Atari 1200xl Keyboard Repairs

[Jeffrey Worley]

I just got a SIIG clicky keyboard.  It isn't my old IBM keyboard but it is very nice.  Keyboards in late years have gotten really flimsy.  They work ok, but no heft and no click.  This is an AT/XT version, so no Atari for it.  

 

Have you considered applying your skills to the ST series?  Mouse and keyboard would be nice.

 

Best,

 

jeff

[+mytek]

3 hours ago, Jeffrey Worley said:

I just got a SIIG clicky keyboard.  It isn't my old IBM keyboard but it is very nice.  Keyboards in late years have gotten really flimsy.  They work ok, but no heft and no click.  This is an AT/XT version, so no Atari for it.  

You would probably like the IBM Model M. It's original PS/2 and very solidly built.

 

3 hours ago, Jeffrey Worley said:

Have you considered applying your skills to the ST series?  Mouse and keyboard would be nice.

No need really. They have something called the Eiffel 3 which does both.

 

[Jeffrey Worley]

On 1/14/2020 at 2:18 PM, mytek said:

Bob have you developed a stutter? BTW, they make some good traps for those things .

 

MacRorie has created some PS/2 to Mouse adapters for the A8 or the ST based on my MouseTari chip. The Logitech M-SBF96 works very well with it, and it's an optical mouse so no fussing with cleaning your mouse balls.

 

Obviously the stock 1200XL keyboard is a great keyboard. I use a TK-II-PB board in my 1200XL as the means to play around inside without having to constantly plug the stock keyboard in to test something.

 

Here's what that looks like, using the RF jack hole for access to the Mini-DIN6 jack.

 

 

The new board I'm working on will be even more compact.

 

 

I see you haven't upgraded the 1200xl, have the original roms and sockets.  Mine came with 28pin sockets!  Very cool.  I'm gonna stick a U1MB in there.

[Airshack]

I recently picked up an Atari 1200xl with one broken key switch stem. The previous owner attempted to glue it together. The top of the stem is stuck inside the keycap, with the bottom half remaining attached to the keyboard.

 

Any ideas on where to get a new key switch replacement? Recently, I replaced the ALPs key switch on an Apple //gs keyboard. Wondering if the Atari 1200xl has removable key switches?

[+mytek]

1 hour ago, Jeffrey Worley said:

I see you haven't upgraded the 1200xl, have the original roms and sockets.  Mine came with 28pin sockets!  Very cool.  I'm gonna stick a U1MB in there.

Yeah I've kept it pretty much stock, only installing a UAV and the TK. Eventually I'll upgrade it to a newer OS and built-in Basic, which will require socket and jumper changes. Not sure what I'll do after that. Don't really need to go the whole U1MB route, since I have other better suited machines with that already built-in.

 

[+mytek]

8 hours ago, Airshack said:

I recently picked up an Atari 1200xl with one broken key switch stem. The previous owner attempted to glue it together. The top of the stem is stuck inside the keycap, with the bottom half remaining attached to the keyboard.

 

Any ideas on where to get a new key switch replacement? Recently, I replaced the ALPs key switch on an Apple //gs keyboard. Wondering if the Atari 1200xl has removable key switches?

bob1200xl is kind of the best thing to a 1200XL expert you're likely to find, perhaps he can provide an answer to this question.

 

[+bob1200xl]

9 hours ago, Airshack said:

I recently picked up an Atari 1200xl with one broken key switch stem. The previous owner attempted to glue it together. The top of the stem is stuck inside the keycap, with the bottom half remaining attached to the keyboard.

 

Any ideas on where to get a new key switch replacement? Recently, I replaced the ALPs key switch on an Apple //gs keyboard. Wondering if the Atari 1200xl has removable key switches?

I have a bunch of keystems. You might be able to drill the center out of the keycap and remove the four pieces left in. Don't drill through the top of the keycap. Use something like a 1/16" drill at low speed.

 

Take out all the tiny screws and remove the k/b PCB. You may have to fix your k/b when you do this.

 

PIC: Remove the old keystem by prying it up with a suitable tool - I use a pair of curved tweezers. Insert the new keystem and press the keycap onto the new stem.

 

Put it all back together. Test each key.

 

Good luck!

