Intermittent buttons

[ljcox]

I have 3 devices:- a printer/scanner, a multimeter and my granddaughter’s expensive calculator (that does everything except make coffee) that intermittently do not turn on when the on/off button is pressed. In the case of the printer/scanner, it is the scan button rather than the on/off button.

They work well if turned on & off frequently, but they fail if left off for some time. In the case of the calculator, it can be as little as 30 minutes.

Often it takes many presses turn them on - assuming you don’t give up in the meantime.

Has anyone had experience with repairing such devices?

Any assistance will be appreciated.

My recollection from some years ago - when I did electromechanical design – is that the rule of thumb was, in order to be reliable - switches and relay contacts needed at least 5 Volt across them when open, and at least 1 mA through them when closed. In those days the contacts were metal, so I assume the voltage was necessary to break down the oxide film that formed on them when open and the current was required prevent them from re-oxidising while closed.

But as I understand it, the modern buttons are made of conductive rubber. So the above mentioned rule of thumb may not apply. Alternatively, the rule of thumb may have been lost in the mists of time & needs to be reinvented.

 

[alec_t]

Sometimes cleaning the conductive rubber contacts, e.g. with alcohol, helps.

 

[ljcox]

Thanks Alec,
That rings a faint bell.

I faintly recall that it has to be a particular type of alcohol - possibly the one the medical people use to wipe your arm before giving an injection.

 

[alec_t]

I think the recommended alcohol is isopropyl.
Rubbing alcohol (surgical spirit) contains a small percentage of castor oil, so might make things worse rather than better

 

[jpanhalt]

Addition of castor oil to "rubbing alcohol" may be common in some countries; it is certainly not that way in the US.

To remove oils from your switch contacts, 70% isopropyl alcohol will work. As referenced above, that is also called rubbing alcohol. You can check the MSDS to see if the material available in your country has any oil in it. Alternatively, put a drop on the shiny side of a piece of aluminum foil and let it evaporate completely. You need to wait for the water in it to evaporate too. A residue would indicate some contaminate, probably oil.

You can also use 98% isopropyl alcohol. It is a better solvent for oils and should evaporate cleanly leaving very little water, if any, behind. It is a poorer antiseptic than the 70% formulation.

Neat ethanol (≥95%) will also work. In fact, absolute alcohol (100%) is a very good solvent for fats. Denatured alcohol has chemicals added to make it unsuitable for drinking. Unfortunately, some of the denaturants are much more aggressive solvents than ethanol and can harm plastics; although, rubber is probably safe. The problem is that there are many different formulations used for denatured alcohol, which are usually determined by government regulation. It can be quite difficult to determine exactly which formulation is used for the product you are buying.

John

 

[alec_t]

Perhaps use vodka? If you polish off the whole bottle you won't remember whether you cleaned the contacts or not, though

 

[tvtech]

Perhaps use vodka? If you polish off the whole bottle you won't remember whether you cleaned the contacts or not, though

LOL

Seriously though.

Tac Switches can cause problems of note. I am seeing it more and more now on TV's. You can be watching the set and suddenly it decides to start auto searching....looks like the Micro has lost it's marbles

Normally you will find one Tac switch only that is causing the problem....you cannno't test it with a meter though. You need to have the set on and then disable them one for one until the nonsense stops. Then you have nailed it....Don't replace it unless it is physically broken though.

99% of the time (if not physically broken) all it needs is some cleaning. I use a Switch Cleaner from Spanjaard called lubricating switch cleaner on this page:**broken link removed**

It does not melt or damage plastic. It cleans, flushes and lubricates. It is very mild so requires more work than the others that eat.

Be patient and work the switch after spraying into it. Multiple times.

Regards,
tvtech

 

[ethelknox]

How to Repair Electronic Circuits

Before repairing a circuit board one must diagnose the problem and identify the failing parts. In most cases a component-level repair is required, that is, one or more components must be replaced with new ones to make the circuit work again. Most circuits are composed of passive and active components. Passive components are the ones that do not require a power source to work, for example, resistors and capacitors. A failing passive component can be identified easily but a bad chip is much harder to diagnose.

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[ljcox]

LOL

Seriously though.

Tac Switches can cause problems of note. I am seeing it more and more now on TV's. You can be watching the set and suddenly it decides to start auto searching....looks like the Micro has lost it's marbles

Normally you will find one Tac switch only that is causing the problem....you cannno't test it with a meter though. You need to have the set on and then disable them one for one until the nonsense stops. Then you have nailed it....Don't replace it unless it is physically broken though.

99% of the time (if not physically broken) all it needs is some cleaning. I use a Switch Cleaner from Spanjaard called lubricating switch cleaner on this page:**broken link removed**

It does not melt or damage plastic. It cleans, flushes and lubricates. It is very mild so requires more work than the others that eat.

Be patient and work the switch after spraying into it. Multiple times.

Regards,
tvtech

Thanks for this suggestion. I had not thought of using contact cleaner. I have a can of cleaner made by an Australian company rf Chemical Technology.

So I'll try that.

 

[ljcox]

Before repairing a circuit board one must diagnose the problem and identify the failing parts. In most cases a component-level repair is required, that is, one or more components must be replaced with new ones to make the circuit work again. Most circuits are composed of passive and active components. Passive components are the ones that do not require a power source to work, for example, resistors and capacitors. A failing passive component can be identified easily but a bad chip is much harder to diagnose.

