Keyboard input not working / diagnosis / repair

[mrvan]

After leaving my TI99/4A powered on for multiple weeks, but using it here and there, I pressed a key on the keyboard with no response. I powered cycled it but the keyboard remains unresponsive. I cleaned all the keys on the keyboard a few months back, getting all to under a few ohms resistance so thought all was great.

 

Multiple power cycles and a few of them didn't even get to the main screen and that's new behavior. I found that with joystick 1, pressing the fire button and 3 of the 4 directions moves to the second screen. Not sure why not the 4th direction. 

 

I had found an old post on this forum, maybe 10-15 years old, about diagnosing the keyboard input. I can't seem to locate it again, might have been better to post there, so sorry about that.

 

For diagnoses, this is what I've done:

- Checked pressing every key with no input detected.

- Removed the keyboard connector and shorted pins 1 and 15 which should have resulted in 'U' being detected, but no go.

 

The post seemed to point to the 9901 after this degree of diagnosis. If so, I have perhaps a partial failure. The failure to read keys and the few times it didn't boot -- maybe makes some sense. 

Reading the TI99 university pages, the 99 uses the 9901 to read the keyboard directly but uses a 74LS138 to decode the 3 9901 lines to 1 of 8, with six lines then going to the keyboard strobe lines and two to the joysticks. 

 

Given what I've seen it's clear that both the 9901 and 138 are at least partially functioning, although the 9901 seems the most suspicious. I see two 138s on the board that from a layout perspective seem misplaced and more oriented toward the expansion port, but I've not studied the circuits.

 

I have a new old stock 9901 and socket ready to go, along with a high end soldering iron and solder sucker. Seems it's time for surgery. If this is needed, will I have any problems with clearance installing a socket? I have the F18A VDP replacement also in a socket and that seems to be ok. Are the traces fairly solid on these boards? Back in the late 80s I replaced many 4xxx chips on various circuit boards and some were horrible with the traces pulling up and eyelets breaking off. Wondering just how fragile this gal is.

 

Any thoughts for further diagnosis or considerations before I begin? 

[+dhe]

FWIW - I would swap the keyboard to a know working console to verify it isn't fiddly mechanical hardware.

 

If you DO replace the TMS9901, I'd clip the old one out so it's easier to desolder, and replace with a machine pin socket (you only want to replace that socket once). I've never had a clearance issue with a socket. The 9918 is socketed and STILL requires an extra riser to make contact with the shield to form a heat sink.

 

Probability that the problem is the 9901 is high.

 

Then scribble a note somewhere, having a console with a socketed 9901 is a very nice thing to have. 😃

 

 

[mrvan]

7 hours ago, dhe said:

FWIW - I would swap the keyboard to a know working console to verify it isn't fiddly mechanical hardware.

 

If you DO replace the TMS9901, I'd clip the old one out so it's easier to desolder, and replace with a machine pin socket (you only want to replace that socket once). I've never had a clearance issue with a socket. The 9918 is socketed and STILL requires an extra riser to make contact with the shield to form a heat sink.

 

Probability that the problem is the 9901 is high.

 

Then scribble a note somewhere, having a console with a socketed 9901 is a very nice thing to have. 😃

 

 

Thanks dhe. All good to know. I just prepped the garage work area, so next is prepping the gal. And then surgery.

[mrvan]

The socketed 9901 replacement is complete and the 9901 was in fact the problem. The keyboard is again working fully. 

 

To my surprise the joysticks no longer move from the boot screen to the second screen. Perhaps that was a feature of the failing 9901. But using Parsec I found the up state continues to not register, so not sure if my joystick is bad, the connector port dirty, etc. Can anyone confirm joysticks do not advance from the boot screen? But for the failed joystick up, knowing now how the joysticks work, similar to the keyboard, I can work my way through that another day.

 

This surgery was more difficult than I expected, primarily because I found I had tools that were all one notch too big. The soldering tip was one size too big and the cutters were a little too large. But I made it work and the added reward is a couple new tools on order. Thank you, dhe, for the tip on cutting out the chip--that definitely protected the islets and traces from breaking or raising.

 

As to how the machine failed in the first place I think I now know. A few weeks back I was at work and we were preparing for a demonstration. We were told the lab humidity was low and we could not power on the equipment due to increased likelihood of electrostatic damage. That happening didn't register that my TI at home was in danger as well.

[+dhe]

Back when millions of TI's were in use in homes, winter was always a dangerous time for TI's as (typically kids) would run around very dry houses and touch joystick ports as they were plugging in the joystick. that tiny static spark didn't do any favors for 9901's pins directly connected to the joystick port. 😃 

 

Having the caps lock key engaged will mess with the operation of the joysticks.

[mrvan]

12 hours ago, dhe said:

Back when millions of TI's were in use in homes, winter was always a dangerous time for TI's as (typically kids) would run around very dry houses and touch joystick ports as they were plugging in the joystick. that tiny static spark didn't do any favors for 9901's pins directly connected to the joystick port. 😃 

 

Having the caps lock key engaged will mess with the operation of the joysticks.

LOL, yes, the caps lock did does mess things up (for up direction) and was my problem! So my TI99 is 100% functional again.

[+OLD CS1]

13 hours ago, dhe said:

that tiny static spark didn't do any favors for 9901's pins directly connected to the joystick port. 😃 

I now have humidifiers stationed around the house partly for this very reason.  But now the Christmas tree is in place, they will be staying idle most of the time.