Power Circuit (Atari Jaguar)
The Atari Jaguar similar to the Lynx is powered by a 9V inverted DC jack (with positive on outside).
Similar to the Lynx requiring a game to turn on, the Jaguars main voltage regulator (the MC34163, U38) has its secondary feedback (pin 2) pulled up to the 9V rail through a 10k resistor, through an internal voltage divider in the regulator keeping that pin at around 6V. This pin is labelled CART_IN.
The CART_IN pin needs to be grounded for the regulator to output voltage and turn on the system and power LED.
The CART_IN pin only goes to the cartridge connector and when a game is inserted it shorts the CART_IN (pin B34) to the CART_OUT (pin B35).
The CART_OUT pin is shorted to ground when the power switch is turned on.
What this means is when the power switch is on, the cartridge CART_OUT (pin B35) is shorting to ground. When a game is inserted this shorted pin is connected to the CART_IN (pin B34) which grounds the feedback (pin 2 of U38), turning on the main power regulator.
To bypass this system and allow the console to power on without a game (which is very useful for diagnostics) we can simply short the B34 and B35 pins together.
This turns on the power regulator as soon as the power switch sends CART_OUT (pin B35) to ground.
To power the Jaguar from a bench power supply (super useful for current monitoring, broken or dead shorted connections and diagnostics), apply ground and 9V to the underside of the DC jack as shown (black is ground, red is 9V).
The expected power draw of a working system when turned on with or without a game is around 500mA @ 9V.
The main system regulator (MC34163, U38) is located at the bottom right of the console by controller two port.
The 9V DC power comes into the system through the choke near the DC power jack, where both the ground and 9V are filtered through the choke.
Check the choke has continuity through each side (from pin 1 to 2, and pin 3 to 4), but not continuity to all 4 points (otherwise you have a shorted choke causing a dead short).
If your chose does not have continuity between pins 1 to 2, or 3 to 4, it is open circuit and needs to be replaced or just pins 1-2 and 3-4 shorted with wire.
After the choke are some smoothing caps and a voltage suppression diode before power enters the U38 regulator at pin 7.
The large green resistor R169 in the image above is a current sense 0.1R resistor used to detect how much power is being drawn and to shut down the regulator on over-current situations.
Measure it with a multimeter and it should measure just over 0R if your meter is good enough to measure it.
The regulator should output around 5V (usually between 4.9V and 5.3V) on pin 3.
The large L29 inductor should have continuity, and it connects to VCC (pin 3), as well as the Switch Emitter (pin 15).
The top side of the inductor (Switch Emitter, pin 15) if you probe with an oscilloscope should have a pulsing 9V signal as the inductor stores the energy for the output.
The bottom side of the inductor (VCC, pin 3) should be the steady 5V system output.