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mbbrutman
December 26th, 2008, 02:50 PM
This is a continuation of a thread I started back in March. The subject machine is a TRS-80 Model 4 but I am sure that there are many machines out there with the same problem.

Back in the spring I picked up this beautiful Model 4 which looked to be in wonderful condition except for the keyboard which only had a handful of working keys. Today I got a keyboard from a donor system that Yzzerdd had, and this keyboard is significantly better but it still has some bad keys on it.

I've been able to desolder a keyswitch cleanly and disassemble it - here is what we are looking at:

http://www.brutman.com/pics/TRS80_Model_4_Keyswitch.jpg

So the big question is, how does one improve the conductivity of the pad in the center of the rubber dome? On a good keyswitch the measured resistance is under 100 Ohms. On this particular bad keyswitch, the resistance varies from 6000 to 10000 Ohms, and only after you press down for a while.

In a previous thread in another universe somebody suggested using the graphite from a soft pencil. I am looking for something a little more robust than that - after I button this machine back up I never want to go inside of it again. (Getting inside of a TRS-80 Model 4 without damaging it requires the skill and patience of a safe cracker.)


Mike

Chuck(G)
December 26th, 2008, 03:10 PM
GC Electronics used to carry little bottle of colloidal graphite solution to repair aquadag coatings on CRTs. That would be just the ticket for your application.

Maybe a lister can spare some or someone knows of some old stock...

Yzzerdd
December 26th, 2008, 04:07 PM
Similair to what ChuckG posted, you could try to locate this metal paste... I don't recall the name, but it was a paste that you spread on to something, and when it dried it was solid metal, or at least had metal properties, including being picked up by a magnet.

This paste, as far as I know, was actually desigined for tool and dye work, like for reparing a dye. People at one point in time used this paste to do body work on their vehicle, such as applying it on minor rust spots. They said it smoothed out fine. I'd imagine it's metal conductivity properties might help out your problem.

Is the metal not making proper contact? I know an old trick for lightbulbs in sockets(GM's brake light/turn signal socket in particular) was to gently with a pocket knife pull up on the contacts a bit. This fixed brake lights that sometimes worked and other times didn't. The key problem isn't all that different. Maybe the switch isn't hitting the metal well enough?

--Jack
As you can tell, most of my repair and diagnosis is in cars. For the sort of thing you are working with, the best I can do is relate to vehicle repair.

mbbrutman
December 26th, 2008, 04:15 PM
The contact is hitting the metal contacts just fine. I suspect that it is supposed to have a conductive coating on it and that coating has seriously deteriorated.

The colloidal graphite that Chuck suggested sounds like the right material for the job, if it stays put for a few years. I don't want anything that will readily flake off the contact after it has dried.

patscc
December 26th, 2008, 04:39 PM
This is sort of a red-neck solution, but you can always pull apart enough junk remotes, calculators, etc. and use their contactors.
Or, get ahold of some composition gold leaf and cement it on the rubber.
There used to be conductive trace paint (well, still is, like http://www.mcmelectronics.com/product/CAIG-LABORATORIES-CW100P-/200-175), that might work. Try to find something with a brush instead of a tip.

I've actually tried all three methods with varying degrees of success.

The composite gold leaf( or other thin metal film ) sometimes causes problems since the resistance is so much lower than the conductive rubber.

The paint seems to work reasonably well. It does change the feel if you're picky, but on keyboards with a mechanical aspect ( as opposed to remotes & calculators, where the keypad is a sheet of dimpled plastic with the rubber nodules, and nothing else )

I refuse to comment on pulling apart junk remotes, but, if you've had a bad day...

