TIA Chip (Atari 2600)
The TIA chip on the Atari 2600, also known as the Television Interface, is responsible for generating the audio and video output signals directly as Luma, Chroma and Sync, which ultimately through passive components on the motherboard then generate RF outputs.
With the right work, it is fairly easy to reconstruct a clean composite signal directly from the Luma, Chroma and Sync pins of the TIA chip.
The TIA is a chip that can be tested with pretty high guarantee to be working without the need for anything except the oscillator pin, ground and 5V.
Even without the RIOT chip or MPU installed, so long as the TIA chip as 5V power (pin 20/23), ground (pin 1/22) and a clock signal (pin 11) then operation can be checked easily.
To check functionality of the TIA chip just get an oscilloscope on pin 2 of the chip (SYNC) and you should see a mostly high signal with low pulses every line of video data it is generating. If you see this signal, then you can have a pretty good assurance the TIA chip is running.
The signal should be approximately every 60us and 5V signals.
Confirm you have 5V and ground on the correct pins, and then next up you should check you have a valid clock signal on pin 11. The clock signal should be 3.54MHz, coming from the Y200 clock and network.
The TIA chip should generate the CSYNC signal (line blanking signal which will ultimately form part of the composite/RF signal) regardless of anything else.
NOTE: The CSYNC Pin is an open-drain pin so pulls the line low, and requires an external pull-up resistor to work. The Atari 2600 provides the pull-up resistor on the board.
As well as the CSYNC pin, you will get the color burst information present on the COLOR pin 9 regardless of the other chips or a game present.
The COLOR pin requires both a successful game load and the COLADJUST pin to be above 2.4V before the low periods pulse at the crystal frequency.
If the COLADJUST pin is lower than 2.4V (controlled via the 2 diodes usually CR201/CR200, and the color adjust potentiometer wheel), then the COLOR pin will only pulse low as a solid low signal without the color burst pulses.
NOTE: The COLOR Pin is an open-drain pin so pulls the line low and requires an external pull-up resistor to work. The Atari 2600 provides the pull-up resistor on the board.
The color burst is the composite video color burst information. Without a game in you will only see the color burst starting pulses and then blank until the next line.
When zoomed in you can see on a PAL console you get 19 pulses of 226ns pulses for example.
If you have a game loaded, and Black and White switch is not turned on, you should see color information in the form of pulses in between the color burst signals.
The LUM0/1/2 pins on the TIA chip output the 3-bit brightness information about the pixels.
During the pixel data portion of the composite signal generation the three luma pins set the brightness value by being pulled to 5V or sinking the pins low to generate a 3 bit value (where 000 is black, 111 is white for example).
NOTE: The LUMA pins are an open-drain pin so pulls the line low, and requires an external pull-up resistor to work. The Atari 2600 provides the pull-up resistor on the board.
The LUM pins even without a game usually show activity and pulses. If the MPU is bad, then the LUM pins typically remain at ground. This could be a sign of a bad MPU.
The RDY pin (presumably meaning Ready) stays high 5V when there is no game loaded. This pin will toggle only when the MPU has a game loading or loaded and is telling the TIA it has video data it can process.
If you do not see activity on the RDY pin then either a game is no loaded, the game is bad, the RIOT chip is bad, or the MPU is bad. Many other things could be wrong but if you do not have a RDY pin signal then do not expect the TIA to do anything of use except output a SYNC signal.
List and screenshot all other important signals