Atari 800XL keyboard issue

[kynikos]

Hi all, I picked up my first Atari 800XL and everything works great aside from the whole top row of keys (escape -> backspace). None of them register at all. Is there anything I should investigate once I open up the computer? At least it's made in Hong Kong, so everything should be socketed from what I've read, in case it comes down to replacing chips. I've also heard of the mylar going bad as well? Any help would be appreciated to point me in the right direction!

[Beeblebrox]

@kynikos  given a select few keys in a row the keyboard is likely a mylar with a line of em knocked out by one of the mylar tracks having gone bad. Loadsa advice on AA about painting mylar tracks back on with conductive paint.

 

It could also be a bad trace on the keyboard's pcb if it is that type of keyboard knocking out a line of keys which is fixable. 

 

Open up the lid and post a few pics of the keyboard and cross ref it with this thread :

 

 

 

[erichenneke]

Permatex defogger repair kit conductive paint is always my go to. And a continuity tester, trace by trace.

[xrbrevin]

here is the schematic:

 

[erichenneke]

One thing to emphasize... After you take the backing plate off the bottom side of the keyboard assembly, do NOT NOT NOT ever flip it over. You'll be hunting for, and picking up, tiny little springs for the next hour of your life if you do!  

[bfollowell]

Having several keys in a row or column, according to the keyboard matrix, go out is also a clear indication of a bad 4051 multiplexer. There are only two and sometimes just swapping the two to see if the problem moves to different keys will tell you if one or both are bad. If you have another known good machine, swapping the 4051s between the two will rule this out as well. A whole row of keys is quite a few to be out though, and like the previous posters have said, assuming you have a mylar keyboard, having dirty or broken mylar traces is probably the more likely culprit in this particular case.

 

[erichenneke]

19 minutes ago, bfollowell said:

Having several keys in a row or column, according to the keyboard matrix, go out is also a clear indication of a bad 4051 multiplexer. There are only two and sometimes just swapping the two to see if the problem moves to different keys will tell you if one or both are bad. If you have another known good machine, swapping the 4051s between the two will rule this out as well. A whole row of keys is quite a few to be out though, and like the previous posters have said, assuming you have a mylar keyboard, having dirty or broken mylar traces is probably the more likely culprit in this particular case.

 

I agree, the last 800xl I fixed required one 4051 to be replaced, and also some mylar traces (broken traces that were open) to be fixed.   Then it was perfectly fine. 

[bfollowell]

27 minutes ago, erichenneke said:

I agree, the last 800xl I fixed required one 4051 to be replaced, and also some mylar traces (broken traces that were open) to be fixed.   Then it was perfectly fine. 

 

Yeah, bad 4051s AND bad mylar traces. What a combination! I bet that was a pain in the butt to troubleshoot.

 

[bfollowell]

31 minutes ago, erichenneke said:

I agree, the last 800xl I fixed required one 4051 to be replaced, and also some mylar traces (broken traces that were open) to be fixed.   Then it was perfectly fine. 

 

Yeah, the more I look, all of those keys have lines 1 and 2 in common, so it could easily be the mylar or a 4051. The mylar would be easy enough to check out with a multimeter if you have one. If the mylar checks out, swap the two 4051s and see if the problem moves to a different set of keys. It's almost certainly ones of these two problems.

 

[erichenneke]

1 hour ago, bfollowell said:

 
Yeah, bad 4051s AND bad mylar traces. What a combination! I bet that was a pain in the butt to troubleshoot.
 

Yeah, I've had to do both before but I think that was the first one that required both to get all the keys working! And also 2 RAM chips (some pins had oxidized and cracked in half on 2 memory chips so they just weren't making contact with the socket).

Sent from my KB2005 using Tapatalk
 

Edited May 14, 2022 by erichenneke

[kynikos]

14 hours ago, Beeblebrox said:

@kynikos  given a select few keys in a row the keyboard is likely a mylar with a line of em knocked out by one of the mylar tracks having gone bad. Loadsa advice on AA about painting mylar tracks back on with conductive paint.

 

It could also be a bad trace on the keyboard's pcb if it is that type of keyboard knocking out a line of keys which is fixable. 

 

Open up the lid and post a few pics of the keyboard and cross ref it with this thread :

 

 

Thanks all! I'm pretty sure I have a type 4 keyboard just going visually but won't have time to open everything up until later this weekend. The seller claimed that row of keys were working before packing it up so I'm hoping its just a matter of something coming loose during shipping. Will keep you all posted!

