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I'm not experienced in electronics or soldering.

I find myself soldering the components of a PCB from a mechanical keyboard, all right and smoothly, except for two keys.

When I use the multimeter in V mode over the diodes, they all return values like ~1.628 V. When I do it over the two ends of the hot-swappable switch, the value amounts to ~2.794V.

However, on these two keys that don't work, neither the diode nor the switch gives a value, so I take it for granted that the problem is in the diode.

I have tested the diodes individually using ohm mode, and the value in all is ~66.1, including those that give apparently error, so I can conclude that the diode works.

The diodes are through hole type. Each key shares section for the holes diodes and for SMD diodes. In my case, I bought the through hole parts.

enter image description here

I have several questions:

  • How can I corroborate that the problem is the board connector for the through hole diodes?
  • Can I use, in this particular key and only and SMD diode?
  • Can I solder a through hole diode on the board for SMD?

What is the best solution?

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  • \$\begingroup\$ sorry, it's probably a bad translation, I'm not a native English speaker. It is the diode that is observed in the image, which is usually connected by means of holes, differentiated with smd diodes @jsotola \$\endgroup\$ – balon Apr 14 at 17:35
  • \$\begingroup\$ Do you have a diode test function on your multimeter meter? You need to test a diode out of circuit in both directions to 100% verify function, though if they measure resistance in one direction and overrange in the other, they're almost surely okay. Check the switch function too. And (obviously) make sure the diodes are installed the right way 'round. \$\endgroup\$ – Spehro Pefhany Apr 14 at 17:46
  • \$\begingroup\$ bridge over the switch contacts (black part) and the diode with jumper wire and see if either of those bypasses makes the button work. \$\endgroup\$ – Jasen Apr 15 at 4:59
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Those look to be standard 1N4148 or 1N4448 “Silicon axial signal diodes . Almost any signal diode will do as they serve to select the switch during a row or column scanning.

  • use the Ohm mode across the switch terminaLs to confirm. if it is an often used key , compare the mechanical feel and check the diode solder joints as well. Do all these with power disconnected.

Normally Diode test mode is best way to confirm which uses a small current source to get 0.6 to 0.7 is forward bias (ON) with red on Anode and black on cathode (bar) with OFF being 1xx over scale for reverse voltage. Not exactly the same as resistance measurements but as long as they show high R in reverse and low R <<100 in forward, they are probably OK. And not <10 ohms in both polarities.

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Considering since you are repairing a keyboard, I would first conclude it was some sort of mechanical failure than a electrical one. The first thing I would do is inspect the board for any signs of physical damage. Since the device is surface mount, and physical flexing of the board might happen, I would then try touching up the contacts of the non working switch(s). Then I would inspect the circuit by continuity checks of the PCB traces while slightly mechanically flexing the board. Then I would test the parts, but the chances of the diode failing would be rare. You only need to lift one lead out of the circuit to test it.

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  • \$\begingroup\$ although surface mount pads are present the diodes are all mouted in the thru-holes instead. \$\endgroup\$ – Jasen Apr 15 at 4:58
  • \$\begingroup\$ Diodes rarely go out on a keyboard. Thru hole diodes are better on a board like this because they can mechanically flex. That is probably why they installed them instead of surface mount. \$\endgroup\$ – David Mikeska Apr 15 at 5:04
  • \$\begingroup\$ Surface mount switches are a bad design here. As they are prone to get cold solder joints from flexing. That is why those old IBM keyboards a long time ago lasted so long. Other than the high quality cherry switches. \$\endgroup\$ – David Mikeska Apr 15 at 5:16
  • \$\begingroup\$ these are circuit mount switch sockets. \$\endgroup\$ – Jasen Apr 15 at 19:32
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bridge over the socket (black part) and the diode with jumper wire and see if either of those bypasses makes the button work.

if bridging the diode makes it work it's either broken or backwards.

1N914 or 1N4148 would be suitable replacement parts.

If bridging the socket makes a keypress then it's either the socket or the switch itself that's broken.

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