The PC Engine GT has the high voltage (backlight tube) generation circuit, HSYNC/VSYNC and data generation on the main motherboard, and the LCD contrast circuit voltages (12-22V) on the LCD motherboard.

The LCD ribbon has various signals sent from the motherboard to the LCD motherboard through the ribbon.

All signals below are present and sent to the screen even when nothing is connected to the main motherboard at all.





On the motherboard the contrast wheel goes to pin 2 and 8 of the LCD ribbon, and passes in a variable resistance between 0 and 4.7k (0 is over bright, 4.7k is under contrast/black).

The actual high voltage (12 to 22V usually) voltages for the LCD contrast circuit are generated on the LCD motherboard through the circuits on there, related to the OKI M6370 chip that is on the LCD board.

The only real way you can monitor if the motherboard is sending the signal to the LCD board for contrast is to measure the contrast wheel pins 1 and 2 (middle two) go between 0 and 4.7k resistance.

Next check the pins 1 and 2 go to the LCD ribbon connector pins 2 and 8.

Finally make sure the -25V DC is present on the LCD ribbon connector pin 3.

The -25V signal is generated by a small 3.8V AC pulse going into pin 4 of the T500 transformer (bottom right pin), which generates a 35V AC pulse on the output pins of the transformer (left bottom and left middle pin).

This pulse goes through a lot of circuitry to the left and underside of the board to generate an adjustable -10V to -30V DC.

The 3.8V pulse going into the transformer T500 pin 4 connects to C501 capacitor on the other side of the board near the top, through R501 at 680R and through R500 at 6.8k (which connects to the 9V input voltage).

The tap off point between the two resistors goes to the top left and bottom right pin of Q501 just below it.

The T500 transformer is a 10:1 transformer with a center tap having a top and bottom ratio of 8:1 and 2:1 respectively.

The primary side is driven by a 3.8 VAC input. This input also triggers the gate of Q500, which shorts the center tap of this transformer to ground via the collector of Q500 during the pulses.

The tap off for the 8:1 (top left pin) is 9V.

The top off for the 2:1 (bottom left pin) goes to many places, including a ton of components to the left of the Q502, and eventually includes VR501. This is the main output for the contrast circuit.

A semi-common issue is the capacitor CC500 leaking and rotting the bottom left pin of D502, which should have a solid DC output of around -40V to -70V.

If the trace is corroded it can spike up to -120V or more, which ends up outputting an unstable and usually less than -25V on the final output circuit.

The final adjustable DC -25V signal (pin 3 on the LCD ribbon), goes to the negative of the 22uF 35V capacitor CC503 (the side with the main large black chips on).

C506 is also in parallel with CC503.

Pin 3 also goes through a 2.7k resistor R502 (next to CC503) then through VR500 potentiometer just above it, on the bottom left pin.

The top of VR500 goes through another 47k resistor R503 which goes to ground.

Turning VR500 adjusts the negative voltage between -10V and -30V which adjusts the maximum contrast range.

Set the voltage to as close to -25V as you can, or based on your LCD and its visual output.

The generation of the -25V comes from the capacitors, transistors, and resistors at the top left (opposite to the power switch) of the motherboard as mentioned above.

Both the front and back of the PCB components are involved in it. For example the large capacitors CC500/501 and Q500 and D502 on the front of the motherboard are all part of the circuit.

On the back, D501, R504, C505 and many other components are involved. I haven't reverse engineered further than this, but if you are not getting the full voltage range or no voltage at all on the -25V pin to the LCD connector go through this circuit.