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Rob Wilco

TI99/4A Soft Resets When 6,7,8,9,0 pressed

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My girlfriend introduced me to her TI-99/4A from when she was a kid, it has been stored in her family's garage since the mid 1980's.

We took it back to our house, plugged it in, and I've fallen in love with this computer. I purchased a FlashROM99, a TiPi, and a sidecar 32K expansion.

The only thing I have found that is not functioning correctly with the computer is that instead of only Function + 9 resetting the computer,

the keys 6,7,8,9,0 reset the computer, regardless of any other key being pressed / alpha lock on or off, joystick connected or disconnected.

I have run memory test and all banks show OK.

I tested the voltage from the external and internal power supply with a multimeter and all seems to be normal there.

To rule out the keyboard, I disconnected the keyboard and when shorting the appropriate pins for numbers 6,7,8,9,0 the same reset behavior persists.

Interestingly enough, when first powering on the computer if it's been sitting, and I open up TI BASIC, I can get those numbers to show up briefly when pressed,

though a lot of other garbage (letters and numbers not pressed) appears and then the reset behavior reemerges and stays.

I would be tempted to ignore this problem, however it is necessary to use parentheses in order to call on certain devices and functions when using the TIPI,

and any press of the 8 key will reset the computer.

 

I have read other posts regarding the TMS9901 being a problem for some, but when searching I can't seem to find any similar behaviors described that prompted their replacement.

Has anyone else seen anything quite like this? I don't see any obvious signs of corrosion or leaking / bulging capacitors. I have replacement capacitors and TMS9901s (eBay seller recommended in these forums!) just in case but I wanted to see if anyone has any suggestions before I start replacing parts just in case it's a common problem and I can focus on the right spot first.

 

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Posted (edited)

Have you cleaned the cartridge port?

http://www.mainbyte.com/ti99/minimem/cart_fix.html

Does the TI reset on 6,7,8,9,0 key presses when no cartridge is inserted as well?

Do you have a true mechanical keyboard in your TI or the later membrane-style keyboard?

The membrane is known to go bad over time, could cause the type of behavior you describe (letters and numbers not pressed by you appearing on screen), but i don't think this in itself could cause a reset.

Maybe a bad trace or short on the keyboard's circuit board somewhere?

It might also be a system ROM chip going bad.

Edited by jrhodes

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Is it actually a 4A, and not a 4?

It should be FCTN-= to reset, not FCTN-9.

 

Either way, sounds like a short or a bad 9901. How are you loading a memory test? What banks?

 

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11 hours ago, Tursi said:

Is it actually a 4A, and not a 4?

It should be FCTN-= to reset, not FCTN-9.

 

Either way, sounds like a short or a bad 9901. How are you loading a memory test? What banks?

 

My apologies, it is a 4A, FCTN-= is supposed to be the key combination to reset, I mistyped.

I visually inspected the board for any defects and I don't see anything obvious. I also cleaned the board with some compressed air.

The two memory tests I tried were:

BANKTEST.BIN (md5:029c29dede08bafbc2c365d98d224494)

!banktest-v2.bin (md5:493c95a0580d469fe89880fd04dc6110)

 

I think as a fun exercise I might just replace the TMS9901 and put in a socket and report back. Thank you!

 

11 hours ago, jrhodes said:

Have you cleaned the cartridge port?

http://www.mainbyte.com/ti99/minimem/cart_fix.html

Does the TI reset on 6,7,8,9,0 key presses when no cartridge is inserted as well?

Do you have a true mechanical keyboard in your TI or the later membrane-style keyboard?

The membrane is known to go bad over time, could cause the type of behavior you describe (letters and numbers not pressed by you appearing on screen), but i don't think this in itself could cause a reset.

Maybe a bad trace or short on the keyboard's circuit board somewhere?

It might also be a system ROM chip going bad.

I got lucky and it has a mechanical keyboard, it has also been cleaned. As a troubleshooting measure to rule out the keyboard I disconnected and shorted pins directly on the motherboard header using a paper clip - all other inputs work normally when the appropriate pins are shorted with the paper clip, but the same behavior persists with 6,7,8,9,0.

 

I'll report back and let you know if surgery went well or if I blew everything up.

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SOLVED!!!

Installed a socket and new TMS9901 AND THE PROBLEM IS GONE.

 

Even after letting it sit for a while and testing, ALL KEYS WORK. This is so exciting, this is also the first time I've ever desoldered and soldered a new chip onto a motherboard.

Next step will be getting the TiPi up and running again and trying some demos and online BBS'ing!

 

Thanks so much for your help, this community and its members are a real treasure.

IMG_20210528_105005.jpg

IMG_20210528_105903.jpg

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Welcome to the ti99 self repair club. We may do it right, we may do it wrong, but regardless we'll do it!!! Lol.

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That's great to hear!

 

Just one clarification -- I believe that the banktest ROM you tried tests the cartridge ROM port only, it's meant to prove out the banking chip on the PCB is good. It doesn't test RAM... ;)

 

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Congratulations on the repair. So much harder to live the 99 life when you can't type 99.

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On 5/28/2021 at 5:54 PM, Tursi said:

That's great to hear!

 

Just one clarification -- I believe that the banktest ROM you tried tests the cartridge ROM port only, it's meant to prove out the banking chip on the PCB is good. It doesn't test RAM... ;)

 

That is very good to know, thank you! I don't suppose there is any way to test RAM just in case? I don't have any symptoms of failure so far, but it would be fun to test if it's possible.

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On 5/29/2021 at 8:56 PM, Rob Wilco said:

That is very good to know, thank you! I don't suppose there is any way to test RAM just in case? I don't have any symptoms of failure so far, but it would be fun to test if it's possible.

ti diagnostics has a simple ram test

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We should probably do a group project to put together an ultimate ROM-based diagnostic cart that anyone can burn and run... we can probably come up with at least one test for each peripheral ;)

 

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