 

Bob

 

[Jeffrey Worley]

On 1/12/2020 at 6:49 PM, bob1200xl said:

Uuummmm... (oh, my goodness)

 

I don't want to start anything here but my experience with 1200XL keyboards is somewhat different than yours. The point of failure is the interconnection between the mylar and the PCB. The mylar contacts lose continuity and need to be re-coated or replaced. You sound like you have a third option. (nice writeup)

 

Best Electronics makes a replacement mylar that works very well. Pull off the old mylar, clean the PCB with GOOF-OFF or alcohol (90% +), and position the new mylar. Six zillion little screws later and you're good to go.

 

Even the new mylar may resist making good contact, so you may have to re-position it a little. Don't bother with that little interposer board. Works perfect without it once you get happy.

 

Or, you can just touch up the original mylar. Instructions are around here somewhere...

 

Bob

Don't start nothin' won't be nothin'.  ?

 

I've not run into this with this keyboard, but you've handled, like, ALL of them, so I defer to you. ? 

 

For the group, diagnosing this is pretty simple and involves removing the screws from the bottom of the keyboard and separating the halves.  Once done, plug the keyboard (sans keys of course) back into the machine and test the pads on the board using BASIC or the keyboard test in rom to give you some response.  If the keypads produce keystrokes reliably using your fingers to depress them (a lot more powerful stroke than the spring-driven keys themselves), then your Mylar is OK, and you need to follow the instructions in the 1200xl keyboard post a few messages back in this thread.

 

If they DON'T, then you have a mylar problem or, less likely, the IC's mounted to the PCB in the 1200xl, (which are almost universally mounted on the motherboard for other Atari 8-bits).

 

If you have great trouble getting the tested mylar to reliably put down keystrokes when fully assembled, a last resort is to GENTLY and CAREFULLY peel up the mylar from the PCB  WITHOUT TOUCHING the area where the contacts are connected to the PCB.  You want to leave those contacts (lower left-hand side of mylar) well stuck.  Use a flat shim, like a playing card or other stiff, thin item to aid you in this.  The underside of the mylar is coated with a latex barrier which may peel a bit in the difficult spots.  If it starts to peel, stop and work at that spot from the opposite direction.  The places the latex sticks are around the screw holes, and the strakes which press the mylar down on the PCB.  Gentle, careful shimming will peel the mylar away from the top, completely but for the several-contact fingers at bottom.  Once done, examine the mylar for stray bits of latex and remove them gently.  Do the same with the PCB.  Once you are sure the mylar and the PCB are free of debris, lay the mylar back down on the PCB so that it is smooth and even.  It does not stick.  It doesn't have to, really shouldn't.  Now, plug the keyboard in and test the pads again to make sure you have not damaged the mylar and inactivated any keys.  If you have good function, put the keyboard back together and test.  Since the mylar is now free, it will flap on reassembly.  You will of course account for this tendency and, by holding the PCB in its natural, installed orientation, make the keys to it, ensuring a square fit and flat mylar.  Now you can, holding the two halves together tightly, invert the assembly and install a cross of screws.  Install and test the keyboard.  If you have good function, install the rest of the screws without over-torquing any.  You should be done.  It may be gripey for the first few keystrokes.  Operate all keys until you have no gripiness left.

 

If this does not work, you will have to replace the mylar as Bob, the 1200xl god indicates. ?

[Banquo]

Most of the keys on my 1200XL would not work, so I took it apart and when I tried just plugging in the board without the keys on top I found the same thing as the OP said. When I pressed the contacts with my finger every single key worked perfectly, just the slightest touch. Put it back together and they stopped working. It seemed really puzzling but it makes sense now that the springs just aren't pressing down hard enough anymore. I'm actually disappointed about that because it sounds like a nightmare trying to take out every spring and stretching them out. Do the new mylars from Best Electronics work ok even with weak springs? That would be a much easier fix, though a bit expensive.

[Colleton]

13 minutes ago, Banquo said:

Most of the keys on my 1200XL would not work, so I took it apart and when I tried just plugging in the board without the keys on top I found the same thing as the OP said. When I pressed the contacts with my finger every single key worked perfectly, just the slightest touch. Put it back together and they stopped working. It seemed really puzzling but it makes sense now that the springs just aren't pressing down hard enough anymore. I'm actually disappointed about that because it sounds like a nightmare trying to take out every spring and stretching them out. Do the new mylars from Best Electronics work ok even with weak springs? That would be a much easier fix, though a bit expensive.

