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Continued Sol-20 Keyboard Issues

boxerbomb

Member
Joined
Oct 3, 2018
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Sorry for yet another Sol-20 keyboard thread, although I feel like my problem is unique and might provide some help for others. When I first got my SOL-20, the TexElec keypads were out of stock so I decided to punch out my own foam-and-foils. I tried my best to follow guides but when I installed them, but most of the keys did not work and some of the keys would press multiple keys at a time.

I then decided to remove them and I bought a French-Canadian New Old Stock Sun Type 4 keyboard and I harvested the pads. I installed all of those and not a single key is working. Interestingly enough, all of the keys work properly when the bare PCB is touched by my finger. If I use some of the loose foil-foam pads I can only activate the keypress if I completely compress the foam pad on the key. Is that normal, or should it be when it first touches? I have a few possible ideas/theories:
  1. The PCB should be sanded down, with fine grit sandpaper. (I have tried this on a few keys but do to the nature of capacitive switches I am unsure if this would help)
  2. The PCB should be cleaned
  3. One of the 18 screws is somehow shorting a connection on the PCB
  4. A capacitor responsible for the "capacitive tolerance" of a valid keypress is damaged and should be replaced
Any other ideas? Now that I see that the TexElec pads are back in stock, I would be willing to purchase a set of those, I was just thinking that these Sun Pads should do just as well.

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I assume you have read my article about the sol keyboard and pads as I have posted it a lot on other threads:

https://www.worldphaco.com/uploads/HARDWARE_DIAGNOSTIC_TOOL_FOR_THE_SOL.pdf

It explains the dielectric properties and capacitance the pads require to activate the key detect circuitry, and how the key detect circuitry works.

Touching the circuit tracks where the pads go and getting a response is not a very good test to determine if the key detect circuitry is ok, because the resistance of your finger also triggers the circuitry for non capacitance reasons.

To test if the circuitry is working well, get some low value capacitors in the range of 10 to 20pF. Discharge them by shorting them out and connect them across the pcb pads. Generally if the electronics is working as it should, the threshold where the keyboards produces a reliable response is about 10 to 15pF. Over 15pF should reliably be detected if the circuit is in good order.

(I would not recommend sanding anything. Any surface oxides on the pcb act as a dielectric and they are super thin compared to the pad mylar and won't decrease the capacitance significantly, and if you sand them you risk exposing the bare copper, this should be a last resort, but cleaning the pcb with contact cleaner is ok).

To check the keyboard you can use a meter to check the DC conditions in the transistorized capacitance detector circuit, a scope or logic probe for the various pulses. It would seem strange that all of the SUN 4 pads were non standard, so likely a keyboard fault.

Also remember that if one of the IC's on a key row detector drops out, or an IC driving the columns it will affect multiple keys that seem to have a random relation with each other. You can look up the key numbers on the matrix in the manual to see if any defective keys are linked by being on the same row or column, indicating its an electronic fault there.
 
This is an interesting development. I'm just about ready to harvest the pads from a Sun Type 4 keyboard myself for my reconditioning effort. I think it was Hugo Holden that mentioned a while back that the texELEC pads did not perform well with the Sol keytronic-based keyboard. Of course, I read that post right after I had purchased the texELEC pads. :) I'm not sure they have been improved or not. Hugo recommended harvesting pads from the Sun 4.

My Sun 4 keyboard came today, so despite your findings, I will still remove the pads, but it sounds like I will need to do some preliminary testing with the keyboard after a few are replaced. I already have a few key with the textELEC pads, so will be able to test both pads before total reassembly of the keyboard to determine which ones work best - at least with my keyboard. I'm still waiting on the thin, double-sided tape to arrive first.

I wish I had a more definitive recommendation to share, but it seems different people are/have experienced different results. So, it may just be trial and error, unless of course there's something more electrically mysterious going on with, as you suggest, a short or defective part.

