Video Output (Atari Jaguar)
The Tom ASIC is responsible for processing the video output to the rest of the video circuit on the left of the console.
Start by getting to RSB (Red Screen Boot) by observing the Q5 data pin of the BIOS.
Have no game inserted, cartridge pins B34/35 shorted together and a good BIOS. Steps on this are detailed in the BIOS Boot section and shown at the end of this video.
In short, Q5 pin should toggle rapidly for about 0.5 to 1 second, then stay high with short pulses once every 16ms (60Hz) to 20ms (50Hz) depending on region.
Next up, probe the RGB pins 3 to 28 on the Tom chip.
As you can see, the RGB pins all go into a resistor ladder on the left to be converted to a single analog signal per color.
All of the R, G and B pins should looks similar to this if set to 20us time division on the oscilloscope to capture a few lines of data.
After the digital pins from the Tom chip, they pass through a resistor ladder to convert to analog values, per color.
If you probe with your oscilloscope still set to 20us time division, still on Red Screen Boot, for the resistors R30 (red) you should see the following.
You can see a low pulse back to high for each line on the screen drawn. A high value is full color, so in this case all red. The pulses low on some lines as shown in the image will be the black areas around the Jaguar text.
The resistors R37 (blue) and R46 (green),should be a solid DC voltage of around 2.1V (as the RSB has no other colors except red in it).
After this, the RGB values go to two places.
Firstly they go through a capacitor and inductor (red C14 / L15, green C17 / L11 and blue C18 / L14) which go straight out of the AV cable port at the back of the console.
īģŋRed goes to pin 4 on the bottom of AV edge connector.
īģŋGreen goes to pin 7 on the top of AV edge connector.
īģŋBlue goes to pin 5 on the top of AV edge connector.
If you probe them with an oscilloscope they should look identical to what you saw when probing the analog resistors.
Secondly, the same RGB values get passed through 75R resistors (red R29, green R34 and blue R43) which then go off to the MC13077DW (video encoder chip) to be converted to composite, RF and S-Video.
The MC13077DW is responsible for taking the pure RGB, the HSL and the VSL from the Tom chip and converting them to Composite, S-Video and RF.
There is a lot of circuitry involved in generating the inputs into the video encoder.
The first is the RGB analog values we just covered.
The second is simple, it is VSL from the Tom chip pin 32, through resistor R4, and into the video encoder pin 7.
This is a CSYNC style pin which outputs horizontal line pulses that pulses low once every 63.8us. and every 50 or 60Hz depending on region it outputs a series of longer low pulses several times.
Finally the other input required into the video encoder is the 4x Subcarrier clock input.
The goal is to have the pin 8 of the video encoder receive a sine or sawtooth wave 1Vpp signal that is 4 times faster than the carrier signal.
For composite video the subcarrier frequency is 3.58 MHz for NTSC and 4.43 MHz for PAL. This means the input into the pin 8 should be 4x that, so 14.32MHz for NTSC and 17.72MHz for PAL.
Probing pin 8 on a working PAL system shows this.
This generation of this signal starts in the Tom chip on the HSL pin 31, which is the same as the VSL pin except the VSL pin has periods where it outputs longer low pulses signifying the start of a new frame.
After this the signal travels through a whole lot of circuitry to generate the final output, including U6, U8, U11 and crystals Y1 and Y2 and all supporting components.
The main components to test for when the circuit is faulty is that you are getting a 14.32MHz for NTSC or 17.72MHz for PAL sine wave on Y1, and 26.6MHz signal on Y2.
The video encoder has a band pass filter circuit that it uses on the Chroma before its used in the final outputs.
On the encoder, the Chroma Bypass (pin 17) outputs the 1Vpp Chroma wave and passes it through the band pass filter U1, and back into the Chroma In (pin 20).
The filtered output going back into pin 20 is slightly lower in amplitude but cleaned up of any high frequency and low frequency noises.
If this filter is bad, you will see Chroma at the input to the filter (pin 17 of encoder), but not at the output of the filter (pin 20 of encoder).
The same as the Chroma, the Luma has a filter that has its input coming from the encoders Luma Out (pin 10).
And then through U2 filter (long grey self-contained filter above the encoder chip). The output of the filter goes to the encoder Luma In (pin 6), and is similarly to the chroma about half the amplitude but slightly cleaned up.
If this filter is bad, you will see Luma at the input to the filter (pin 10 of encoder), but not at the output of the filter (pin 6 of encoder).
The composite output comes directly from the Video Encoder chip, through a few transistors and resistors/capacitors to get the correct level and impedance, and then out through the inductor L6 and to pin 11 on the bottom side of the AV edge connector.
TODO: Scope image of working signal
RF is simply composite signal with audio modulated on top of it.
The RF output is tapped off the Composite pin of the video encoder, through a 47uF capacitor (C45), and into the RF modulator box.
The audio is passed into the RF box also from the audio circuit.
TODO: Scope image of working signal
S-Video is again just composite, but with its Luminance (Y) and Chroma (C) separate on two wires. Composite has these to pins muxed together.
S-Video is generated separately by the video encoder chip.
Luminance (Y) comes out of pin 3, through 75R resistor R7, through inductor L10 and out of the pin 8 on the top of AV edge connector.
Chroma (C) comes out of pin 4, through 75R resistor R8, through inductor L8 and out of the pin 8 on the bottom of AV edge connector.
TODO: Scope image of working signal