Edited May 14, 2022 by kynikos

[00WReX]

Good advice with checking the 4051's.

I had a 600XL that lost a row of keys...swapping the 4051's and the fault moved to another row.

4051's are still easy to find too.

[kynikos]

Ok, finally got a chance to open it up. The case screws were still torqued down so I don't think anyone has been inside until me. Everything is in good shape and very clean. It seems like a type 4 keyboard with the metal plate on the back from what I can tell. The mylar has some black coloring near the connector and part way up the top ribbon that goes all the way through one of the lines. I'm not sure if that's normal or indicates an oxidized trace. I planned to try swapping the 4051 chips as suggested but accidentally broke a pin off getting getting one out as the sockets were pretty tight so I have some more on the way to continue testing.

[erichenneke]

38 minutes ago, kynikos said:

Ok, finally got a chance to open it up. The case screws were still torqued down so I don't think anyone has been inside until me. Everything is in good shape and very clean. It seems like a type 4 keyboard with the metal plate on the back from what I can tell. The mylar has some black coloring near the connector and part way up the top ribbon that goes all the way through one of the lines. I'm not sure if that's normal or indicates an oxidized trace. I planned to try swapping the 4051 chips as suggested but accidentally broke a pin off getting getting one out as the sockets were pretty tight so I have some more on the way to continue testing.

If a 4051 pin broke off really easily, it may have been deteriorated/oxidized and making poor contact with the IC anyway. So probably not a bad thing you are needing to change it now anyway. Those are cheap and easy to get. 

 

The coloring on the ribbon cable could be a sign, but not necessarily. Do you have a continuity tester? You can start checking the portions of the traces on the ribbon exposed right now (those first couple inches from the edge end) while you are waiting on the 4051 to show up.  Often that area will be a source of an open or two and at least you'd have a headstart there (and/or confirm they are all good up to that point at least).

[Beeblebrox]

@kynikos  Great it is a socketed XL. I have a couple of 800XLs with that type of keyboard. Not the nicest one to type on unfortunately. (I'm kinda spoilt with the ALPS and better AWC manufactured keyboards).

 

Yup as others have suggested 4051s could be to blame. Incidentally to settle a chip or remove one after decades of it being in situ, best practice I've read is to push down on it first. This breaks any oxidisation bond the chips legs might have with the socket. If the leg broke off and the chip was tight it might indicate that oxidisation has set in and maybe that is part reason for the keys issue, with some bad connections between the chip and socket. (And also why the leg then subsequently broke off whilst trying to remove it).

 

However it is clear there is definitely some oxidiation occurring with the edge of mylars ribbon as you illustrate -with the one you mention going all the way up is likely the cause IMHO. Having said that I've seen mylars with the same level of oxidisation still working. If it is the cause - as suggested by others you can paint the broken traces with conductive paint if you extract the mylar. (Get a flexible type of conductive paint - I wish I'd done so when I was fixing the mylar on my 1200XL a few months ago!)

One thing I've heard others doing is cutting the end of the mylar ribbon off to remove the bad end so you leave enough of the good traces in place to act as the new edge. (Obviously taking care it will reach and not to cut too much off. I've not done this myself so if you do  - do the research and don't just take my word for it).

 

Do you have a spare 4051 or two btw then? If so swap em out for sure in the first instance.

 

If it transpires the old 4051s are confirmed working at it is the mylar alone causing the issue, you can fix the broken pin on the one you mentioned providing there is still some leg left protruding at the break. One fix is that you can get a spare double wipe socket, plug the 4051 into said socket, solder a metal bridge to the stub of the broken leg on the 4051 and the corresponding socket leg - then just plug it into the PCB socket on the XL. Profile is raised a little as effectively two sockets in height then the 4051 chip - but it shouldn't make and difference or get in the way of the keyboard once the casing is back together. Learned of this simple fix on AA from another member and used it to fix an ANTIC chip once with 3 broken legs. Doesn't look pretty but saved me discarding a rare ANTIC chip at the time.

Edited May 15, 2022 by Beeblebrox

[Forrest]

Those 5 lines on the edge don’t look good. Is that a piece of loose tape near the 5 lines? I would pull the Mylar out of the connector and wipe it off with some IPA. Then check the traces with a magnifier for cracks and repair any with the defogger repair kit mentioned above.

[erichenneke]

I agree on the alcohol rub. Be gentle with it though. But that often helps. 