The key to adding a layer of whatever on the rubber is to keep it thin, since there isn't much clearance between the contactor & pad, and reasonably uniform in thickness.

patscc

patscc
December 26th, 2008, 04:41 PM
Umhh, looking at the pics, have you tried using the green pad alone, without the framework ?
It might just be something jamming in the plastic whatsits the push the pad onto the contacts.

patscc

mbbrutman
December 26th, 2008, 05:01 PM
I have put the dome over the contacts on the base of the switch and actuated it by hand. Nothing is jamming it. Putting multimeter probes directly on the conductive disc in the green dome shows that it is not much of a conductive disk anymore.

The CircuitWriter solution looks even better than the colloidal graphite. I read the description on it and it looks like it is designed for circuit boards and similar surfaces, but I'm willing to take a chance that it will adhere to the conductive disc well enough to make a lasting repair.

Mike

patscc
December 26th, 2008, 06:19 PM
Sticks to most anything, although I'd put a dab of acteone on a q-tip, wipe, dab of iso, wipe, dry, before you use the conductive ink, if it's a really gunky keyboard. Depending on the brand of ink (pen), if it comes out to, well, if the drop is too "round", smoosh the droplet against a piece of parchment paper, and the dab the contact against it.
A thought, you might want to build up a couple of layers for longevity (This I haven't tried)

So why do they fail, anyway ? Random plasticizer failure ? Abrasion ? Can't really mention preferential wear & tear, seeing as how it's a 'Q' key. Or maybe because it doesn't get used (i.e. flexed) enough ? Wish I knew more about plastics.

patscc

mbbrutman
December 26th, 2008, 06:36 PM
I have no idea why some keys fail. The behavior is interesting too .. if you press hard on the key while it is hooked up to the meter you can see the resistance stays at infinity for a few while. After about 10 seconds it starts to show 12000 Ohms of resistance, and then it settles to around 6000 Ohms.

It's much easier for me to understand on and off .. non linear behavior is confusing. ;-0

I'm going to check Radio Shack to see if they have the conductive ink. If not, then I'll be mail ordering it. I really want to get this machine usable, and then learn how to use it! (I am a complete Tandy newbie ...) This one has:


First generation keyboard (arrow keys are not clustered)
Two floppy drives
128K
Serial and parallel interfaces


I need to figure out which operating system to use, and then I need to setup a development environment for it. A Z80 box like this is a bit more restrictive than what I've been using.

Druid6900
December 26th, 2008, 06:54 PM
The rubber is conductive in most of those keyboards, so, swabbing it off with a q-tip and acetone should resurface it.

patscc
December 26th, 2008, 06:55 PM
Urgle.
Sounds wierd. I don't suppose you have another pad open that you can swap out against it ?
Are there multiple affected keys ? (I'm curious about the resistance readings, but want to know about other keys before I blather on...)

RatShack certainly has conductive ink. It's what they use in the pen you use to sign the service agreement to the cell phones which is all they seem to be selling these days.
If from RatShack, try it out before you leave. They don't have a long shelf life.

For dev info, have you started digging around in http://www.trs-80.com/ ?
patscc

MikeS
December 26th, 2008, 08:02 PM
As Dru says, some of these only need a *good* cleaning. For the rest, some people use a hole punch to punch little disks out of aluminium foil and glue them on. You can also use the conductive ink sold to repair automotive rear window defrosters.

m

Chuck(G)
December 26th, 2008, 08:08 PM
You can also get conductive nickel and copper paint from GC Electronics (http://www.gcelectronics.com/order/catdisplay.asp?CatID=3). (Look under "thinners, solvents and coatings").

I'd go with the nickel for this application--less likely to corrode.

mbbrutman
December 26th, 2008, 08:41 PM
As Dru says, some of these only need a *good* cleaning. For the rest, some people use a hole punch to punch little disks out of aluminium foil and glue them on. You can also use the conductive ink sold to repair automotive rear window defrosters.

m

I went through the effort to desolder, disassemble, take resistance measurements with a meter and take the picture in the first post. I think I got the cleaning part down ... Wasn't it a pretty picture? :-)

Way back when I've done repairs like this using aluminum foil, but I really dont want it disintegrating or shredding after I seal the machine back up. I think that the best idea so far is the conductive ink, used both for automotive rear window defrosters and for fixing traces on circuit boards.

