Epson WF-7720 Won’t Power On After Surge? How to Diagnose and Repair a Dead Motherboard

Question:
My Epson WF-7720 stopped powering on after a lightning strike. I tested the power supply using your video guide and confirmed it's working. I then disassembled the printer and removed the motherboard. However, I don’t see any large, replaceable transistors—just tiny surface-mounted components. I’ve watched your videos on testing A2222 and similar transistors, but this board seems very different. With only a basic rework station, I’m unsure how to proceed. Do you have any suggestions for refurbishing this board, or should I consider buying a used replacement?

Answer:

Why the WF-7720 Mainboard Might Be Completely Dead

In your case, the power supply has been ruled out—great first step. Since there are no visible A2222 or similar transistors, what you’re seeing instead are Surface-Mount Devices (SMDs). These compact components take the place of traditional through-hole parts and are trickier to identify, test, and replace—especially after surge damage from something like a lightning strike.

Lightning strikes typically cause:

• Power surge damage to MOSFETs or voltage regulators

• Burned or lifted traces

• Shorted or corrupted EEPROMs

• Completely dead processors (SoCs)

Because these are SMDs, you’ll need to approach repair like an electronics technician would—with careful tracing and measurement.

Step-by-Step Diagnostic Method: “Follow the Power”

To diagnose a dead board like this, gather:

• A digital multimeter with diode and voltage modes

• Your hot air rework station

• Kapton tape or foil for shielding

• Optional but helpful: a magnifying lens or microscope

Step 1: Confirm Power Entry

• Identify the DC-IN jack on the board.

• Common voltage levels: 24V, 42V, or 5V depending on the segment.

• With the power supply ON, probe for voltage at the entry point.

• If no voltage is present, recheck the connectors and any input fuses or solder joints.

 Step 2: Diode Test for Main Power Path

• Use diode mode:

  • Black probe to ground

  • Red probe to power rail (after any entry diode)

• Normal forward voltage drop: 0.2–0.6V

• 0.0V or near-zero in both directions = short

• OL (Open Loop) = blown diode or broken trace

 Step 3: Check for Shorted MOSFETs

• Look for three-pin or eight-pin SMD rectangles (black with markings like A2SHB, AO3400).

• These are your switching transistors.

• Test between gate–source and drain–source.

• If drain–source reads low resistance both ways, the MOSFET is likely shorted.

Step 4: Test Voltage Regulators

• Look for parts marked AMS1117, MP1584, or XL4015—these regulate voltage to 3.3V, 5V, etc.

• Pinout usually includes:

  • VIN (input)

  • GND

  • VOUT (output)

• Use DC mode to check:

  • Does VIN have voltage?

  • Is VOUT producing the expected output?

• If input is fine and output is zero, the regulator is likely blown.

 Step 5: Evaluate the Microcontroller and EEPROM

• If all voltage levels look good but the board is still dead:

  • No lights

  • No USB recognition

  • No sound or carriage motion Then the main processor or EEPROM chip may have failed. These are tough to diagnose without advanced tools and are generally not user-replaceable.

 Step 6: Try the “Thermal Finger” Test

• Power up for 3–5 seconds and lightly touch ICs with your finger.

• If something feels hot almost immediately, that may be the failed component.

• You can also:

  • Drip isopropyl alcohol over the board—watch where it evaporates fast

  • Use a thermal camera if available

 Want to Learn More About Board Repair?

You're already ahead of the curve by getting this far. If you want to build on this knowledge, here are two excellent learning tracks:

 Self-Guided Courses

• Coursera: Introduction to Electronics (https://www.coursera.org/learn/electronics)

• Udemy: Electronics for Beginners (https://www.udemy.com/course/electronics-for-beginners/)

Topics you’ll explore:

• Ohm’s Law and circuit theory

• Schematic reading

• Transistor logic and signal paths

• Basic and advanced troubleshooting techniques

 College-Level Topics (EE Foundations)

• Circuit analysis (RLC networks)

• Analog vs. digital systems

• Microcontroller architecture

• Embedded systems and diagnostics

With this background, you'd be well-equipped to refurbish even modern multi-layer printer motherboards.

 What If It’s Not Fixable?

If your board has internal damage beyond repair:

• Consider purchasing a used board (like the eBay listing you shared), but check seller ratings and return policy.

• Or, you can send your board to us at BCH Technologies. We’ll inspect it and let you know if a repair or swap is viable. Email us at support@bchtechnologies.com and include photos or questions.

Repair Service and DIY Support Availability

Board-level repair requires hands-on testing that’s difficult to manage remotely. Because of this, we cannot offer detailed remote support for component-level repairs. However, we do offer full evaluations and in-shop service through our printer repair facility (https://bchtechnologies.com/printer-repair-service).

A few things to keep in mind:

• Repairs are handled first-come, first-served. Wait times can be a few weeks.

• We can work on entire printers or specific components.

• We understand our rates may not be the lowest, so we encourage DIY solutions whenever possible.

Start by exploring more videos on the BCH Technologies YouTube channel (https://youtube.com/@bchtechnologies). Use the search bar next to the "About" tab to look up any topic you need. With over nine years of videos, there's a good chance you’ll find what you need—or something equally helpful from another creator.

Thanks again for your question and your passion for diving into this repair. You've already done a lot of the hard work. If you'd like, send me your motherboard photos and I’ll be happy to highlight where to probe next. Let’s get your WF-7720 back to life—together.