First post, by wsy2220
TLDR: A solder joint is broken under SMD resistor R68.
I recently aquired a very dustySB16 CT1740. After some testing, it's apparent that the right FM channel is not working.
Looks like other people have similar problems:
Sound Blaster 16 CT1740 - Left Channel Not Working - Repair or Return?
Sound Blaster 16 CT1740 - one FM/MIDI channel almost dead
After some probing and testing, I successfully fixed it.
I first checked the output of YAC512 with an oscilloscope, both channels are working correctly. I then traced the outputs of the YAC512, they go to opamp U15. Output of each channel then go through a 10K resistor to the input of another opamp U22 as the image shows.
I found that the right channel resistor R68 is broken. Visually it's still intact but the right solder joint is broken inside, with a resistance over 2M. I resoldered it and made sure the output of U15 to the input of U22 have a resistance 10K Ohm. It works perfectly now.
