Semi-dead black keyboard repair?

[retro486]

Black on 1, red on the rest. They're all negative when I did it that way though I suppose if I put red on 1 they would be positive.

 

The enter key doesn't work either, unfortunately. I saw pun 1 leads to the joystick port down the line so I tried desoldering the joystick port but it was no help.

 

I'll take a look at the pin 12 path since the rest seem fine. I've been fixing solder as I find rot/air bubbles. This is the most I've soldered in my life so getting a lot of experience! Including to pay attention to how I'm holding the iron, as this lovely small burn on my palm will show.

[SignGuy81]

Black on 1, red on the rest. They're all negative when I did it that way though I suppose if I put red on 1 they would be positive.

 

The enter key doesn't work either, unfortunately. I saw pun 1 leads to the joystick port down the line so I tried desoldering the joystick port but it was no help.

 

I'll take a look at the pin 12 path since the rest seem fine. I've been fixing solder as I find rot/air bubbles. This is the most I've soldered in my life so getting a lot of experience! Including to pay attention to how I'm holding the iron, as this lovely small burn on my palm will show.

 

 

Thanks.

 

Pin 1 is positive then which gets sent through the key switches to the other pins. I wanted to know how it was setup up so now I know.

 

The voltage you are seeing with unit powered on, no keys pressed between pin 1 and 12 sound normal, it is actually the rest that don't seem fine.

EDIT: You should be able to verify that by doing the same test on another column of keys on the schematic. Example from pin 2 to the number keys 0, 9, 8, 7, and 6 with whatever pins they correspond to(edit I wasn't thinking it will be pin 2 to all the same other pins you checked earlier), you should have 3.5V on all of them.

 

Keys not pressed should see voltage and pressed should see 0V. And the 3.5 is normal and the .12 or .15V on the others is probably do to the chip being faulty. If it were just one of them I'd say maybe a bad cap in between but that many of them it is probably that chip. Plus to you have already done continuity test on keyboard with connector unplugged all seemed fine. And also with unit off and plugged in. But when plugging it in and power on the way the voltage checks out it seems as though the chip has shorted out. I don't know how it is done because I don't have a schematic but the inputs should all be pulled low through a pull down resistor not sure though if done internally by chip or externally. Pin 1 is positive, so whenever no button is pressed the chip would see 0V through pull down resistor, and when button pressed the input on the chip for whatever key then sees 3.5V. So when you check voltage each input to pin 1 should be 3.5V without keys pressed. Since the others you don't then my next best guess would be to replace the chip because I doubt you would have a problem such as that many shorted caps(for input debouncing) or blown pulldown resistors on the board and that is just really not even going to be a scenario not enough current, pretty high resistor values. If I had a schematic I might would see some other possibility but as far as I can tell it is the chip.

Edited June 25, 2017 by SignGuy81

[retro486]

Dang. Well I have a replacement chip on the way and I already added a socket so at least the tedious bit is already done.

 

I'll update here if it works. Hopefully my socket work isn't garbage!

[SignGuy81]

okay I just thought of another possibility which may simply be a broken ground that might be going to pulldown resistors like I mentioned, for those inputs on the chip. Maybe a trace on board corroded away that was leading to those. If so that would make those inputs not pulled down to ground like supposed to and the inputs could float high. And if the trace is open that would explain why seeing around 0v on those pins to pin 1 instead of the 3.5V. And if all of those inputs are stuck high all the time that would cause them not to work. Just something to look for, trace the inputs from the connector to where they go to before the chip and then on those same lines check for other components like pulldown resistors, caps, and look for a trace that may be open.

[+Lee Stewart]

... If I had a schematic I might would see some other possibility but as far as I can tell it is the chip.

 

Shematics are in this WHTech directory.

 

...lee

[SignGuy81]

 

Shematics are in this WHTech directory.

 

...lee

 

 

Thank you for the reply, looking here ftp://whtech.com/datasheets%20and%20manuals/Hardware/99-4%20%20console%20specification%20and%20schematic.pdfat page 21 and page 22 of the PDF document the keyboard wiring is different than just the keyboard schematic I posted earlier, "ENT" is not on the same line in this schematic. Not sure yet which one is right or wrong. I'm assuming the one I posted earlier may be wrong I went back and looked and it didn't include the 4A so I guess the TI-99 and TI--99/4A the keyboard wiring is different. I might crack mine open and look but I'll probably just wait and see if his chip fixes the issue since he has one ordered already, and if not go from there.

[+mizapf]

ftp://ftp.whtech.com/datasheets%20and%20manuals/Hardware/99-4A%20Console%20and%20peripheral%20Expansion%20System%20Technical%20Data.pdf

[+Lee Stewart]

 

 

Thank you for the reply, looking here ftp://whtech.com/datasheets%20and%20manuals/Hardware/99-4%20%20console%20specification%20and%20schematic.pdfat page 21 and page 22 of the PDF document the keyboard wiring is different than just the keyboard schematic I posted earlier, "ENT" is not on the same line in this schematic. Not sure yet which one is right or wrong. I'm assuming the one I posted earlier may be wrong I went back and looked and it didn't include the 4A so I guess the TI-99 and TI--99/4A the keyboard wiring is different. I might crack mine open and look but I'll probably just wait and see if his chip fixes the issue since he has one ordered already, and if not go from there.

 

As Michael (@mizapf) posted, the correct document has “99-4A” in the title. The one you referenced is for the TI-99/4, the keyboard for which is definitely different.

 

...lee

[retro486]

Hah I was wondering why my joysticks "right" signal was unresponsive and there you go. looking at p11 of the 4a doc I see the enter row and joystick right share an interrupt on pin 7 of the 9901. I'm no expert but that's a strong implication the 9901 is indeed at fault, right?

Edited June 25, 2017 by retro486

[SignGuy81]

Yeah I was also wrong about the voltaqes you should see, it looks like milivolt range is normal on that. I was checking mine just earlier and had around 300mV when I was checking. I thought it would be higher.

[retro486]

Alright, got my replacement 9901 and popped her in and that was it! The whole "enter" row is working as is the joystick "right" direction (well, working as well as it can with my super hacked up adapter that I made from the original completely broken joysticks).

 

Sadly, I also got a MUCH nicer "spare" silver TI-99/4a today, so this project machine will be my new spare but I wanted to share that replacing the 9901 solved the original problem and for very little money.

 

I got the replacement 9901 on ebay for $12 + $3 S&H in the US here: http://www.ebay.com/itm/Texas-TMS9901NL-Peripheral-Interface-Adapter-PIA-DIP40-/151579633037?roken=cUgayN&soutkn=oGUlAE. I got it in less than a week.

 

The annoying part is desoldering the 40-pin from the board. If someone is going this route you ABSOLUTELY should solder in a socket. Hell, do that while you're waiting for your replacement like I did.

 

Thanks everyone for all the help and narrowing down the issue! I learned a whole lot about this thing and greatly improved my soldering confidence.

[SignGuy81]

Glad you got it working!

[Sinphaltimus]

Awesome.