Audio Circuit (PC Engine GT)

audio amp is njm2073m , labelled u10 or ic100 , near volume wheel audio from 6280 to amp the audio comes out of the hu6280 chip, left audio on pin 17, and right audio on pin 18 mono audio can also be provided by the game cartridge itself these three channels all come through capacitors and resistors, merging the mono into both left and right channels audio inputs to amp then the final left and right audio is sent to the volume wheel, resisted down to the volume desired by the wheel position, and into the amp the left audio always goes to the amp left input the right audio, when headphones are inserted , is send to the amps right channel the right audio, when headphones are removed (so speaker output) is mixed with the left audio by pins 6 and 7 of the headphone socket this means when there are headphones inserted, the left and right input to the amp received each audio channel when there is no headphones (so speaker) the amp receives a mixed left and right audio signal to its left channel only, which is then sent to the speaker through the headphone joined pins 4/5 headphone output path the audio comes out of the amps output channels left and right, through a capacitor and inductors, to the headphone left and right channels speaker output path the audio amp output a (left, pin 1) goes through the headphone left (pin 5) and joins to the speaker (pin 4) pad when headphones are not inserted this pin 4 then goes 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 left out 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 also solder a speaker directly between pin 11 and ground (pin 12) to confirm if the speaker is ok front board ribbon pins 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 11 speaker + 12 gnd (speaker ) volume wheel pins (dual 5k pot) pin 1 right audio full (originating 6280a pin 18) 2 left audio full (originating 6280a pin 17) 3 right audio (audio amp input b+ pin 6) 4 left audio (audio amp input a+ pin 7) 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 headphone jack the headphone jack on the pc engine gt has the following pins pin number pin name function ic pin 1 ground ground 2 right headphone right audio to headphones audio amp pin 3 (out b) via cc104, l101 and l104 3 not connected headphone out right audio headphone inserted floating 4 speaker audio audio to speaker when headphones are out headphone out speaker audio headphone inserted floating 5 left headphone left audio to headphones audio amp pin 1 (out a) via cc103, l100 and l103 6 amp left in / left audio from 6280 headphone out joined to pin 7 headphone inserted floating 7 right audio from 6280 headphone out joined to pin 8 as stereo 8 amp right in headphone out floating headphone inserted joined to pin 7