Almost certainly it's the keyboard connector not making good contact with the motherboard.  It isn't the springs.  Go back to the 1st page and watch FJC's repair video.  It's a scary process the 1st time you do it, but take your time and use care and you'll be okay.

[jamm]

33 minutes ago, Banquo said:

Most of the keys on my 1200XL would not work, so I took it apart and when I tried just plugging in the board without the keys on top I found the same thing as the OP said. When I pressed the contacts with my finger every single key worked perfectly, just the slightest touch. Put it back together and they stopped working. It seemed really puzzling but it makes sense now that the springs just aren't pressing down hard enough anymore. I'm actually disappointed about that because it sounds like a nightmare trying to take out every spring and stretching them out. Do the new mylars from Best Electronics work ok even with weak springs? That would be a much easier fix, though a bit expensive.

The springs are there to raise the keycaps up, away from the PCB, not push them down towards it, so it's weird to describe the issue as the springs not pressing "down hard enough".  I guess it's possible that "weak" springs could be causing keys to register contact when nothing is happening, but I imagine that would result in either a.) constantly repeating keys, or b.) no keys ever registering because too many circuits are closed.  Someone who understands the electronics of the Atari8 keyboard matrix should be able to answer definitively whether that's even possible.

 

The "classic" problems with the Mitsumi keyboards on the 800 and 1200XL are:

• The connector interface point between the PCB and the Mylar sheet becomes deteriorated. This requires the repair with conductive paint that flashjazzcat illustrates in his video on the first page
• The contact pads on the Mylar sheet for each individual key become oxidized and stop making good contact.  This is where "pressing lots of times" can sometimes fix the issue as the oxidation might get cleared up.  If that doesn't work, you'll have to *gently* scrape each contact to remove the oxidation.  I used the end of a flat-head screwdriver successfully for this.

 

My 800 had both these problems.  The conductive paint repair restored most of the functionality, but I still had a couple of keys that only registered very intermittently.  Very gently scraping the contact pads with the end of a flat-head screwdriver turned them from a sooty black color to a slightly silver/lead color and they worked perfectly after that.

 

This is the first I've seen anyone suggesting the springs are at fault, but please do report back if that ends up being the case!

 

 

 

 

 

 

Edited April 2, 2020 by jamm

[Jeffrey Worley]

3 hours ago, Banquo said:

Most of the keys on my 1200XL would not work, so I took it apart and when I tried just plugging in the board without the keys on top I found the same thing as the OP said. When I pressed the contacts with my finger every single key worked perfectly, just the slightest touch. Put it back together and they stopped working. It seemed really puzzling but it makes sense now that the springs just aren't pressing down hard enough anymore. I'm actually disappointed about that because it sounds like a nightmare trying to take out every spring and stretching them out. Do the new mylars from Best Electronics work ok even with weak springs? That would be a much easier fix, though a bit expensive.

The fix is to remove the mylar from the pcb and put it back on.  It is stuck on right now and that makes for too much tension.  It is not giving enough to the keypresses.  Avoid peeling the part to the lower-left, the 'fingers' that connect to the pcb.  Those are fine as stuck, are working and it is best to leave well-enough alone there, but free the whole rest of the mylar such that it can flap back, then lay it down, test the keys, then reassemble.  If you gently peel up the mylar, unsticking it, and then lay it back down (as described in my post above), you will have a working keyboard in minutes.  I used a small shim to peel the mylar off the pcb (it is SUPPOSED TO BE FREE, like a playing card, to slip between the mylar and pcb and work it free.  Once free, it will work fine.