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Thanks for the reply.

When I open the case, my first action will be to prove whether the fault is due to oil on the rubber pad or not.

I could be due to tarnish on the contacts, a dry joint on one of the associated components or even a crack in the PCB.

The problem is in making it turn on. It works perfectly once it is on.

 

[Mr RB]

Corrosion of the contacts is common in Australia with the heat and moist salt air etc. In the repair shop we used to open them up, and clean the PCB contacts with toothbrush and warm soapy water, followed by clean tap water, then dried with paper and then a hair dryer. (Alcohol can cause issues by disolving PCB flux or coatings and spreading that over the contacts.)

The back of the rubber contacts was cleaned with warm soapy water and clean water, and not too much brushing as they can be delicate.

For the "soap" don't use dishwashing detergent it is acidic. The best thing is car wash detergent, it has neutral PH and you just need a drop.

 

[ljcox]

Thanks Mr RB.

It is good to hear of your experience in cleaning the contacts & pads.

 

[jpanhalt]

For the "soap" don't use dishwashing detergent it is acidic. The best thing is car wash detergent, it has neutral PH and you just need a drop.

1) Why do you consider acidic bad and alkaline good? (I know why I would, but it is hard to find acidic detergents, see bellow.)
2) Please name the acidic dishwashing detergent to which you refer.

Most detergents are alkaline, and carwashing detergents are often quite alkaline. The commercial ones in particular have sodium metasilicate as a replacement for phosphates, all of which are alkaline. The commercial carwash concentrate I get locally for my implements is pH 10 to 11. Sodium metasilicate is quite alkaline and, in my experience, is harder to thoroughly rinse off than the phosphate-based detergents. The problem with all detergents is getting them completely rinsed off. Moderately polar solvents, (e.g, alcohols and "glymes"*) do not need to be rinsed.

Have you actually checked the MSDS's? Here are some for common brands in America:

Cheer free (laundry**) pH 10.0 to 11.5
https://www.electro-tech-online.com..._Free_-_Gentle_Granular_Laundry_Detergent.pdf

Dawn (dishwashing) pH 9
https://www.electro-tech-online.com/custompdfs/2013/04/Dawn20Dish20Soap20Proctor2020Gamble.pdf

Turtle Wax Zip (automotive) pH 7.7
http://www.turtlewax.com/upload/msdp/T075-0910$54e185fa3c64d380.pdf

Cascade (dishwashing) pH 10.7 to 11.3
https://www.electro-tech-online.com...hing20Detergent20powder20EN20MSDS20110110.pdf

John

*I am using "glyme" to describe a class of alkoxyalkanols, e.g, 2-methoxyethanol and 2-butoxyethanol. They combine the functionalities of ethers from the alkoxy moiety with an alcohol. They are excellent, water miscible solvents and are/were used in a lot of household cleaners until the VOC cops caught up to them.

**I had a close friend whose father was a chemist/executive at P&G. He clued me in to the detergent game. You can buy diluted detergents with perfumes for several times the price of stronger detergents without perfumes. Cheer free makes an excellent liquid dishwashing detergent. I keep a squirt bottle with a 1:4 dilution by my sink.

 

[Mr RB]

Great info, thanks!

I used a car wash detergent labelled as "PH neutral" because I had heard that dishwashing detergents were quite corrosive to metals, due to being "acidic" as that is needed to dissolve some of the food residues.

Maybe it was my memory or my source that got the "acidic" and "alkaline" mixed up, but the main point was not to use a corrosive dishwashing detergent but instead use a gentle PH neutral car wash. Sorry for any confusion!

 

[ljcox]

Thanks jpanhalt & Mr RB, I'll have a look for a PH neutral Carwash fluid.

I'll let you know what I find.

 

[andrewblog]

The problem is in making it turn on

For that reason I think the failing part is not the on/off button. Otherwise you should get in trouble in making it turn on and off.

 

[ljcox]

For that reason I think the failing part is not the on/off button. Otherwise you should get in trouble in making it turn on and off.

Thanks for the reply.
I thought about this possibilty also. My conclusion was - not necessarily.

In circuits that I have designed in the past that have an on/off button, more current passes through the switch when turning on than when turning off.

Therefore, if the contact has some resistance due to tarnish and/or oil on the rubber, it is possible that it will turn off but not on.

However, I have not opened any of the faulty devices yet, so I am not able to prove what the fault is yet.

 

[KeepItSimpleStupid]

There are conductive eleastomer repair kits. Suggary drinks spilled on things like keyboards require a bit of work to fix. For a KB, I washed it, rinsed it, blew it off as good as I could and hung it on the clothes line to dry.

 

[ljcox]

There are conductive eleastomer repair kits. Suggary drinks spilled on things like keyboards require a bit of work to fix. For a KB, I washed it, rinsed it, blew it off as good as I could and hung it on the clothes line to dry.

Thanks for that info. I did a search for "conductive eleastomer repair kits" & found that they are also called "Keypad repair kits".

One hit had a video of how to do it using such a kit.

 

[Mr RB]

Cleaning generally does the trick. It's rare that they have such high wear the rubber needs to be repaired.

But the cleaning has to be thorough. Sometimes a quick clean just spreads the insulating contaminant as a finer layer, so it looks clean but still does not work.