DimensionDude
December 26th, 2008, 08:44 PM
You can also get conductive nickel and copper paint from GC Electronics (http://www.gcelectronics.com/order/catdisplay.asp?CatID=3). (Look under "thinners, solvents and coatings").

I'd go with the nickel for this application--less likely to corrode.



They also have (or had) gold paint for an exorbitant price. I've used the nickel paint for repairing circuit traces, haven't tried it for balky keypads.

I also have, hanging on the wall, a "Rubber Keypad Repair Kit" from CircuitWorks, part no. CW2605. It seems to be an epoxy as there are two parts to be mixed. Unfortunately, it's rather old and the substance in the tiny bottle has dried up. It claims to repair up to 50 pads per kit.

mbbrutman
December 26th, 2008, 08:45 PM
The rubber is conductive in most of those keyboards, so, swabbing it off with a q-tip and acetone should resurface it.

I cleaned the contact areas with rubbing alcohol and a Q-tip. Acetone seems a bit risky because I really don't know what the materials are, and I don't want them being damaged. I know that acetone can be brutal on some plastics from personal experience.

I stabbed the rubber pretty hard with the test probes from the meter and I wasn't getting any joy. I'm suspecting that there was a coating which is no longer with us, and the suggestion to use the conductive ink pen probably is the safest thing to try for this particular situation.


Mike

patscc
December 26th, 2008, 09:13 PM
My memory's a bit fuzzy on this, but the "rubber" is actually a conductive elastomer. The acetone serves to 'roughen' the surface, the iso stabilizes it after the acetone. (Something like that, it's been a while since organic chem)
But if you're playing it safe, conductive-pen it first. The worst that'll happen is that it won't stick ( doesn't stick to oils/grease)
patscc

BillT356
December 27th, 2008, 06:24 AM
MCM Electronics has a kit to make repairs.
www.mcmelectronics.com


20-3890 - Rubber Keypad Repair Kit - Small
Enables repair of rubber button contacts on telephones, TV/VCR remotes, keyboards and more. Restores conductivity to worn carbon contacts by applying a perm...
Mfr Name: Chemtronics
Mfr Number: CW2605

Billt356

mbbrutman
December 27th, 2008, 07:21 AM
That was a very good link ...

Going a step further, if one searches for "rubber keypad repair kit" this looks like a pretty common item. I just didn't have a proper name for it!

Given my desire to get this right the first time, I think I will be patient, do a little more shopping, and get something like this by mail order if I can't get it locally. The conductive ink pen probably works well, but this type of kit seems designed to fix this exact problem.

Thanks again!
Mike

patscc
December 27th, 2008, 04:52 PM
Nice find. I can't wait to try it. It looks like it should work quite well. mbbrutman, can you post your results with it ?
patscc

tezza
December 28th, 2008, 02:11 AM
Good luck Mike.

Sounds like a very fiddly and demanding project, which will make it all the sweeter if you can pull it off successfully.

Let us know.

Tez

mbbrutman
December 28th, 2008, 06:26 AM
Tezza,

As I get into it I don't think it is all that bad. Getting into the Model 4 and removing the keyboard requires the skills of a safe cracker, but after that the keyboard is a pretty simple device. (Well, it is not that bad, but you have to be careful with the neck of the CRT tube.) I have gotten some help on the Classiccmp mailing list about how to get started, and the pointer to the rubber contact repair kit is just golden.

The problems with this particular keyboard are:


The keyswitches are soldered to a PCB at four points instead of the usual two points.
Removing a keyswitch from the middle of the keyboard is challenging because besides being soldered to a PCB, they are also pushed through a big metal plate which holds them firmly in place.
Disassembling the switches requires a very light touch


The original keyboard in the machine was garbage .. I could count on my fingers the number of keys that worked. The thought of desoldering, removing, and reconditioning 70 keys really didn't thrill me.

