Read Harman Kardon ONYX STUDIO 2 (serv.man2) Service Manual online
3.6 Charging current
1. Connect good DC board/ USB board and KB to MB.
2. Set power supply 3.8V, connect to MB J700
3. Set another power supply to 19V/ 2A and plug in DC jack
4. Monitor power supply 19V/ 2A voltage if it drop to very low (close to 0V), it must short in somewhere;
else go to next step
5. Check if CHG voltage is 0V, if not U300 is defective else check below circuit
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