 

Here's a paste of my post above:

 

<SNIP>

 

If you have great trouble getting the tested mylar to reliably put down keystrokes when fully assembled, a last resort is to GENTLY and CAREFULLY peel up the mylar from the PCB  WITHOUT TOUCHING the area where the contacts are connected to the PCB.  You want to leave those contacts (lower left-hand side of mylar) well stuck.  Use a flat shim, like a playing card or other stiff, thin item to aid you in this.  The underside of the mylar is coated with a latex barrier which may peel a bit in the difficult spots.  If it starts to peel, stop and work at that spot from the opposite direction.  The places the latex sticks are around the screw holes, and the strakes which press the mylar down on the PCB.  Gentle, careful shimming will peel the mylar away from the top, completely but for the several-contact fingers at bottom.  Once done, examine the mylar for stray bits of latex and remove them gently.  Do the same with the PCB.  Once you are sure the mylar and the PCB are free of debris, lay the mylar back down on the PCB so that it is smooth and even.  It does not stick.  It doesn't have to, really shouldn't.  Now, plug the keyboard in and test the pads again to make sure you have not damaged the mylar and inactivated any keys.  If you have good function, put the keyboard back together and test.  Since the mylar is now free, it will flap on reassembly.  You will of course account for this tendency and, by holding the PCB in its natural, installed orientation, make the keys to it, ensuring a square fit and flat mylar.  Now you can, holding the two halves together tightly, invert the assembly and install a cross of screws.  Install and test the keyboard.  If you have good function, install the rest of the screws without over-torquing any.  You should be done.  It may be gripey for the first few keystrokes.  Operate all keys until you have no gripiness left.

 

Best,

 

Jeff

Edited April 2, 2020 by Jeffrey Worley
Clarity.

[Jeffrey Worley]

3 hours ago, jamm said:

The springs are there to raise the keycaps up, away from the PCB, not push them down towards it, so it's weird to describe the issue as the springs not pressing "down hard enough".  I guess it's possible that "weak" springs could be causing keys to register contact when nothing is happening, but I imagine that would result in either a.) constantly repeating keys, or b.) no keys ever registering because too many circuits are closed.  Someone who understands the electronics of the Atari8 keyboard matrix should be able to answer definitively whether that's even possible.

 

The "classic" problems with the Mitsumi keyboards on the 800 and 1200XL are:

• The connector interface point between the PCB and the Mylar sheet becomes deteriorated. This requires the repair with conductive paint that flashjazzcat illustrates in his video on the first page
• The contact pads on the Mylar sheet for each individual key become oxidized and stop making good contact.  This is where "pressing lots of times" can sometimes fix the issue as the oxidation might get cleared up.  If that doesn't work, you'll have to *gently* scrape each contact to remove the oxidation.  I used the end of a flat-head screwdriver successfully for this.

 

My 800 had both these problems.  The conductive paint repair restored most of the functionality, but I still had a couple of keys that only registered very intermittently.  Very gently scraping the contact pads with the end of a flat-head screwdriver turned them from a sooty black color to a slightly silver/lead color and they worked perfectly after that.

 

This is the first I've seen anyone suggesting the springs are at fault, but please do report back if that ends up being the case!

 

 

 

 

 

 

The springs do raise the keys, but they also provide the interface between the key and the silicon-rubber contact button.  The key is depressed, compressing the spring, which moves away in sympathy to the pressure, which moves the rubber button, which contacts the mylar, which gives in sympathy and collapses a hair's -breadth do touch a contact on another piece of mylar beneath it.  Expanding the springs is one way to fix it,  The other is to peel the mylar up and lay it back down.  It is stuck to the PCB from age and time, and that is making it more rigid than the design accounts for.  Expanding the springs an eensy bit makes up for that additional rigidity, or, you can remove the source of the rigidity, which is the better fix, really, by freeing the mylar from the pcb, thus allowing for the play that was a requirement of the original specification for the amount of pressure the springs can exert.

 

Best,

 

Jeff

[Banquo]

Success! Thank you guys so much, that was exactly the problem. I held it in front of my little space heater for a few minutes to warm it up and used a thin card to slowly work the mylar loose from the board; it took about ten minutes. The screw holes were the worst spots but I eventually got it all loose without damaging any traces. The worst area was the finger connectors, some of the conductive stuff lifted off and I was really worried about that being a problem. I've read about people using silver paint but I wonder if cutting thin strips of conductive copper tape would work better? I don't have any of that stuff at the moment so I figured I'd just try it as is and see if it worked. I cleaned the board and laid the mylar back down. Then I cut a piece of electrical tape and put it down over the finger connectors to give it a little more thickness for the keyboard frame to press down on. Plugged it in and wow, every key works perfectly now! I hope it will last but I may eventually buy one of those new mylar sheets for it. That white coating Mitsumi put on them is awful but I guess it seemed like a good idea at the time.

