TI-99/4A FAQ: Maintenance Repairs and troubleshooting

My TI isn’t working right! help!

repair fix rebuild troubleshoot etc

• • Keyboard issues
    • The keyboard is connected directly to the TI motherboard with a pin connector. This can become loose/dirty and may need cleaning/reseating. There is no logic within the keyboard is is merely shorting the keyboard lines when you hit the keys.
    • If your cable is damaged there are replacements available at http://www.arcadeshopper.com/under hardware/cables
    • Alpha Lock issue: With the alpha lock key on you are unable to use the up direction with the joysticks. Release alpha lock to play games OR do this fix: http://mainbyte.com/ti99/console/alpha_lock.html
    • What keyboard do I have? There were at least 5 manufacturers of keyboards for the 99/4a. http://mainbyte.com/ti99/keyboard/keyboard.html has good pictures/info/schematic.
    • 
      • Mitsumi: These keyboards do not have any solder points except for the alpha-lock key connection and the ribbon cable.
      • 
        • These keyboards have a membrane under the keys that tends to go bad with age and can sometimes be revived by separating the layers (CAREFULLY) https://atariage.com/forums/topic/256662-repairing-a-mitsumi-keyboard/ 
      • Alps and others: These keyboards have solder points on the back of the PCBoard
        • these keyboards sometimes have dirt/corrosion in the switches that can be cleaned with contact cleaner or alcohol.
        • Example of square keyboard switches with key caps removed:
    • Console just BEEPS (no display or all one color display)
      • The start up routine of a 99/4a initializes the sound chip with a tone and then the rom boot routine shuts off the tone.. If it just beeps either there is a bad connection to a peripheral OR a board level issue that is causing the startup routine to “lock”. Check all socketed chips for good connection/corrosion etc and reseat.  Sometimes this is caused by a damaged 9900 and it will need to be desoldered and replaced.
    • Scrambled/Garbled screen graphics/text
      • 80% of the time this is VDP ram and it will need to be replaced.
        • Here is a great page on how to determine what RAM chip is bad: http://www.ninerpedia.org/index.php?title=Troubleshooting
          • Don’t use 4116 ram, it costs more than 4164 and uses more power/generates more heat. 4164 will last longer.  See this post for more info https://atariage.com/forums/topic/257923-replacing-4116-ram-with-4164-for-reliability/
          • VDP Ram is soldered into the motherboard and will need to be desoldered and replaced (suggest it is replaced with sockets and new ram inserted into the socket) this ram is TMS4116 static ram. Located in number 6 in this photo: http://mainbyte.com/ti99/hardware/big_mother2.jpg
          • The F18A VGA VDP replacement contains it’s own VDP ram so replacing the TMS9918a VDP in your 4/a with a F18A will also replace bad memory without having to solder..
      • On NTSC/USA machines TMS9918a VDP is the hottest component on the motherboard and this tends to end it’s life prematurely. Daily use consoles tend to have the VDP fry sooner or later. This chip is socketed so it is relatively easy to replace. It is covered with heatsync compound and there is a metal slug that sits on top of it under the RF shield on a original 4/a. On a QI model there is a metal heat-sync clipped to the 9918a.
    • Cartridges don’t work or don’t work reliably.
      • The TI cartridge port is the most used part of a console. It tends to get dirty and gummed up. Info here on cleaning carts and the port: http://mainbyte.com/ti99/minimem/cart_fix.html
      • Replacement cartridge port boards are available on ebay and from vendors.

repair fix rebuild troubleshoot