While testing the new keyboard I had a good idea pop into my head - measure the resistance on each key instead of just checking for continuity. So now not only can I tell if a key works, but how well it works. Which also lets me prioritize which keys need immediate repair, and which I can defer for another 20 years.

I'm going to have to mailorder the rubber contact repair kit, so that is slowing me down. I went looking in Radio Shack for either that or the conductive ink pen, and you should have seen the look on the store manager's expression when I told him I was restoring the keyboard on a Model 4. (He has probably been around long enough to remember selling them in the store.)

I'll hold onto that keyboard for when I have spare time. I never trash parts that I know can be reconditioned and reused. You should see my stockpile. :-)

mbbrutman
December 28th, 2008, 06:30 AM
RatShack certainly has conductive ink. It's what they use in the pen you use to sign the service agreement to the cell phones which is all they seem to be selling these days.
If from RatShack, try it out before you leave. They don't have a long shelf life.

For dev info, have you started digging around in http://www.trs-80.com/ ?
patscc

No more conductive ink, at least at my local Radio Shack. The store manager has been around long enough to know, and he thinks it is discontinued.

The TRS-80 site is great - I've been doing my background reading there.

tezza
December 28th, 2008, 09:22 AM
I went looking in Radio Shack for either that or the conductive ink pen, and you should have seen the look on the store manager's expression when I told him I was restoring the keyboard on a Model 4. (He has probably been around long enough to remember selling them in the store.)

You mean that "What, are you nuts!??"-kind of expression? Yea, I've got that at my local electronic store when I've told them I'm repairing an old computer. One guy said "Why don't you just get a new one??"

I think he was missing the point..lol


I'll hold onto that keyboard for when I have spare time. I never trash parts that I know can be reconditioned and reused. You should see my stockpile. :-)

Hmm. Stockpiles. Yes, I have a designated area for that and it seems to be getting bigger ever month :)

Tez

patscc
December 28th, 2008, 11:14 AM
tezza said...Stockpiles. Yes, I have a designated area for that and it seems to be getting bigger ever month

My stockpile is in boxes in the basement and in storage. I keep telling my wife the boxes are just emtpies for the next time we move. There may be a spot of trouble when the time to move comes, and I'm busted. But, as they say, the future will take care of itselfs.

patscc

Terry Yager
December 28th, 2008, 11:14 AM
I was going to suggest copper tape, but since there's a product designed for the purpose...
You'll let us know how it works out, Mike? (BTW, I'll pull apart the 4P board this afternoon and see how it compares, parts-wise).

--T

mbbrutman
December 28th, 2008, 11:35 AM
Geesh guys, if I went through all of this effort to carefully disassemble, disect and figure out the cause of failure, don't you think I'll be telling half the world when I get it working? :-)

On the 4P - I'm interested in seeing what the keyswitches look like, just out of curiosity. Some of the Model 4 keyboards have keyswitches with four pins (like mine), while others are of the two pin persuasion.

Yzzerdd
December 28th, 2008, 06:27 PM
Mike, was your Model 4 that hard to get in to? I just removed the necessary screws, picked it up a few inches off my bed and gave it a push downward and a jolt a few inches up the the case opened undamaged.

As for the keyboard, how was yours held down? Mine was a few screws and a plug. Don't mean to make you sound bad or anything, I'm just curious as to if there are design differences in mine and yours. To me, the Model III was no easier to open, except it's cover slid off without needing any work whatsoever.

--Jack

Druid6900
December 28th, 2008, 07:48 PM
Oh, after you've opened up a few hundred Model IIIs and 4s, it just become reflex. It's the same as getting the casetop back on the Model IIs and 16As without snapping off those lugs.