Rick
 
T

I wish I had a more definitive recommendation to share, but it seems different people are/have experienced different results. So, it may just be trial and error, unless of course there's something more electrically mysterious going on with, as you suggest, a short or defective part.

Rick

It is not surprising, due to the way the capacitance detect circuits works. If the capacitance increase provided by the particular pads is borderline and in the range of 7 to 10pF, some keys will activate and others not. The density of the foam has an affect too because with more pressure the capacitance increases a few pF more as the mylar surface is better approximated over a larger surface area to the pcb track . As noted in the article both the thickness of the dielectric film and its dielectric constant determine the capacitance increase, all other things equal. So what that film is made out of and the foam that it is attached to, both have a contribution as to whether the pads will work well.

Then likely there is a degree of variation on individual keyboards as the current gain (hfe) of the transistors there has a significant effect. Many of the transistors appear to have been graded at the factory for hfe, because they have colored paint on them. So it could be that a set of marginal pads might work in one particular keyboard and not reliably in another as a set of borderline conditions conspire together to make the key closure detection marginal.

There is another alternative that will work. When I was experimenting with the keyboard I found that any resistance below about 68k across the pcb pads is also detected as key closure. I bought some 10mm diameter conductive rubber discs on ebay (the type used in calculator repairs). These trigger the keyboard well, the output pulse from the key detect circuit is a little longer in time, but it does not affect the keyboard's function. So the way the keyboard works is amenable to change, but since the SUN-4 pads worked well in both keyboards I had fitted them to, I didn't use the rubber discs and kept the capacitive detection method original and because, I guess, I just like the clever nature of it.

PS: about the actual key numbers in the matrix shown in the manual, they are actually on the keyboard pcb's rear surface near each "switch". I could not find a listing which married those up with the numbers/names of the actual keys. So those are the numbers that need checking, if some keys are not working, to see if they are associated on a row or column of the circuit, or it is easy to blame the pads when it is not their fault, when it seems like random keys are not working.
 
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Thank you both for responding.

Hugo, I have been following your posts for quite some time now, you are quite the expert on Sol keyboards and it shows, we are lucky to have you.

Rick, I have been seeing your posts as well, I wish both of our Sol systems luck, we seem to be about the same spot right now.

I just took a look at the keyboard again and I have both good and bad news. I tested the contacts with a range of capacitors and Hugo was spot on, the key would trigger at 20pf and higher but it would not at 10pf. I then decided to remove the foam from one of the keys and make a "double-decker" foam keypad. I have only applied that to one key, but it went from not being able to trigger at all no matter the force on the key or pressure on the back of the PCB, to being extremely reliable.

I hope that I just got unlucky and your Sun Keyboard will serve you fine, I purchased mine from MemoryX:
SunType-4 Mini-Din French 320-1018 2001276 Canadian Keyboard (p/n 320-101)
 
It is possible to globally alter the sensitivity of the capacitance detect circuitry and lower the capacitance value it triggers at. The input capacitance of the transistor amplifier is set by the base-to emitter 220pF capacitors there C9 and C10. These create a voltage divider with the key's capacitance. The sensitivity can be increased by lowering their value, though the risk is if it is too low cross coupled signals and interference on the pcb could trigger the detector. I have not done it, but probably the value could be reduced to 180pF, or possibly even 150pf and get away with it.
 
Well, Hugo, we're glad to have your deep knowledge here. And, to know we have some alternatives to corrective actions if necessary. I will wade in slowly. :)
 
​Today, the roll of double sided tape arrived, but after looking at it and reviewing it, I could easily determine, it would not be very conducive to punching. What I really needed were thin sheets with peel-off on both sides. I went looking in my DIY box of adhesive pads and strips (I use for everything), and I saw I pad some 3M VHB little square pads. They punched super well, and I was able to put a few on the Sun 4 pads. See photo of my setup. I can get 4 adhesive pads out of each square. While the 'tape' part of the pad appears to be thin enough, I don't think it's more than .1mm thick. In appearance, it's not quite as super thin as the original I scrapped off some of the Sol keypads. I think these will work fine.