 

I'm really thrilled to have it all working now, everything else is in perfect condition. I've always thought the 1200XL was one of the most beautiful computers from the time. ?

[jamm]

4 hours ago, Banquo said:

Success! Thank you guys so much, that was exactly the problem. I held it in front of my little space heater for a few minutes to warm it up and used a thin card to slowly work the mylar loose from the board; it took about ten minutes. The screw holes were the worst spots but I eventually got it all loose without damaging any traces. The worst area was the finger connectors, some of the conductive stuff lifted off and I was really worried about that being a problem. I've read about people using silver paint but I wonder if cutting thin strips of conductive copper tape would work better? I don't have any of that stuff at the moment so I figured I'd just try it as is and see if it worked. I cleaned the board and laid the mylar back down. Then I cut a piece of electrical tape and put it down over the finger connectors to give it a little more thickness for the keyboard frame to press down on. Plugged it in and wow, every key works perfectly now! I hope it will last but I may eventually buy one of those new mylar sheets for it. That white coating Mitsumi put on them is awful but I guess it seemed like a good idea at the time.

 

I'm really thrilled to have it all working now, everything else is in perfect condition. I've always thought the 1200XL was one of the most beautiful computers from the time. ?

 

That's great!  I had the same issue you did with the Mylar sticking very thoroughly around the holes in the PCB.  I used a hair drier to get things really hot, but bits of the Mylar still tore for me.  For whatever reason, the Mylar on my 800 was *much* easier to deal with.

 

I used a folded-up piece of paper to improve the connection between the PCB and the Mylar where the connectors are, but electrical tape sounds like a better idea.

 

If you end up needing to go back in and add that conductive paint, here's the best source I found after looking around quite a lot:

https://www.chipquik.com/store/product_info.php?products_id=480001

 

[ijor]

On 1/12/2020 at 4:31 PM, Jeffrey Worley said:

Since my return from the dead all I've been hearing about the 1200xl keyboard (and the mitsumi 800 keyboard) is that they are failures.  I have decades of experience with these machines and I deny that these keyboards are anything but excellent.

 

This keyboard design likes to be used.  If you don't use it, two things will happen:

1)  The mylar, over years, will become ever so slightly stiffer.  You would need a special gauge to test this but it is so.  I know.

2)  The springs under the silicon rubber contactors - these springs actually provide the pressing force on the mylar - will collapse ever so little, reducing the effort they can exert on the mylar.

To fix this fault:

Take the 30 odd screws out of the bottom of the keyboard.  It will separate cleanly into two sections, the key part and the pcb.  No parts will be loose to worry about.

...

 

 

Very useful information, thanks Jeff.

 

I have a 1200XL that is brand new, or at least in brand new condition. The keyboards was completely dead. No key worked at all, not even the Reset key. I followed Jeff instructions, disassembled the keyboard, found the mylar looking pristine and connected the naked pcb. Indeed it works fine without the actual keys.

 

Quote

I have two Atari 1200XL computers and both keyboards did not work anymore.  I thought the mylars were bad.  When Best Electronics released their new ones I bought two.  When I had time to work on these I found that if I hit each key hard many times they started to work!  I then spent a while hitting each key really hard 50 times or so until it started to work. I did this to each key and eventually they all were working!  I still have my mylars incase I need them later, but for now this method worked for me!

 

I would prefer to avoid messing with the springs. So I reconnected the whole keyboard, tried the "tjlazer" method and started working out the keys. After many hits, a few keys started to come alive! Still a long way to go, it feels like this keyboard will need something like 500 hits per key, not just 50. But, oh well, things we do for Atari!

 

[Jeffrey Worley]

19 minutes ago, ijor said:

 

 

Very useful information, thanks Jeff.

 

I have a 1200XL that is brand new, or at least in brand new condition. The keyboards was completely dead. No key worked at all, not even the Reset key. I followed Jeff instructions, disassembled the keyboard, found the mylar looking pristine and connected the naked pcb. Indeed it works fine without the actual keys.

