06 February, 2015

Repair of electronic tracks on an old keyboard: Conductive silver liquid

In this post we will repair CNC keyboard that is no longer being manufactured
Contacts on the keyboards have evolved from mechanical systems to more modern with membrane. In this case we will repair an intermediate type; this keyboard from a Gemini 8050T FAGOR around 15 years old carries a screen printed aluminum membrane for tracks and graphite contacts, on which a metal piece with flexible concave shape closes the contact when pressed.
The technology of switches on keyboards usually are of two types: contact , where some kind of spring (metal or plastic or flexible rubber, etc) keeps separate tracks of the conductive surface (graphite or other) of the button, which when pressed blend tracks; and secondly, the induction, where pressing the button, a small magnet creates a small electrical pulse in the coil of the membrane (ultra durable modern musical keyboards).
Repairing first type of keyboards often requires all your ability of "handyman"; some with copper tracks like TV remotes are often easier to repair welding on the tracks a small micro-switch if the button is completely useless, but most cases are plastic membranes that can not be welded and we can use either glue threads on them as in this entry, or use the useful liquid silver, and you will always get a more professional work to build the appropriate cable connections.

Equipment needed

  1. Cutter to open the damaged contact (in other keyboards will not be necessary).
  2. Cleaning liquid or solvent which does not attack the material of the keyboard; I used Ardrox 9PR5 or you could use isopropyl alcohol.
  3. Liquid silver conductive, with which "will draw" deleted / damaged tracks with a fine brush after cleaning them well.
  4. A tester to verify that the track drawn conducts electricity.
  5. Scissors, PVC tape to cover the contact again, brush, permanent marker to draw the button again, etc.

The contact I had to repair was heavily damaged for its heavy use; the ENTER in a numerical control lathe, with even the mechanical button damaged, so I extracted the metal part of another unusable keyboard by cutting the top of the plastic protector gently; we can see the tracks and the thin layer of graphite on contact in good condition; sometimes we have to simply clean carefully the contact (without scraping to not erase graphite) and replace the metal sheet to get it working again:

We do the same with the contact to repair, we will cut the plastic being careful to not cut the tracks, then we can see that graphite is practically erased; as we will replace it with conductive silver, there is no problem; we can clean it with a cloth and cleaning fluid to leave visible tracks thoroughly cleaned free of grease and impurities.

Be extremely careful not to erase the tracks; are extremely fine.

Opening you can see the tracks, separated by a green paint to isolate:

This "Kemo" glass jar of conductive silver with 3 g. silver powder, is a very good conductor (between 0.02 and 0.1 ohm per cm2), depending on the amount. As you can see almost the entire bottle is solvent (only 15% of the content is silver).
The conductive silver serves to repair the tracks of anti-fogging in the rear of the car, circuit and generally to repair electronic tracks membrane.

The trick I use to make sure to leave a good amount of material with the brush: I beat the little boat and let stand 5 minutes with the cap down; thus accumulates in sufficient amount in the plug, then I can open the little bottle and start using the amount deposited there.

We give a first layer being careful not to join the different electronic tracks, and let dry well, applying little amounts until we get the appropiate conduction:

We give a second layer, and once dry (takes half an hour), check with the tester that conducts electricity well and we have not gathered contacts, testing in different points of the circuit. If not give good conductivity in some area, you can then re-apply more liquid, or clean it and start again in that part:

This circuit works joining the center point with the outer one when the contact is closed

Finally put the metal button and check that it works with the tester with a pole in each circuit and in position of testing conductivity, and press the contact.

If everything is ok, we will cover with tape to protect the button and circuits; I have placed thin transparent tape first, and then a couple of coats of white PVC tape to give durability, I painted the original text and have put transparent tape again to protect the text that is not erased by hand so easily.

Keyboard repaired
I took to protect another pair of keys widely used in the lathe beginning to damage (once the protective plastic starts to get fat begins to fail breaks y), with some PVC tape, painting the text and another layer Transparent:

Another more complicated case

In the other case, the keyboard of an old Num 750, the problem is different; contact is made ​​by pressure between sheets (longer to be more elastic) and the upper track is serigraphed on the bottom, so we have to "paint" the track so that it passes over the edge to the part above, and using another metallic contact pad.

We cut circle partially upper contact , but not exceeding the diameter of the outer contacts of the new metallic pressure switch:

Paint track, connecting with the bottom side edge by:

We waited 10 min. to dry and put the new contact pressure once we have checked conductivity with the tester between the lower and upper side:

And we cover them protecting with PVC tape or a tape that resists wear and tear, paint the buttom function with permanent marker and re-cover with transparent tape; I've left a little uncovered corner, so that the activity LED is seen:

And now it can be placed again in the machine, to last another 15 years). Happy repairs!


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