Audio Circuit (Sega Mega Drive)
Depending on the version of Mega Drive the audio circuit is slightly different, but the overall design is the same.
The Mega Drive has two main sources of sound. The FM syth and the PSG sound. Both get mixed together before forming the final audio that gets amplified and output.
All FM audio is produced via the Yamaha YM2612 FM synthesis chip.
The 68k processor generates the audio data, sends it to the Z80 co-processor, which then sends it on to the YM2612.
In order for the two processors to communicate, and thereby produce digital audio to drive the YM2612, all of the inter-communication chips must be functioning.
For VA0, VA1, and VA2 the IO Controller (315-5309) and Bus Arbiter (315-5308) and Clock Generator (315-5345) are used.
On VA3 and VA4 it is done via the same IO Controller (315-5309) but the Bus Arbiter (315-5308) and Clock Generator (315-5345) are combined into a single chip (315-5364).
On VA5, VA6, VA6.5 and VA6.8 this is done by combining both chips from the VA3/4 together into one. On VA5 it is 315-5402 and VA6 it is 315-5433 (on VA6) The only difference is the 315-5433 also contains the trademark screen TMSS ROM. These chips combine the IO Controller (315-5309) and Bus Arbiter/Clock Generator (315-5364)
On VA7 all this functionality is combined into a single ASIC, the 315-5487.
The PSG audio is a 4-channel Programmable Sound Generator capable of 3 wave form generators and 1 noise generator.
Techincally it is done via the TI SN76489 chip. However on the Mega Drive this chip is directly integrated into the 315-5313 Video Display Processor (which is a Yamaha YM7101).
On the VA7 models, both the FM and PSG audio is generated inside the single ASIC 315-5487.
There is no PSG audio or YM2612 chips on the VA7, the audio simply comes straight out of the ASIC.
The YM2612 sound chip receives its data from the Z80, and outputs analog audio as Mono Left and Mono Right outputs.
The output audio should be about a 1Vpp signal. Here is the Sonic 1 intro screen coming out of Analog Left Out.
If you don't have an oscilloscope, you can probe the RCA audio jack wires for left or right audio directly onto the Analog Out pins of the YM2612 and you will hear the audio.
It will be a little quite but you will definitely hear it.
After the FM audio comes out of the YM2612 is when all the sources of audio get mixed together.
Firstly the edge connector SL2 (pin B29) SR2 (pin A29) mix any analog audio coming from the Mega CD connected to the edge connector, into the analog audio.
Secondly, the 315-5313 mixes its PSG analog audio output (pin 95) into both the left and right audio channels.
Finally this forms the full left and right audio signals. This is mixed to mono through the LM358 Op Amp and sent to the CXA1145 Video Encoder as Mono Audio to send out of the AV port.
The left and right channels are also sent through the Sony CXA1034 Headphone Amp and out to the headphone port.
The CXA1034P audio amplifier is used to output audio to the headphone port.
I have seen a few times, if you get random crackling on the audio both headphone and AV port, like a radio static, it can be failing 7805 LDO regulators.
To confirm, join both outputs of the 7805 regulators together, then power the console from a bench power supply on the 5V outputs of the regulators, directly with 5V.
If the noise goes away, your regulators are bad and need replacing, or upgrading to a CleanPower USB-C mod.