Why Your Printer Power Supply Works but the Motherboard Still Won't Turn On

Question:
I recently sent my printer's power supply and motherboard to be tested. The motherboard checked out fine, and while one of the power supplies was shorted (I already suspected this due to a loose bolt), the other one tested as functional. However, when I reconnected it to my printer, the unit wouldn't power on. I even swapped out the on/off controller, power cords, and outlets, but it still wouldn't come on. What could be the next thing to check?

Answer:

1. Check the Power Switch and Signal Path

Even though you swapped the power switch controller, it's still possible that the power signal isn't reaching the mainboard. Many printers have a small control board that sends a low-voltage signal to the power supply unit (PSU) to initiate startup. If that signal line (often a soft-on or standby pin) is broken or not making contact, the PSU will remain in standby mode even if it's functional. Check for any loose connectors, bent pins, or corrosion on the signal cable between the power switch, the control board, and the PSU.

2. Verify Output Voltages from the Power Supply

Use a multimeter to measure the output voltages directly from the power supply terminals. Even though it was tested and confirmed functional outside the printer, it's worth verifying that you're getting the correct DC output (often 24V and sometimes 5V, depending on the printer model). Some printers have internal fuses or relays that open the circuit under load conditions, meaning the PSU might show good voltage without a connected load but fail when installed.

3. Inspect for Hidden Shorts or Blown Fuses on the Mainboard

Sometimes, a shorted component on the mainboard-like a regulator, capacitor, or driver IC-can cause the board to pull too much current at startup, triggering the PSU's internal protection mode. Even though the board tested fine during your initial check, re-inspecting it with a multimeter for resistance across the main power rails can reveal hidden faults. Also, look for small fuses or surface-mounted jumpers on the mainboard that might have opened due to previous power issues.

4. Check Ground Connections and Chassis Continuity

A poor ground connection can also prevent a printer from powering on. Ensure the ground between the PSU, chassis, and mainboard is solid. Loose screws or oxidation on grounding points can interrupt the circuit.

5. Test the Standby Voltage and Logic Power

Many printer power systems provide a standby voltage (often 5VSB) to the logic board even when the printer appears "off." If this standby power isn't present, the board won't initiate the power-on command. Check the standby line using your meter; it should be live as soon as the printer is plugged in.

6. Evaluate Secondary Components

If all else checks out, other potential culprits include:

• Defective ribbon cable or connector between the power supply and mainboard.

• Damaged MOSFET or voltage regulator that's preventing the mainboard from distributing voltage to key circuits.

• Firmware or EEPROM corruption, though this is rare-it can prevent startup sequences even when hardware is intact.

Addressing printer issues can be a complicated affair due to the hands-on nature of the problems. So, we're not able to provide remote troubleshooting, suggestions, or support for printer repairs. We offer an in-person evaluation and repair service via our local diagnostic facility (https://bchtechnologies.com/printer-repair-service). Given the high demand, we operate on a first-come, first-served basis, so it might take a few weeks before we can check your printer in. Our services are structured to repair either a whole printer or specific parts, with detailed instructions on how to proceed. However, we recognize that our rates might not be the lowest available, so we encourage self-help through online research. You can begin by visiting our YouTube channel (https://youtube.com/@bchtechnologies). Use the search icon next to "About" on the top menu bar to look for videos related to your printer's model and issue. With over nine years of video content, this is often the quickest way to find exactly what you need-and you might even discover helpful content from other creators as well.

Thank you again for reaching out and for your continued support of BCH Technologies. We truly appreciate your trust and involvement as we work to make printer repair knowledge accessible to everyone.