Contact Material Replacement

[bruce0]

I repaired a roland keyboard that had a cup of coffee.

The coffee etched away certain circuit board traces, which I replaced with jumpers (tedious).  And it all works.  But there is one problem.

The circuit boards have pairs of "contacts" that are under little silicone bubbles that are pressed by the keys.  The bubbles have pairs of little contacts molded into them too.  And when the key makes the contacts strike the circuit board contacts the circuitry measures the time between the two contacts to get how hard the key has been hit.

Here is the problem:

One contact has been corroded by the coffee.  I need to replace the contact material but don't know what to use?  Any ideas?

It is a grayish oxide looking stuff.  You can see the one bad contact clearly in this picture, in the right hand half of the shot.

Any idea what I can use to resurface this contact?

 

[buildafriend]

I recently fixed a synthesizer with bad/cheap traces for a friend.. I did what you did. Used wires hahaha. I'm curious about any trace repair methods that are out there too.

Thanks for asking, I get to learn from your inquiry  8)

 

[bruce0]

Thanks...

My question is about those contacts in the picture.  They are part of a switch.  The little contacts come down on them and make contact.  THere is a bad one, and I need to paint it with the same stuff but I don't have any idea what it is. 


As the the trace repair.... Here is what I did:
They were cheap traces... very thin, board was poorly coated (a 20 year old roland piano) and I used wire jumpers too.  Cleaned off the conformal coating, flux, and tack soldered the wires to the trace.  Then conformal coated it again.  Then I hot glued down the wires so they wouldn't move or buzz and put strain on the trace.

 

[sahib]

Have you tried conductive silver paint?

 

[RuudNL]

That's the kind of stuff they sell to repair broken traces on printed circuit boards.
Not very cheap, but it might work.

 

[joaquins]

There is a paint for TV remote controls repair and this kind of contacts... This is the product here in Argentina, http://www.edelta.com.ar/edelta/index.php?page=shop.product_details&flypage=shop.flypagem&product_id=572&category_id=75&manufacturer_id=0&option=com_virtuemart&Itemid=1

JS

 

[JohnRoberts]

My only experience is gold plating, or carbon ink. That isn't gold, and the carbon ink I am familiar with is dark black.

I am not aware of aftermarket conductive ink products for repair, so good luck.  You might scrape the rest off and tin it with solder, or try it as is after cleaning with the remnants left. The pad beneath it should be conductive so when it wears away the switch could still work, perhaps intermittently.

JR

 

[audiomixer]

did you test this?

https://www.sparkfun.com/products/11521    conductive pen

or the stuff you can buy at car shops to repair the rear window heater traces.....

cheers - michael

 

[bruce0]

I have a conductive pen (to repair traces on circuit boards) with mostly silver in it I think.  But the existing contacts are dark, and "flat black/flat grey", and seem very durable.

It will get pounded on by the piano player, constantly.

I didi think that the conducive pen would be good enough.

Anybody know what they use for such contacts?

Is it some sort of oxide?

 

[CJ]

de oxit contact cleaner?

there is a company that sells silver powder for plating copper, but the prices went so high for silver that they had to start charging for free samples,

see here>  http://www.cool-amp.com/cool_amp.html

maybe try a dremel tool with a sanding attachment like scothbrite or something,

maybe just try to plate the contacts with solder, scrape with an exacto and solder,

 

[audiomixer]

I think the working contacts are dark, not the coffee addict ones....

what is making contact? is it comparable to the soft switches? JR mentioned carbon ink, might be what is used regularly. if the contact is 'soft' then silver could work well.... the mechanical stress is low because of the silicone intermediate that absorbs the impact.

- michael

 

[pyjaman]

I've made experiments for a project a while back, mixing some acrylic lacquer with graphite powder. you can go from highly resistive to highly conductive depending on the proportions. graphite powder could be mixed with softer material if needed (epoxy glue? hot glue ? silicone for bathroom? I know that some conductive paint use this principle (containing a certain amount of graphite powder) .
Laurent.

 

[bruce0]

audiomixer - Yes, The speckled warted looking contact is the bad one, the dark coffee coloured ones are normal.

The moving contacts are pairs of hard black things, possibly carbon or something (they look like little magnets, but they are not), held in a bubble of white silicon, and as mentioned cushioned by the silicon on top.  The moving contacts themselves are hard, as are the circuit board contacts.

I attach a picture of the top of the contact bubbles.  (Irrelevantly...The keys hit them asymmetrically and the contacts come down in a a "one-two" punch.  The key velocity is determined by the time between punches)

CJ - I did try to clean that contact, but the corrosion of the underlying copper seems to have lifted and destroyed the material that was coating it. 

Pyjaman - That is interesting.  And your source of graphite powder?  (grind up pencils?) I don't need more than a pinch of it.