 

 

I would prefer to avoid messing with the springs. So I reconnected the whole keyboard, tried the "tjlazer" method and started working out the keys. After many hits, a few keys started to come alive! Still a long way to go, it feels like this keyboard will need something like 500 hits per key, not just 50. But, oh well, things we do for Atari!

 

You can truncate this process considerably by opening the keyboard and carefully peeling the mylar down from the top to the bottom, not peeling the tab that connects the traces of the mylar to the PCB, but everywhere else, the keypad portion.  Once peeled back, just lay it back down and test.  It should still work nicely but will now be nice and loose instead of bonded to the keyboard PCB.  It is this bonding that makes the mylar so tight and resistant to the keystrokes from the strikers above.  Peeling is a pain, but it will make the keyboard new without having to strike each key a bunchazillion times.  Once you put it back together you will need to type a little on it to settle things in, but it is a sure fix.

 

[adam1977]

I've recently repaired a Mitsumi keyboard using this conductive electric paint, it's actually graphite based and looks similar to what was originally used on the mylar.

 

https://www.bareconductive.com/news/what-is-electric-paint-the-composition-and-application-of-conductive-paints/

[+bob1200xl]

I will give this one more try...

 

Look at the k/b circuits. The keypress makes contact between the mylar and the PCB. The PCB  side is gold, so just clean it with alcohol. The mylar side is graphite or some such that you can wipe out by playing with chemicals. Leave them alone. I have not had much luck removing the mylar from the PCB with or without gentle heat. It always tears. If you manage/minimize the tears, it will all be good. The mylar insulator holds itself in place.

 

Once the PCB/mylar contact is closed, the signal has to make it thru the k/b connector and out to the flat cable.

 

So, two contacts. If you get out your meter, you will measure a couple of hundred ohms (or more) when the contacts are made. The path is a series circuit with two points of failure, the PCB mylar and the k/b connector. Both of them usually measure high, but not open. So, if you fool around with the connector, it may all work. Or, if you clean the PCB, it may all work. It's a series circuit... If you move the point of contact, it may be happy, or not. Press the keys 50 times...

 

You are all pretty much right.

 

Bob

[Jeffrey Worley]

1 hour ago, adam1977 said:

I've recently repaired a Mitsumi keyboard using this conductive electric paint, it's actually graphite based and looks similar to what was originally used on the mylar.

 

https://www.bareconductive.com/news/what-is-electric-paint-the-composition-and-application-of-conductive-paints/

The XE keyboards are terrible for the traces frying.  I had one of the first 130xe's and it failed in just a couple of months.  I had it fixed in warranty and it failed a few months later, out of warranty.  It was then I discovered Nickel Print, the conductive paint to which you refer.  It works well but requires some finesse to work with, especially to repair trace breaks more than an inch or so long, or breaks in crowded areas of the board.  I keep it in my kit for just such repairs:

 

https://www.amazon.com/gp/product/B01M28SRH0/ref=ppx_yo_dt_b_search_asin_image?ie=UTF8&psc=1

 

I've used it on the XE a zillion times, but only a handful of times on the XL, and then mostly to fix the ends of the fingers that mate with the connector on the main board.  The 1200xl, never, but I've only had a handful of 1200xl's to fix.  All of those responded to keystrokes to reawaken the keyboard, or, at most, peeling up the mylar and laying it back down.  What 'tears' on the 1200xl mylar is not the mylar, if you are careful, it is the pesky intermediate layer between the mylar and the PCB, which is a sort of latex rubber.  You can just try to preserve this layer as best you can, peel up the mylar with a shim, paying careful attention to the areas around the screw holes and strakes, where pressure has bonded the latex to the PCB.  Once the mylar is flapping free, but for the part that mates with the PCB to carry the signals to that, in the lower-left-hand side of the mylar, clean any little bits of latex left laying around on the PCB and the mylar and then just lay the mylar back down.  It shouldn't be bonded to the PCB.  This bonding is an effect of time and pressure, like coal :-).  The bonding makes the mylar too stiff and it won't flex enough to allow the light pressure the springs on the keys exert to close the contacts.  With the mylar free again, the play needed is restored.  That is why folks have such good luck with replacement mylars.  It isn't the new mylar that does the trick, it is that the new mylar is nice and free, as your old one will be if you follow the above instructions.

