Backlight Driver (PC Engine GT)
The backlight driver circuit that generates the high voltage (900V+) needed for the fluorescent tube to light up, is located on the motherboard (Back board that reads the game cartridge).
The circuit used in this console, and the Game Gear, Atari Lynx and many other backlight tubes from the era are variations on the Royer/Jensen Oscillator circuit (you can read more about that circuit theory here).
This means it is a self oscillating circuit (without the need for any MCU or digital control). It relies on Q900 and Q901 to be very closely matched (so replace them in pairs) otherwise they will burn out with magic smoke.
The backlight circuit will not be turned on/activated without the front board being plugged in, or shorting pin 8 to ground on the LCD 12 pin connector (which is what the front board does).
There is a pad labelled BLH that is bridged at the factory. When it is bridged the backlight circuit is enabled.
If you remove this bridge, the backlight will not turn on. This disconnects the 9V power from the batteries to the backlight.
The final output should appear on the red wire (bottom pin) of the backlight 2 pin connector.
It should be a sine wave of 900V to 1,600V depending on load (usually 900V when loaded down with a backlight, and 1600V free swing if no load). This drives the fluorescent tube.
The main component that generates the high voltage is the T900 transformer. This is under a metal cage.
On the other side you can find the pins. The low side has approximately 30V pulses, that then transform into over 1600V when not loaded down.
The third pin up (16V pulses) is the center tap pin of the 3 pin transformer coil, which should go directly to 9V. Sometimes this trace breaks and you want to restore it with a wire to the left of L500.
Ultimately the backlight circuit (the pulsing 30-55kHz 1000V signal) is generated from the self-oscillating Jenson circuit.
The circuit is going to be very similar to the Game Gear backlight circuit. The difference is the driving voltage is 9V not 5V, the Inductor (Current Fed Royer circuit) is not present, and the transistors are in another layout. But it is a good start.
In the case of the PC Engine GT the following components would be replaced from the Game Gear designators.
No Inductor (L500 is not for this circuit)
The actual circuit is more laid out like an actual Royer circuit.
This becomes evident when you probe the Q900/Q901 layer and find the Emitter pins are to ground, the Collector goes through a resistor, though the inductor and to VCC, and the bases
The 30V pulses from the T900 transformer second pin up from the bottom left, also connects to Q900 middle pin.
The 1.2V circuit when unloaded and in certain spots can be up to 3.3V, so where referencing the 1.2V pulse, it can be 3.3V in places. This is normal based on load.
A quick probe around shows that Q902 top two pins are joined to the 1.2V pulse of the transformer pin.
This pin joins to the bottom left pin of Q902, which is connected to the top middle pin of Q903.
The Q903 signal is joined to the bottom left pin which is driven at 1.6V pulses.
This is connected to resistors and capacitors below it, and to D901 on the top middle pin.
If you remove D900 the backlight still works, so this signal path is not worth investigating further.
Removing Q902 also allows the backlight to work. So we know wherever this part of the circuit goes, its not directly related to the backlight working. It is more likely a feedback signal going to the ASIC to monitor the backlight circuit perhaps.
The Q900 or Q901 transistor pair form the main loaded components of the oscillator circuit and they take all of the brunt of the power. If the transistors wear out unevenly and start to take on different characteristics, or one fails so there is no oscillation, the transistors can overload and burn out of the transistor (the magic smoke), which results in no backlight.
Without Q900 or Q901 the transformer sits at 9V DC on the bottom left 3 pins, top left pin, and top right pin. The pin second down on the right (which is usually 25V pulse) is not at 9V DC but at 0V. The 900V is also at 0V.
The Q900/Q901 are BJT NPN transistors in SOT-89, left to right pins BCE. Vbe 0.7V and Ic 5.8mA. This transistor pair can also be taken off a Game Gear Q3 and Q4.
The Collector of Q900 goes to R900, then the other side of R900 goes through the transformer to 9V.
The Collector of Q901 goes to R901, then the other side of R900 goes through the transformer to 9V.
The Emitters both go to Ground.
The smaller transistor Q501 is SOT-363 package, containing two NPN BJT transistors.
The PCB underneath joins the collectors together.
The yellow emitter is connected to ground.
This is not related directly to the backlight driving circuit but here for useful reference and testing.