Some shorts within the DAC circuitry is probably happening. DAC circuitry is located under the vertical ATMega 28-dip socket. This part of the PCB is highly crowded and you may have touched some SMD component while trying to solder components such as SW2, RV2, D3, D9 and D21.
Please have a look at their solder joints, reflow them and keep an eye on the surrounding SMD parts looking for bridges and tombstones and try to reflow the following resistances and capacitors: R12, R19, R20, R21, C19. Please keep in mind that if an SMD problem has been found as mentioned above, the SMD LED (D7) it’s likely burnt because of that issue and a replacement is needed. That’s actually a minor problem since the calibration mode perfectly works even without that LED, which just acts as a visual reference for you (you can check if you are in calibration mode by looking at the CV LED: it’s constantly on when calibrating and off when in operational mode with unconnected probes). If you feel confident about desoldering it and replacing it by your own, you can easily take any SMD 0805 LED and use it: please remember that it’s cathode (negative terminal) must be oriented pointing to the lower part of the PCB, it must be close to the anchoring pins of the RV1 potentiometer. If you chose to replace the LED by your own, please keep in mind that we’re not responsible for any damage that you could bring to the PCB and/or components.