 

best,

 

jeff

[Jeffrey Worley]

14 hours ago, adam1977 said:

I've recently repaired a Mitsumi keyboard using this conductive electric paint, it's actually graphite based and looks similar to what was originally used on the mylar.

 

https://www.bareconductive.com/news/what-is-electric-paint-the-composition-and-application-of-conductive-paints/

There are two common types of conductive paint.  One is carbon particles in an aromatic solvent and the other is nickel particles.  The nickel is more conductive so I prefer it for fixes.  Usually you can tell what the maker used for the printjob on the mylar by the color.  Gray would be nickel and black would be carbon, but experience has shown me that you don't have to match the types to get a good repair with nickel print.  It just works.

 

The only downside of this sort of repair is that it does not flex very well, so repairs made at points that flex a LOT, like the shoulder of the part that mates with the main board of the computer, up high where it leads into the keyboard body.  There the angle is pushing 180 degrees.  You can still get a good repair, but you need to be careful to paint thin traces. Thin flexes more than thick, which latter may crack and flake.  On parts that don't flex at all the fix is good as new and will probably outlast any other part of the circuit.

 

To make fine lines, I use a couple of different 'brushes'.  One is a paper match stick, cut at a 45 degree angle, and the other is a fine wire, I use 30 gauge solid-core hookup wire because I have a roll on my workbench...  Clip a short section of wire into the jaws of a pair of hemostats, dip the end in the paint, draw a trace, throw the wire section away, clip another, and repeat.  The reason you pretty much have to start fresh with a new 'brush' for every little trace application is that the paint is highly aromatic, dries quickly, and it will clump at the end of the brush, which makes subsequent applications draw thick, messy traces.  A fresh brush will draw the cleaner trace, yields the best results and avoids bridging traces.  When you create a bridge, often the paint will dissolve the factory paint on the bridged trace, which will require you to repaint THAT trace as well, so avoid spreading the stuff around as much as possible as doing so may double or triple your workload.  ?

 

Best,

 

Jeff

[ACML]

When I first started out repairing 1200XL keyboards, I used the expensive Craig Circuit Writer conductive pen.  It was hard to work with it's applicator and once opened, it didn't have a long shelf life.  Radio Shack started to carry BARE conductive paint which I've used on the majority of 1200XL and three 800 Mitsumi keyboard repairs.  It's thicker and seems to match the dark color of the original traces (Carbon based?).  The shelf life, once opened,  is much better.   My preferred method of application is a toothpick.  As they say, "a little dab will do ya".  I just pull the paint off the dab like an artist's paint pallet.  I do find that BARE has a high resistance.  This can be confusing at first.  If you are bridging a gap say more than 1/8", the resistance is high enough that once dry, a multimeter set for continuity will still see it as an "open".  Trust me, it will still work.  Even though the repaired trace won't give you a "beep" with the multimeter, it will work.

 

The multimeter is very useful prior to repair as it is excellent at detecting where the trace is broken, even if it's under the white silicon spacer.  You just march down the trace for the errant key touching exposed traces that are not covered by the silicon spacer and you will find the break.  Usually, the breaks will be obvious as they are generally where the white silicon spacer has torn and separated from the mylar during removal.

 

I agree, this stuff dries hard and is brittle when dry.  I'd avoid trying to touch up the mylar at the key touch pads as repeated contact will increase the likelihood of flaking particles and causing potential shorts.

 

[adam1977]

I tried to use a brush with the conductive paint, but I just didn't have one fine enough, in the end I used the leg of a resistor and an eye loop ?

[Jeffrey Worley]

On 1/16/2020 at 12:02 PM, Airshack said:

I recently picked up an Atari 1200xl with one broken key switch stem. The previous owner attempted to glue it together. The top of the stem is stuck inside the keycap, with the bottom half remaining attached to the keyboard.

 

Any ideas on where to get a new key switch replacement? Recently, I replaced the ALPs key switch on an Apple //gs keyboard. Wondering if the Atari 1200xl has removable key switches?

Ask Bob1200xl on Atariage.  He has some keysystems he can let go of.

[+bob1200xl]

Yep. Millyions and millyions of 'em.

 

Bob