Recently, I've acquired an old SNES, specifically the SNSM-CPU-1CHIP-03 model, with no audio output. I traced the issue to a dead S-Mix chip, essentially a buffer between the audio DAC and the line output.
While the DAC in question, an LM6379A, is technically capable of driving the line output on its own, I thought I'd try my hand at making a replacement buffer, using an LM358 op amp as a voltage follower and taking advantage of the passive components already in the board. The resulting circuit looks a bit like this, for each stereo channel:
Now, this does seem to work, at least for a little while - but eventually, after a minute or two, the output gradually lowers, before disappearing completely, or being drowned out by power supply noise. Placing my hand next to the amp did occasionally make the problem go away, making me suspect it's some sort of coupling issue, but I am not experienced enough with analog circuitry to pinpoint what exactly might be going wrong.
Is there anything I might be overlooking in this?