the suggestion! Though I like the idea of just mending the gap, I don't trust my hands to do such delicate work without shorting something, too shaky!

In my previous post here I show that I identified the problem! A nick on the traces for the Q and T keys! And the Fn key was just not being registered in Switch Hitter, but it was functioning! I'm so happyyyyyyyy 

I'll just jump the trace and as far as I know, that should be it! Thank you so very much for helping me identify the problem with this board!

I'll order the switches now that I know it's good for use. I'll go with Akko switches because they are the best ones I can afford for now and they are available where I live. From what I've seen online, people find them to be good budget alternatives. Should be better than the LOUD, chattery Outemu blues that used to be on this board, but who knows!

You're the best, _rubik! Thank you for being kind and helping me out 

[suicidal_orange]

That's a nice little nick - straight through two traces.  Was visible in the pic I zoomed in on to add dots too so should have seen it!

Glad you found it and the PCB has been saved

[_rubik]

I'm happy our advice helped! Enjoy the rest of that project -- be sure to post the results once it's done