Repairing a TV remote control - cleaning

My TV remote control has been a bit hit and miss. Sometimes it works and sometimes it doesn't. I took it apart and cleaned it as
follows:
ONE I carefully wiped some gunky residue (food? drink?) from inside using tap water on a paper cloth to dissolve it. That included:
(a) the rear of the one-piece rubber keypad (b) the part of the circuit board containing the key contacts
Maybe I should have used distilled water but I made sure I wiped the water off until it was almost completely dry.
TWO I then used isopropyl to remove any oils or grease residues on (a) and (b) above.
------
I have read here: http://www.tvantenna.com/support/tutorials/remoterepair.html that the rear of the rubber contacts are coated with some special conductive compound. I thought it was conductive rubber else I expected some little pieces of metal foil behind each key. Oh well!
Is water or alcohol likely to remove the coating that page refers to?
Is it completely crazy to very LIGHTLY sandpaper the contacts on the board? I would using 1,000 grit or even the soft "flour" paper usd to finish off varnishes on furniture. The contacts look like some sort of carbon and might take two or three very GENTLE strokes.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I would have started with just the cleaning using water and isopropyl. That should be enough in most cases.
--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html
Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Battery contacts? New batteries?

One can use a 125 degree F oven to bake it for a half hour to ensure complete dryness. That is below the temp for any plastic parts and matches the "bake out" temp used in many industrial environments. Approximately 60 to 65 C.

Make sure it is 99% IPA as the weaker IPA is cut with water!

They are usually thick pads of what is referred to as "carburized rubber".

I don't know of any current models that are not regular carburized rubber LV resistive "switches".

Bake it out (no batteries of course).

There is sometimes a coating on the PCB pads. One must be careful not to abrade those. If they are bare metal, one CAN abrade them lightly with a pencil eraser (lightly).

NO SANDPAPER. For one thing, it has a huge triboelectric effect, and will introduce electrostatic charges into the PCB assembly, perhaps causing an ESD event, which could damage the circuit.

Pencil erasers are abrasive enough, and remove very little surface molecules. More like a an oxide rubbing, to remove any oxides on the (likely brass or gold) plated contacts.

No. If the PCB has carbon painted (or screened) spot, you do NOT want to do any of the above. They will remain conductive just fine. The bake out is all you really likely need, and good battery contacts, and good batteries, and make sure the receiving end is unobstructed (dust on the window or an item in front of it).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I'm not sure that high octane alcohol is always needed. My point, as alcohol dries, the water percentage goes up rapidly, attaining 0% alcohol when it dries, and the remaining water accumulated from the moisturing grabbing alcohol, stays around until it evaporates. Hot air drying is almost always needed during the process.
greg

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 15 Feb 2006 15:00:43 GMT, snipped-for-privacy@pitt.edu (GregS) Gave us:

That depends on the temperature of the board AND the alcohol.
If both are warm, the alcohol will NOT leave any "dew" behind. It will dry 100% , quickly and free of water.
IPA heats up in a microwave just fine. A small cup for 15 seconds up to as much as a minute or so for about half a cup will get it very near its boiling point. It also works much better as a solvent / cleaner when it is hot like that, and evaporates nearly instantly. I have set up many a production floor with vapor phase degreaser machines using various solvent media.
Hot air drying is NOT "almost always needed" if one starts with things in the right conditions to begin with.
If the PCB and assembly is hot, it will be pushing water OUT of it, not absorbing it. Just make sure that it returns to room temp in a dry box (ie the oven that got it hot). Not required though unless the climate is VERY humid.
The last step should always be the half hour 120F bake out if one REALLY wants all the water out of the hygroscopic PCB..
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I have never thought od heating alcohol for obvious reasons, but this may come in handy for stubborn cases. I had some boards that the caps would absorb moisture or other stuff, and would take days, months to dry without baking. I usually just use a hot air pencil or gun to heat up boards.
greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 15 Feb 2006 15:34:11 GMT, snipped-for-privacy@pitt.edu (GregS) Gave us:

Yet that practice is very dangerous. Even a toaster oven is safer as it would heat the whole board uniformly (relatively). The pencil is way too high a heat, and even feathered across the area desired is a shock to the media, every time. Blow driers and heat shrink guns can be used if they are kept far far away so that the heat has a chance to cool a few hundred degrees as it passes thru the cool air on its way to the target. Still the oven (toaster or otherwise) is better. 120 F... 60 - 65 C.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I have one of those hot air pencils with complete control of temp and flow.
I sometimes use a 100 watt lamp over the board.
greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 15 Feb 2006 15:00:43 GMT, snipped-for-privacy@pitt.edu (GregS) Gave us:

Hahahaha... that's funny. The alcohol with the higher water content (ie 70% or 50%) would have the higher octane number.
Methinks you chose the wrong adjective today! :-]
Higher *purity* is the 99%. But lower octane number (if it even has one).
Octane is a measure of a volatile liquid or mist or gas and air mix to resist detonation by heat or pressure before deliberately being ignited by the spark. So, 99 octane gas actually burns with a little LESS energy than 89 octane.
What the higher octane allowed a racer or engine maker to do is raise compression ratios and combustion chamber temperature without the worry of "pre-ignition" or detonation before the desired point in the stroke.
Not saying that you didn't know that, just iterating.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Snipped a bunch
Uhhh. . . buy a new one? They're about $5 now.
--
Regards,

Paul



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Zak wrote:

I'd avoid the use of solvents like IPA since you'll think the dirty mark is dirt when it may well be the carbon coating coming off!

No - don't do it! There is very little coating present and you don't want to lose any of it.
Once it has been cleaned with water and dabbed dry, put in the airing cupboard or other warm - NOT hot - place to completely dry.
Then reassembe and test.
HTH -- Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps Change 'news' to 'sewn' in my Reply address to avoid my spam filter.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.