[help] missing components on pcb

al.c

15 May 2016, 16:28

hi!

i know this is probably a longshot.

http://imgur.com/nvhX2fA

i've got this piece of a keyboard, and it's missing two (?) components (circled) on the pcb (ignore the missing keys).
there is no text anywhere, no cables (i figured out gnd/vcc by looking up the pinout of the HD74LS05P) and no enclosure.
if plugged into my usb converter (tmk on a teensy) i can switch the num lock led on and off by pressing the num lock key.
but that's about it, one position of the xt/at switch gives me "unexpected signal" for some keys with hid_listen..

i was hoping that maybe some resident electronics wizard could help me figuring out what the two missing parts could possibly be. i can provide more/better pictures and multimeter outputs if needed.

thanks for reading.

User avatar
Muirium
µ

15 May 2016, 17:00

My guess would be another jump wire and a 4th LED. But it's just a wild guess. Some keyboards did have a 4th light up there simply to show system power.

Image

But! None of that should affect the controller. Strange you're getting as far as a lock light showing up right, but no alphas.

al.c

15 May 2016, 23:45

the num lock is special on many keyboards, i think because its very often near the power connector and the switch is right below it.
i believe the missing part near the edge was a capacitor, but i can't be sure. the other one, maybe a resistor or one of these resistor-shaped inductors. but my guess is as good as yours. i just think, why would someone desolder a wire bridge, it would be of no use and it can't really break. and four leds are pretty unusual.
it's probably a lost cause, but i thought it'd ask around.

User avatar
chzel

16 May 2016, 00:31

My first instinct is that the small gap is for a ceramic capacitor (100nF or so) and the other a jumper.
If you follow the traces (as far as I can tell), a jumper there would make sense with the ceramic bypassing Vcc to GND.
Or it could be a resistor with a led.

al.c

16 May 2016, 14:32

i think it might be an otc ok-keyboard, but not the one in this vid:

https://youtu.be/zRKo61_xrSo?t=80

the mounting plate looks very similar though, and the only visible text anywhere on my pcb reads "OTC"..

User avatar
Ratfink

16 May 2016, 23:11

What chzel said (either a filter capacitor+jumper or LED+resistor; judging by position I'd say LED+resistor is more likely) would seem like a reasonable thing to put there. Neither would be critical to the keyboard's operation, and these components almost certainly not populated at the factory. They have nothing to do with why your keyboard isn't working. :)

User avatar
OleVoip

17 May 2016, 01:06

If you follow the traces, you'll find that the pads are connected like this:
VCC-----[large gap]-----[small gap]-----GND
This is a typical pattern for a power LED with a current-limiting resistor.
Since the size and position of the small gap is a direct continuation of the pattern set by the three other LEDs, it seems quite unlikely that the missing components could be anything else.

edit: The combination jumper + ceramic capacitor does not make sense: it could only serve as a blocking capacitor for the IC, but it's unnecessarily remote from it (and there already is another blocking capacitor for the IC near one of the LEDs).
Last edited by OleVoip on 17 May 2016, 01:21, edited 1 time in total.

al.c

17 May 2016, 01:20

omg!! it's alive! i rewired it to try out all the possible combinations one more time, and suddenly KEYPRESSES!! woooohooo!! a few keys aren't registering, but if i connect the pins manually, the controller picks them up! caps and scroll leds are dead as well, but aside from that, it IS an ok keyboard once again!! awww yisss!!

if anyone's interested, the pinout is gnd, vcc, clk, dat (gnd and vcc like in the picture) and the upper position of the switch is at..

thank you! you were right, no missing parts after all!

ps. and of course i'll add that power led ;)

al.c

17 May 2016, 10:23

btw, could anyone point me to a good tutorial on how to repair a dead alps switch? the ones i've found are either 404'd or have missing pictures.. i've cleaned the switches with compressed air and the metal contact plates with alcohol, but no luck so far..

al.c

18 May 2016, 09:59

update. no luck with the power led, but the rest of the leds and all but six of the keys are working now. i think i'll just desolder the numpad keys and use the switches to fill up the rest of the non-working keys.
on a side note, i opened all the letter switches, lubed them and removed the clicker plate. non-clicky tactile white alps, i love how smooth these keys feel and sound now! i might even keep this as my main keyboard.

devucci

18 Jun 2019, 06:13

hey men :) I am almost sure I have the same keyboard... But i don't know the brand of it..
it seems ORTEK close model to that one but i think it is not ortek.. it something else!< but what? :) :lol:
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