Audio Circuit (PC Engine GT)
Audio amp is NJM2073M, labelled U10 or IC100, near volume wheel.
The audio amp output A (headphone left) goes through CC103 and has a pull-down resistor R109 of 47R to ground, and taps off through L100 finally going to the headphone left (headphone pin 5) and to the speaker (headphone pin 4) if no headphones inserted.
Headphone connector pin 4 (speaker out) is sent to the pin 11 of bottom right front board white connector (speaker+) so only receives audio when headphones are not inserted, otherwise the pin 5 (headphone left) is disconnected from speaker output pin 4.
To test the amp is working but perhaps the headphone connector is bad, send a wire directly from pin 1 of the amp to pin 11 of the front board white ribbon connector and see if you get audio. The speaker will never turn off when headphones are inserted but proves if the the audio amp is working.
Pin | Function | Notes |
1 | 5V | With bulk cap CC800 connected to it |
2 | Up / I | Up (Low Pulse), I (High Pulse) |
3 | Right / II | Right (Low Pulse), II (High Pulse) |
4 | Down / Select | Down (Low Pulse), Select (High Pulse) |
5 | Left / Run | Left (Low Pulse), Run (High Pulse) |
6 | Pulse Map | Button Pressed Wave Form |
7 | Trigger | 1us 5V pulse every 133ms |
8 | !Backlight On | Front board shorts this pin to ground to turn on backlight |
9 | ??? Always 5V | Tied to 5V and C702 (470uF) is on that rail |
10 | GND | īģŋ |
11 | Speaker + | īģŋ |
12 | GND (Speaker -) | īģŋ |
Pin | īģŋ |
1 | 5K Pot 1 |
2 | 5K Pot 2 |
3 | Audio Amp Input B+ (Pin 6) - Headphone Right |
4 | Audio Amp Input B+ (Pin 6) - Headphone Right |
5 | Ground |
Headphone connector on front board bottom center pin is ground, the two pins above it get grounded when headphones inserted. Those 2 pins go to the BU5782K IC.
The headphone jack on the PC Engine GT has the following pins.
Pin Number | Pin Name | Function | IC Pin |
1 | Ground | Ground | īģŋ |
2 | Right Headphone | Right Audio to headphones | Audio Amp Pin 3 (OUT B) via CC104 |
3 | Not Connected | Headphone Out: Right Audio Headphone Inserted: Floating | īģŋ |
4 | Speaker Audio | Audio to speaker when headphones are out Headphone Out: Speaker Audio Headphone Inserted: Floating | īģŋ |
5 | Left Headphone | Left Audio to headphones | Audio Amp Pin 1 (OUT A) via CC103 |
6 | 5V Pull Up (Unconfirmed) | Headphone Out: Joined to Pin 7 Headphone Inserted: Floating | īģŋ |
7 | īģŋStereo (Unconfirmed)īģŋ | Headphone Out: 5V = Mono Headphone Inserted: GND = Stereo | īģŋ |
8 | Ground | Headphone Out: Floating Headphone Inserted: Joined to Pin 7 | īģŋ |
In short, the main processor outputs audio, sends both left and right channel audio into the volume wheel, which then go into the audio amps dual inputs.
The outputs are then sent to the headphone jacks left and right channels, and the speaker is tapped off the headphone left via the tip switch (pin 4) that is connected to headphone left while no headphones are inserted.
The outputs have capacitors between the audio amp and headphone/speakers, and a pull-down resistor of 47R to ground.
