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SmakyTaky

Tandy Color Computer 3 Keyboard Troubleshooting

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I just got a "Mexican" Tandy Color Computer this weekend from a local seller. The computer albeit dirty is powering just fine and showing up video (both RF and Composite). However, I cannot get any key to be recognized. Initially thought it may could be an issue of the dirt accumulated both on the computer and keyboard. So dismantled the whole thing and gave a soappy watter + magic eraser treatment (and a bit of retrobiting) to the plastic pieces. The electronics received the usuak IPA rubbing and contact cleaner treatment. I noticed nothing really bad there... no apparent broken traces or anything. However after putting the computer back together I am still not getting any response from the keyboard.

 

Checked Power supply and voltages.. all seem ok.

 

I searched online and got the service manual for this model. However, keyboard troubleshooting is quite skim. Just a few checks:

 

  • Check flex cable and CN2: both seem to be ok, No visual disconnects or broken lines on the flex membrane (at least not at the visible part of it coming out of the keyboard). BTW, I noticed the flex cable has 16 pins and pin 3 is connected to nothing as the whole trace is not there, however that seems to be like that from the factory.
  •  
  • Check Pin 23 (CS2*) of IC5Not really sure what to look for here. With the multimeter this pin seems to be set to 4.47 V
  • Check D8-D11, C18, C19, C22, C23 and IC14 (NAND Gate):  All these components seems to be just fine, tested with multimeter for shorts or expected behaviors. I aksi tested IC14 and given the inputs of the 4 different gates it contains seems to be working just fine.

 

Not sure where to take it from here... I dismantled the keyboard, cleaned thoroughly all the keys, springs and sprungs. Gave the membrane a good cleaning with IPA.. however, the keyboard refuses to give any sign of life. Any pointers? How can I troubleshoot the membrane? Are there any replacements available?

 

Regards.

 

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Edited by SmakyTaky

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Here is the diagram , including the missing trace (normal) half the pins are outputs from the pua chip and should be pulsing   and the others shouod stay high untill connected wirh a button press.

You need a logic probe with the little flashing lights, they are cheap and easy to use to poke around these machine.. you want to see flashing lights to know something might be working.

With the keyboard disconneced  a small jumperwire from pin 1 to pin 16 would activate a "G"    from pin 2 (Pa1) to pin 12 will get you a ""K"

This will tell you if its a keyboard or chip problem.

Screenshot_20210125-103006_Gallery.jpg

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This is the kind of tool you need to troubleshoot

Electronics while they are running...

They are under 20$ for a cheap unit

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Just got my logic probe delivered yesterday... so after learning how to use it I will test as adviced. On the other hand using a multimeter I started the long process of probing continuity on the membrane keyboard traces. if no key is detected, which trace might be the one that could prevent any key from being detected? is that possible? I also just ordered a conductive pen just in case I end up finding broken traces. 


Thank you so much for so much feedback.

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If you look the way it's wired ,,if trace from pin 4 was broken keys. P,q,r,s,t,..w.  would not work.

If trace from 13 was broken ,d,l,t would not work

If you had a basket full of push buttons you could make your own with that diagram

 

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I would not think every trace is broken .. check with the logic probe  on the connector .. alot should be flashing.. the others should give you a high.

 

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One quick question looking at videos online the CoCo3 has a couple of copper clips on the motherboard that touch the keyboard back metal plate... I only have one of those... do they play any role that may prevent the keyboard from working?

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6 hours ago, MrDave said:

Here is the diagram , including the missing trace (normal) half the pins are outputs from the pua chip and should be pulsing   and the others shouod stay high untill connected wirh a button press.

You need a logic probe with the little flashing lights, they are cheap and easy to use to poke around these machine.. you want to see flashing lights to know something might be working.

With the keyboard disconneced  a small jumperwire from pin 1 to pin 16 would activate a "G"    from pin 2 (Pa1) to pin 12 will get you a ""K"

This will tell you if its a keyboard or chip problem.

Following your advice with the jumperwire I was able to get characters into the screen as per the diagram connecting different pinw yield different results. So it seems  IC5 and NC2 are working just fine. So this brings me back to the membrane. I'll start testing continuity on the traces in the evening. 

 

Thanks!

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On 1/26/2021 at 8:47 AM, SmakyTaky said:

Following your advice with the jumperwire I was able to get characters into the screen as per the diagram connecting different pinw yield different results. So it seems  IC5 and NC2 are working just fine. So this brings me back to the membrane. I'll start testing continuity on the traces in the evening. 

 

Thanks!

So how did your Probing the membrane go mate? as I have 2 keyboards that do not work and curious if that conductor pens will fix it..

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Well I am now struck with two Coco3 with dead keyboards... for the fist one I have been using a multimeter to test for gaps.. however, it seems the membrane if full of them... Using conductive silver ink I have been pretty much redrawing most of the traces... however it still does not work. The second seems to be repairable as there are fewer traces not working. However I am not sure the conductive ink approach work, specially for long traces being broken. Not sure how to proceed though.

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Well, you could look to buy a used keyboard.. buy a new keyboard ftom the zipsters zone.. use a ps2 keyboard adapter..or make your own....

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Well I tried contacting the zipster zone.. have been writting to them over the last couple of months with no luck. Probably I will need to go down the build your own route.. a bit daunting as this is beyond my current restoration knowledge.

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There is also foil tape you can use.. i will check with the people on the coco discord if there are any used coco3 keyboards for sale..

 

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Not having any luck finding a keyboard.. i am looking into designing a replacement using cherry mx mechanical keys 

 

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I have a few sample boards in production ..uses cherry mx red switches has extra mapped keys for gaming as well :)

Screenshot_20210329-165639_Gallery.jpg

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It's coming... won't be too much longer..just have to tweak a few things..lol

And no it does not have be orange :)

it has 60 keys.

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