When we once had a shortage of keyswitches for the IIIs/4s, the boys in Fort Worth came up with with the acetone idea and National Parts shipped out a bottle of it and a box of q-tips to every repair depot in North America as an AXX-whatever keyswitch refurbish kit.

Just to show you how anal they were, it even came with instructions to the effect of dipping one end of the q-tip into the bottle, lightly swabbing the conductive rubber (there was even a diagram) and wiping it off with the other end of the q-tip.

Worked well and never caused any damage that was reported.

gerrydoire
December 28th, 2008, 08:58 PM
I took apart an IBM XT Keyboard to clean it inside and out, and it was a nightmare getting it back together....

Not sure cleaning the dust inside was worth it the hassle.

mbbrutman
December 29th, 2008, 05:36 AM
Mike, was your Model 4 that hard to get in to? I just removed the necessary screws, picked it up a few inches off my bed and gave it a push downward and a jolt a few inches up the the case opened undamaged.

As for the keyboard, how was yours held down? Mine was a few screws and a plug. Don't mean to make you sound bad or anything, I'm just curious as to if there are design differences in mine and yours. To me, the Model III was no easier to open, except it's cover slid off without needing any work whatsoever.

--Jack

I'm excessively paranoid about not damaging machines. Removing the top shell needs to be done very carefully because of the CRT neck - it is easy to get it caught and break it off. But yes, once you get the machine cracked in half safely the keyboard is easy to extract.

Compare that to opening an IBM machine - remove 5 screws and slide top of case forward. Except possibly snagging some cables in the middle of the machine while sliding the case, it's a fairly idiot proof operation.

mbbrutman
December 29th, 2008, 05:42 AM
Oh, after you've opened up a few hundred Model IIIs and 4s, it just become reflex. It's the same as getting the casetop back on the Model IIs and 16As without snapping off those lugs.

When we once had a shortage of keyswitches for the IIIs/4s, the boys in Fort Worth came up with with the acetone idea and National Parts shipped out a bottle of it and a box of q-tips to every repair depot in North America as an AXX-whatever keyswitch refurbish kit.

Just to show you how anal they were, it even came with instructions to the effect of dipping one end of the q-tip into the bottle, lightly swabbing the conductive rubber (there was even a diagram) and wiping it off with the other end of the q-tip.

Worked well and never caused any damage that was reported.

It's interesting to hear that Tandy used acetone as part of the repair kit. I'm still a little nervous about it given that acetone is pretty good at both cleaning and dissolving things.

If given a choice between new chemicals to coat the contacts and acetone, and the luxury of time, what would you choose to use?

patscc
December 29th, 2008, 10:45 AM
I'd tear apart a broken remote and try all methods, seeing as how you've got the time, to see which one works best for you. With the acetone, you just want lightly dab, not smother.
To further complicate your choice, check out
http://www.allelectronics.com/make-a-store/item/CK-44/CAIKOTE-TM-44-CONDUCTIVE-COATING-SILVER/CARBON/1.html and available at MCM as part no. 200-315 here http://www.mcmelectronics.com/product/CAIG-LABORATORIES-K-CK44-G-/200-315

It's for carbon contacts as well. It looks about the same as the rubber repair kit except that you don't have to mix it up. Allelectrinics also has the traditional circuit pen.

The acetone is more a thourough cleaning, wheras the rubber repair kit & the Caikote-44 pen actually put a new conductive layer on it. (Not that they have a conductive layer to begin with, the whole rubber plug is conductive, it's just that the surface of it gets gunky, hence the acetone solution.)


Druid6900 said...acetone
I'm glad someone else remembered acetone, although I didn't know that it was part of an "official" Tandy repair kit. How's that for a collectible ?

patscc

Druid6900
December 29th, 2008, 08:59 PM
It's interesting to hear that Tandy used acetone as part of the repair kit. I'm still a little nervous about it given that acetone is pretty good at both cleaning and dissolving things.

