Intro blurb, power supply, video, and basic specs
Keyboard quirks and joystick port, cassette, ROM and expansion ports.
The power connector on the back side of the machine takes AC voltages from an external transformer. The connector will most likely have four pins, although the power supply from the later QI models has only two. Older transformers designed for four pin connectors do work with the newer machines. Voltages are described http://mainbyte.com/ti99/hardware/power_supply.html, but if that looks like something ain't right with those numbers, it's because voltage test points come in pairs and these numbers don't. Probably this is a center tapped transformer. FIXME if you can.
During the life of the console, someone got the idea AC powered appliances needed safety fuses to keep from potentially killing people for some reason, and TI had to hastily add one to the TI-99/4A. Transformers without fuses are in the wild, and they're of the "wall wart" variety. Newer models have a brick with a cord going to the console, and another cord that ends in a plug permanently glued into a socket a short extension cord with an inline AC socket. TI spared no expense fixing this! ;)
Of course that fuse can fail, and if it ever does your transformer is dead. The fuse cannot be replaced and your only option is to cut the transformer's AC plug out of the socket or just cut off the plug and wire a new one. Doing euther is beyond the scope of this beginners guide and if you electrocute yourself to death playing with live mains power, your zombie is not allowed to sue us. If you don't know what you're doing here, don't do it. C'mon, the TI is not a rare and expensive system here, get a new brick or find someone who knows how to fix it right.
It is unknown if European appliances of the day were allowed to possibly contain deathtraps, but your author has read that most people in the UK at least who would have ever seen one of these consoles in their prime would have learned how to wire a power jack in school, including to make the earthing wire longer so that if the others wer somehow pulled free, the device would still be earthed. Because in Europe, they use twice the voltage Americans use, which makes it twice as dangerous, and therefore twice as good an idea to teach children how to start with bared wires and end up shoving something into an electrical socket. Brilliant! :)
US and EU video ports exist, where was each distributed? RF modulator or monitor or...
RF modulator, pinout, Commodore/Atari composite cable compatible, sources for cables
RF modulators and what to look for, pinout of the video port, using the YPbPr output? Are cap/resistor mods remotely worthwhile? Xref to modding RF modulators.
Some troubleshooting info, cap and resistor fixes for US models (only?), xref F18A upgrade
TI wired remotes, WICO (and other) joystick adapters, xref build your own joystick adapter
BASIC programs and cartridge databases, single and dual cassette cables, xref making your own cable. Alternatives to cassettes, xref to using a USB sound device with a modern computer.