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My synth, an AKAI MPK49 worked just fine until recently. Powered by either USB (5V) or a 6V DC input. However, the circuit fails to power up when using USB only now but works fine with 6V DC entry.

Observations:

  • 5V DC input (5V DC adapter 1A)
  • D3 cathode: 4.6V.
  • After D3 and R1: 4.4V.
  • D1 pin 1: 3.3V.
  • TP2: 3.3V.
  • TP3: 3.3V.

Circuit works as expected.

USB only:

  • D4 cathode: 4.8V.
  • Anode (power line): 0V.
  • Circuit does not power up.
  • TP2: 3.3V.
  • TP3: 0V.

Tests Performed:

  • Replaced D4 (MBRS140LT3 → SS32), no improvement.
  • +3.3V is present on D1 (BAV99).
  • Sent 3.3V on D1 pin 1 to power up the system.

The power on D1 seems to power up U7 which send the POWER_EN signal to U3-A when 6V DC is connected, which is not when USB is connected. I removed C4 in case of malfunction. How can the USB tell U7 to power up?

enter image description here enter image description here

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  • \$\begingroup\$ From where the USB comes from? A PC, other computer, power bank, phone charger etc? \$\endgroup\$ Commented Dec 12, 2024 at 17:13

1 Answer 1

Reset to default
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It seems to if the uC has power from USB then "Power"=0 in and "Power_En" out should go low.

enter image description here

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  • \$\begingroup\$ Do you mean USB_DIS or USB+ ? because the POWER from pin 1 of D1 is always low when USB is connected. D4 is blocking the signal comming from D3. I d'ont see any other POWER line coming out of the power supply schematics. D4 is blocking current from USB to reach POWER at D1 pin 1, I can't figure out how the POWER line is suposed to be connected to USB to send signal to U7 to power up the system. \$\endgroup\$ Commented Dec 12, 2024 at 7:34

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