I’ve had the same Sega ST-V motherboard in my collection for quite some time now. Randomly one day, the board booted up with completely scratchy / static audio. At first I suspected the capacitors since the fault developed in my possession, however they were not the culprit. I put the board in storage to return to it another day.
Fast forward a few months, my friend Jassen pinged me asking if I could look at his ST-V. He described his board was having scratchy audio (same fault my board is exhibiting). To my surprise, Jassen informed me he contacted Ken @ irepairsega.com, and was advised that the TDA138 DAC Audio IC was likely the culprit. We all know Ken is a GOD when it comes to Sega repairs, so this information was likely spot on…
We ordered a couple of IC’s (one for each ST-V). Once they arrived from China, I pulled out the hot air gun and went to work:
I successfully installed the DAC’s (one on each ST-V motherboard):
SUCCESS! The scratchy samples are now completely gone (on both boards), and the audio is nice again. Being that I repaired two boards with the exact same fault at the same time, if you experience and ST-V with bad audio, start with the DAC!