I found this service bulletin one time, but I haven't been able to find it online since. I'm uploading it here so others will be able to access it. It is a simple mod for TKR-820 repeaters used for DMR or DSTAR. By removing a capacitor on the display unit, the output waveform to the modem is less distorted. I've seen only minor improvement in error rates, but every little thing helps.
To access the capacitor, you remove the top and bottom covers. Then remove the left and right top screws holding the front face in place. Tilt it down, and you have access to the capacitor. You do not need to take the knobs off, or remove the from face from the unit. Total time is less than 15 minutes to complete.