If given a choice between new chemicals to coat the contacts and acetone, and the luxury of time, what would you choose to use?

Truthfully, I think the acetone route is the best, which is why I suggested it in the first place.

Since you are going to be resurfacing the contact (removing whatever chemical process is preventing proper operation) it follows that the other methods MAY not adher to whatever gunk is on there or flake off in a short time.

Dabbing a little acetone on and then wiping it off may leave a LITTLE residue, but, since the other part of the connection is, IIRC, a couple of metal triangles with a bit of G10 between them, I don't think it's going to eat through the epoxy.

Since it is quite volatile, leaving the key bubble aside for about 10 minutes should allow any residual acetone to evaporate to nearly nothing.

Rubber Renew, used for cleaning belts and idler wheels in a number of tape transport devices (I used it to clean the drive belts on FDDs), is almost 100% acetone and doesn't seem to dissolve the guts of anything.

Druid6900
December 29th, 2008, 09:05 PM
I'm glad someone else remembered acetone, although I didn't know that it was part of an "official" Tandy repair kit. How's that for a collectible ?

patscc

It was a stop-gab measure (which I think they charged us about 18 bucks for).

Like I was going to continue to dis-assemble and clean keyswitches from then on instead of just replacing them when we got more stock in. LOL

AmigaJules
January 15th, 2009, 07:35 AM
I've successfully repaired an Amiga 1200 keyboard with a few bad keys by pressing the rubber/carbon contacts down onto a clean sheet of printer paper and GENTLY dragging the paper out from underneath them.

It left a dark smudge on the paper, then the key worked. I think it was just gunk built up on the contact that was preventing proper functioning.

I read about the technique on one of the Amiga forums.

Good luck!

Lohmeyer
January 15th, 2009, 04:58 PM
Re: Desoldering Keys:

The best way to remove the keys is a blade type soldering iron (two of them, or a tweezer type iron with two blades). For example I have a Metcal Talon tweezer soldering wand with varying width blades which can get in and heat up all for pins on a key contact at the same time. Metcals are expensive, though, so perhaps two Weller soldering irons with blade tips (does Weller make blade tips?) would do the job. But, even the weller stuff can be more than you want to spend.

Alternatively, get a hot air gun (like the kind used for shrinking heat shrink tubing). They have plenty of heat to melt the solder and allow you to just remove the keys. But, note I said keys. A hot air gun isn't a piece of precision desoldering equipment. :-D Ideally, get one that has a necked down nozzle for close work. They should be available from anyone that sells electronics tools, such as Jameco. Also, either cover the keys you don't want desoldered with something heat resistant (like a thick piece of rubber), or just plan to solder the keys back in afterwards if they fall out. You may need to suck some of the solder out of the holes with a good solder sucker first. Plus the hot air gun may blow solder around on the board, so you have to inspect it for solder blobs afterwards.

I have also used a small propane torch, but these can burn the PCB if not done correctly, and I don't recommend it with out practicing on some unimportant PCB's first.



Re: Acetone Repair

It sounds to me from what Druid says, that you want the Acetone to slightly melt the rubber, and that's what actually re-conditions the keys. Applying it sparingly with a Q-tip and then rubbing it off with the other end shouldn't turn a key into goo, but it will soften it up a little.

Acetone doesn't actually melt most rubbers immediately. Instead, it soaks into the rubber and softens it up. Left along long enough, it will turn the rubber to goo. Left alone longer, and the rubber will dry out again and be solid (again, most rubbers, not all rubbers).

But, of course this only makes sense if the rubber itself is a conductive rubber, or there is still some coating left. The test of sticking the ohm meter probes deep into the rubber and seeing no drop in impedance implies that the rubber is not conductive.

mbbrutman
January 15th, 2009, 05:44 PM
But, of course this only makes sense if the rubber itself is a conductive rubber, or there is still some coating left. The test of sticking the ohm meter probes deep into the rubber and seeing no drop in impedance implies that the rubber is not conductive.