My concern with the coating using solder is it is not hard enough and I think it might come off on the moving contact... see these silicon bubbles are no longer available (eventually they fail... and the piano is useless)... So I am afraid to screw up the moving contacts.  The graphite and epoxy idea seems like it might work?

I think what I need is a hard, flat conductive paint.  I would say from looking at these that they spray painted this stuff on.  There are slight signs of wear on the surface of the fixed contacts (which are shaped somewhat like craters.

So Cleaning is out ( I did it, and it worked for a while, but that key now is malfunctioning)  and it is the G just above middle C.  ) because it will just re-corrode.  I think I could try paint, or silver solder, or the graphite epoxy mix.  I will look online for conductive paints.

 

[JohnRoberts]

I am not aware of small quantity sources for the conductive ink.

Perhaps ask a local PCB fabricator, if you can find one locally, as they may have left over ink from some boards they processes.

I believe the ink is thick liquid that gets silk-screened onto the PCB, then cooked to cure.

I would have a low expectation for success for a complete repair at such a basic low level damage.

JR

PS: The round black pucks on the back of the plastic push buttons are made of conductive rubber. They just short between two conductive regions on the PCB. 

 

[0dbfs]

Interesting.

I had to take apart a console with a coffee spill recently and clean. The ON/OFF buttons used this exact same material/method which I thought was pretty neat since from a manufacturing standpoint you don't need to source another unique fairly complex switch assembly.

Anyone know if these are fairly common for piano/keyboard keys? Maybe some PCB manufacturers have something like this as a standard option?

My thoughts was that it was some sort of carbon based adhesive/glue type compound that conducts and is fairly soft yet perhaps somewhat rubberized to conduct a certain amount of current yet withstand a high number of operations/mechanical-stress.

Anyway, I maintain a number of consoles with these button types so am also interested in solutions to this particular problem.

Cheers,
jb

 

[audiomixer]

you might try the different (carbon powder epoxy, silver, carbon powder with acryl) suggestions on a different pcb and measure the contact resistance with your silicone pads. I was quite concerned of the attack / velocity sensing but if it is sensed wit two delayed strokes the absolute resistance of the pads is not so relevant and the pressure vs resistance does not influence the result (think: press harder and get lower resistance sensed by the keyboard as a harder attack, but this is apparently not the case)

you can use silver conductive traces to a 0.1" header and measure the result....

the wear will probably not be to much of a issue with silver trace, as the rubber pad is soft, but maybe oxidation might mess it up in the end. a option would be to combine silver trace with carbon top layer....

- michael

 

[bubbi]

My Yamaha DX-100 had a very similar problem and I used this stuff: http://www.ebay.com/itm/KEYPAD-FIX-Permanently-Repairs-All-Rubber-Keypads-/290658760252?pt=US_Remote_Controls&hash=item43ac9cb63c

It's a lot better now but not perfect though.

 

[PRR]

Powdered graphite is readily available as lubrication for door locks. Home-repair store or car-parts store.

It may be too coarse. A pharmacist's mortar and pestle, or two spoons, will work it finer.

A pretty hard/tough conductive paint is sold for repairing car rear window defogging stripes. When long and thin it has ohms of resistance, but a small dot should be "zero" compared to the carbon-rubber buttons.

Many calculators use similar contact systems, and of course dead/sick/unloved calcs are very cheap, but I don't think transplantation is likely.

 

[sodderboy]

I have used the silicon membrane repair paint and it works very well.  I get it at mcmelectronics.com but their site sucks for searches so I could not find a link.  It is a two-part paint for the membrane side.  No reason why it would not repair the board side.  It is a conductive epoxy made from a mix.

Finally found it here

Usually the board side is a maze of two copper contacts, and the treated membrane makes the connection.  The double contact derives the touch/velocity.  This style contact has always been susceptible to dirt/moisture.  Cordless phones have had such contacts for over 20 years and they get all greezy and mis-fire.

As far as keyboard membranes go, they were standardized somewhat and different footprints are still available through Roland and Radio Shack for keyboards that I have serviced in the past few years.
Mike

 

[mad.ax]

KONTAKT CHEMIE - GRAPHIT 33

From the datasheet:
"As a conductive coating for permanent and safe diverting of electrostatic discharges (ESD)
• Backs of cathode rag tubes
• Electroplating of non-conductive materials
• ESD safe packages
• Repair of graphite-coated pcb’s in keyboard switches (e.g. remote controls)
• To firm the ESD protection of packaging and conveyor tube equipment."

you can find it at Farnell:
http://fr.farnell.com/kontakt-chemie/graphit-33-200ml/revetement-conducteur-graphite/dp/832959

direct link to the English datasheet:
http://www.farnell.com/datasheets/318264.pdf

Axel