Now, the only problem is... Those who have ever worked with VHB material, will know that once it is on, it can be very difficult to remove. Hence the Very High Bond name. :) So, I need to be sure these pads will work before applying them to all the keys! I will have to functionally test it with the Sol prior to full application.

Any final thoughts or comments?

Sun pad assmbly.jpg
 
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I have nothing to add here other than to say I sure feel sorry for those french-canadian owners of SUN computers.. RIP. :)
 
Yeah, I opened up the keyboard yesterday and started popping off the pads late last night. They came off just like a few on my old Sol did - easily. Got them all off, threw the remains of the keyboard in the trash and went to bed. The most expensive foam pads I've ever bought. :)
 
Ever since I put my keyboard back together it's been stuck in upper case when it's turned on, and stays that way. Even without the uppercase LED lite. Weird. What is the most likely 'failure' or anomaly for this symptom? My Dual Personality Module has a command to put the keyboard in lowercase until the next RETURN. And that works as expected. Is there some place I can start looking?
 
Is it CAPS LOCK or a SHIFT key I wonder? What happens if you type a numeral?

The description of the keyboard circuitry is pretty good in https://www.sol20.org/manuals/sol_systems_manual.pdf. There is an encoder ROM on the keyboard that produces the top 4 bits of the ASCII code itself based upon things like the SHIFT key and the CAPS LOCK setting. Here is the place to start looking.

Dave
 
Ever since I put my keyboard back together it's been stuck in upper case when it's turned on, and stays that way. Even without the uppercase LED lite. Weird. What is the most likely 'failure' or anomaly for this symptom? My Dual Personality Module has a command to put the keyboard in lowercase until the next RETURN. And that works as expected. Is there some place I can start looking?

The fact that your Upper case LED is changing state means that your keyboard is very likely sending correct ASCII codes out unless there is a hardware mod there near that circuit, a modified ROM, or a hardware failure on the keyboard. You can easily check this by looking at the keyboard's output codes with a meter or logic probe. (this is where that keyboard diagnostic tool would be helpful and you can see them immediately). But likely, I think these output codes will be ok. If your uppercase LED was stuck, that would be a lot less likely.

I do know there was a hardware mod on some SOL-20 keyboards that cause them to come up in upper case, or lower, depending, after a power cycling event. As I recall it involves the power on reset capacitor circuit being swapped between the set & reset terminals of the flip flop that is toggled by the uppercase key. But with that, the keyboard upper case key and LED still work normally as I recall. Check the upper right hand area of the SOL pcb around the IC's there for any obvious wiring modifications.

Just looking at the schematic, the power on reset capacitor is c14 (a tantalum cap on the upper right of the board) and it controls the clear inputs of U15, it is around the clear and preset terminals of U15 that I had seen a modification on a SOL-20 keyboard.

The SOL-20 keyboard receives no input signals at all from the mobo to alter its function. So the OS cannot tell the keyboard to do anything. It is a one way street with the keyboard sending output signals only.

Off hand, I'm trying to think how the OS would be able to alter the keyboard's ASCII data presented to the character generator so as to be able to alter the CASE by altering the code, there might be some gating there. I would have to study that one on the mobo schematic.
 
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This is a option from a previous thread. Dave has come up with a Cherry MX replacement PCB. I built an early version with incorrect spacing sent to me for the cost of shipping and it worked great

<https://www.vcfed.org/forum/forum/genres/s-100/74223-any-interest-in-a-sol-20-keyboard-replacement>
 
I do know there was a hardware mod on some SOL-20 keyboards that cause them to come up in upper case, or lower, depending, after a power cycling event. As I recall it involves the power on reset capacitor circuit being swapped between the set & reset terminals of the flip flop that is toggled by the uppercase key. But with that, the keyboard upper case key and LED still work normally as I recall. Check the upper right hand area of the SOL pcb around the IC's there for any obvious wiring modifications.