[kynikos]

Well after getting out the multimeter, there are definitely some continuity issues with the traces coming out of pins 1 and 2 (the top two in the photo I posted). Those two correspond exactly to the row of keys that are not working according to the schematic. I guess I should have checked that before messing with the 4051s. Oh well, replacements were cheap. Some of the black oxidation or whatever it is goes up pretty high to where I'd probably need to take the keyboard apart more to access. At that point, might I be better served just getting a new mylar for it? I don't have any conductive paint so I'd have to spend the money on one or the other. If that's the route I go, is the mylar from Best Electronics still the go to? Many thanks to all for the help so far!

Edited May 15, 2022 by kynikos

[erichenneke]

Hmm, tough call. Personally i am a big believer in the conductive paint approach because I have had a 100% success rate after many many fixes.  That said, I have also bought some Best Mylars as well, but I've never had to use them because the paint fix always worked, and so far (in some cases several years later) are still going strong. So I am just holding onto the Mylars for backup if needed some day.  If you don't already have any paint, and if you don't plan to troubleshoot more systems in the future, well then you might just want to go for the Mylar and be done with it.

 

You could try testing just up the trace a little bit, like a half inch, from the edge to see if it is solid. If so, then trimming it a bit could be an easy fix with nothing to lose too.

[kynikos]

34 minutes ago, erichenneke said:

Hmm, tough call. Personally i am a big believer in the conductive paint approach because I have had a 100% success rate after many many fixes.  That said, I have also bought some Best Mylars as well, but I've never had to use them because the paint fix always worked, and so far (in some cases several years later) are still going strong. So I am just holding onto the Mylars for backup if needed some day.  If you don't already have any paint, and if you don't plan to troubleshoot more systems in the future, well then you might just want to go for the Mylar and be done with it.

 

You could try testing just up the trace a little bit, like a half inch, from the edge to see if it is solid. If so, then trimming it a bit could be an easy fix with nothing to lose too.

I'm not opposed to trying the paint fix as I work on a lot of other computer/game console electronic projects so I could probably use it for something else. Do you have a link to a specific product so I can get the right thing?

 

Do you happen to remember how much shipping was on the mylars from Best? I've ordered from them a few times so I know the drill.

Edited May 16, 2022 by kynikos

[erichenneke]

1 hour ago, kynikos said:

I'm not opposed to trying the paint fix as I work on a lot of other computer/game console electronic projects so I could probably use it for something else. Do you have a link to a specific product so I can get the right thing?

 

Do you happen to remember how much shipping was on the mylars from Best? I've ordered from them a few times so I know the drill.

This is the stuff I've always used.  Last time I actually just bought it at Advance Autoparts.  

Permatex 09117 Complete Rear Window Defogger Repair Kit, Single Unit https://www.amazon.com/dp/B000ALBZJY/ref=cm_sw_r_apan_i_DFNW5AK6PD2G184NMNCR

 

https://shop.advanceautoparts.com/p/permatex-complete-rear-window-defogger-repair-kit-09117/7160063-P

 

It's been so long ago that I purchased those Best Mylars that I can't remember what the shipping cost was for that.    

 

-Eric

Edited May 16, 2022 by erichenneke

[kynikos]

Thank you! Replacing the mylar seems like it would be a pain.

[erichenneke]

Thank you! Replacing the mylar seems like it would be a pain.

Any updates?

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[kynikos]

7 hours ago, erichenneke said:

Any updates?

Sent from my KB2005 using Tapatalk
 

I swapped out the 4051s but that didn't make any difference so it must be the mylar. I'm still waiting on the conductive stuff to arrive. If that doesn't work then I'll just get a new mylar.

[erichenneke]

I swapped out the 4051s but that didn't make any difference so it must be the mylar. I'm still waiting on the conductive stuff to arrive. If that doesn't work then I'll just get a new mylar.

Well that's not a bad thing. I think you'll find the conductive paint trick will work.

By the way, from my experience it works best to just use a toothpick to dip a little bit out at time and "brush" it on along the traces that have opens. Usually it will be in areas where there are tight folds or bends in the Mylar, or near the card-edge end. Let it dry really well before you start moving it around a lot again.

Test the continuity again after you let the paint dry for a bit, to make sure you successfully bridged the circuit gap before putting everything back together. Also, if you paint "outside the lines" by accident (easy to do!) don't fret, you can very easily scrape it off gently as needed. I use flat toothpicks as a brush, and they also work great as a gentle scraper if you need to remove some of the excess or stray paint too.


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