Which is why I haven't bothered with the Acetone.

Radio Shack may have had some models of keyboards the Acetone trick would work on, but if it was a matter of being dirty then my attempts to clean black disc should have worked better. And stabbing the disc with the probes from the meter made no difference.

I intend to do the fix using the rubber contact repair kits - I think that is the best long term fix. It's just on the back burner for now .. the FTP client that I'm coding is of immediate practical use to me. FTP from the Model 4 will come later. :-)

Chuckster_in_Jax
January 15th, 2009, 06:59 PM
Reading through the posts I get the impression that the individual keys have to be unsoldered before thay can be disassembled. Is this true for most of these old keyboards? I have an OKI IF/800 with a lot of keys that don't work and I would like to fix it. I also have an old Heath H9 terminal in rough shape I need to repair.

tezza
January 16th, 2009, 01:26 AM
Reading through the posts I get the impression that the individual keys have to be unsoldered before thay can be disassembled. Is this true for most of these old keyboards? I have an OKI IF/800 with a lot of keys that don't work and I would like to fix it. I also have an old Heath H9 terminal in rough shape I need to repair.

It probably does vary. However, my Atari 130XE and Apple IIe Platinum keys were (thankfully) just held on by friction!

Tez

Lohmeyer
February 19th, 2009, 10:03 PM
I just got a TRS-80 Model III with less than 1/4th the keys working. Ugh! I guess it was time to figured out how to fix keys - lots of them quickly and conveniently. I have reconditioned nearly 1/2 of them now, and amazingly it is going very fast.

Regarding my comments earlier about unsoldering the keys, I suggested using a blade soldering iron to get the keys out. On this Model 3, however, I was able to just suck the solder out of the holes, and the pins were free. I normally don't expect this to work because there are always little bridges you have to melt to get the part out. But the holes on this Model 3 keyboard are big enough that the solder sucker does a perfect job the first time. Desoldering the keys has the potential to be the most difficult part of the repair job. I'm using a Soldapullt desoldering tool for this. It has lots of suction.

Once desoldered, I flipped the keyboard over, pulled off all the key caps, and then I grabbed the key by the key cap post with a pair of needle nose pliers, pressed in the little tabs on the side that hold the key to the metal bracket, and rocked it back and forth until it came out. I used a small flat blade screwdriver to press the tabs in. You can press one side in, rock the switch up on that side, and then press the other side in and rock up on that side. Repeat, rocking the switch back and forth until it comes out. You can see that if the pins still had solder bridges, this wouldn't work. You would have to apply heat to the pins while rocking the switch out at the same time. I don't have that many hands.


... I get the impression that the individual keys have to be unsoldered before thay can be disassembled. ...

This is true for all the TRS-80 keyboards I have seen at least. I have repaired a single key before, but never had to do an entire keyboard like this.

After I got the keys out, I took the keys apart - again thankfully very easy to do because of how the keys are assembled. Just flip up two plastic tabs and it comes apart. I then used a pink pencil eraser to clean the metal contacts in the bottom of the key's plastic housing. In my case, the carbon/rubber dome part is still fine. I didn't have to recondition the carbon parts. The problem was the metal contacts had gotten tarnish on them, probably due to being stored for years without use.

FYI - keys that worked measured about 500 ohms when pressed. Keys that didn't work were more like 5000 ohms or much higher. After cleaning the metal contact, all the keys dropped down to the 40-70 ohm range.

Regards,
Mike

chuckcmagee
February 20th, 2009, 12:15 AM
Alright!! <jump> <jump> The ol' pink pensil eraser trick. I keep telling people here about that frequently. Do NOT use on ribbon cables that use conductive paint, as I found out the hard way. I ended up with some nice smoke colored clear plastic with no traces on it.