I think you are onto something. I knew this little circuit was there, but had long forgotten what it did. Is this the culprit? You'll note from the pictures, when I installed it (my hacking skills were pretty sad back then), I was restricted somewhat because of the bucking xformer. So I had to lift the top part of the keyboard up a bit. Note the white nylon spacer. When I looked more closely at it tonight, there was evidence of dried duck tape residue as well. Yup. duck tape! I must have used that for insolation purposes.

I looked at the circuit board closely and did not see anything broken loose, a wire undone. Maybe the circuit has failed? But if you look closely at the annotated pic, where I have circled and placed an arrow, there is a very small part of the trim pot edge that has appeared to have rubbed off. Maybe it was shorting something out? Hard to say because once it's all assembled you can't see anything. I don't have a little inspection(dental) mirror in my shop either. :(

Now there must have been a good reason to do this mod at the time. If, it indeed is causing the UPPER CASE to always be on, I can easily remove it all and restore the one IC pin (green wire on top) back into it's socket. It's worth a try. Maybe that one IC (DM74LS123N) on the perboard has gone bad.

I'll wait to hear your diagnosis. :)

KDB Mod6.jpg KDB Mod4.jpg KDB Mod3.jpg KDB Mod4.jpg
KDB Mod1.jpg KDB Mod5.jpg
 

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That added circuit is connected to U16 (Z16) and it is a one shot , so for some reason you wanted to add a pulse into the part of the circuit where U16 is. I'm not sure if it relates to your current upper case problem or not, it was not the mod I had seen before. If you traced out the schematic of this mod, you might be able to figure out what it was intended to do and why you wanted a pulse there.
 
Unless it was in conjunction with another mod I did, I have no idea why its there then. From the looks of other wires I have running about on the mobo, and lifted IC pins, most of these little mods (or fixes) came from newsletters, or other computer publications of the day. Or possibly ECNs from PT as well. I have no recollection of them, and any personal notes at the time of implementation are long gone. If it's not related to the upper case issue, l think I'll just let it be. Your insight is much appreciated.

One thing for sure... my electronics implementation skills have come a long way in 40 years. And most of that in the last decade. :)
 
In your situation with no documentation on the mods, like getting into bed with something without a manual, I would be inclined to try to put the computer back to standard.

The mods might interact with each other and there might be unexpected protocols from them acting together (sounds a little I-Robot). And if you get a failure it will be harder to fault find without a correct overall schematic and that might make you run in circles (like Mad World). At least if you made the SOL standard, its behavior will be more predictable, then you could add mods in later one by one and document them.

Though I would advise if any of the mods were removed to document them in detail and only removed one by one with the computer checked. It is only after they are drawn out on paper that it would become possible to figure out what they were intended for and more to the point, if they now have any utility value.

I wonder if anyone else would agree with this advice, or not ? interesting to see. I tend to focus on the hardware side of things and like to be in control of that side of it , so maybe I'm more obsessed with accurate documentation on schematics.
 
I appreciate your advice and approach. I know where you're coming from. At this point, my Sol will never become what it once was by way of a productive machine. That is not the reason I resurrected it. I basically wanted an old toy back to see if it still worked. I had no reason to believe that it would not. I assumed all my major upgrades would work, and I was surprised that one of the significant ones (Z80 add-on) was one I had forgotten all about. So far, everything seems to be working as it was intended.

Except for the physical deterioration of the kbd pads over time, I did expect the electronics to still behave properly. The upper case issue is not a show stopper, and doesn't inhibit any operations.

At least 3 of my major add-ons were commercially available products to enhance the operation and feature set of the Sol. They integrated very nicely with the mobo and worked from the get-go and never caused any problems. Hopefully the other smaller (perhaps less important) mods/hacks will not cause me any problems. So, at this point, once I get my serial connection up and running, I'll be happy to get an OS installed, a disk system going - N*DOS, or CP/M - and revisit my command-line days. I'll even fire up Electrical Pencil, or WordStar. I cut my teeth on those two apps. :)
 
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