Epson ET-8550 Turns White Screen, Then Black Screen and Shuts Down: PSU, Mainboard, CPU, and EEPROM Diagnosis

Question

I have an Epson ET-8550 that will not fully power on. When I press the power button, the screen turns white for a few seconds, then the printer automatically goes black and shuts off. After that, it cannot continue booting.

After opening the printer, I tested the PSU output pins while the power supply was connected to the printer. In standby mode, the output is below 3V. After the power-on command, the voltage briefly rises to about 18-22V, and during that moment the LCD screen is white. Then the voltage drops back to around 1.8V, the screen goes black, and the printer shuts down.

I suspect there may be a shorted MOSFET or another downstream component failure, but I have not started the repair yet. Can this type of fault be diagnosed further? What does this usually indicate? Is it a common issue on the Epson ET-8550?

Answer

Based on your voltage readings, I do not think the power supply unit is the main suspect at this stage. In fact, your test result shows that the PSU is responding to the printer's startup command. The standby voltage being around or below 3V is normal behavior for many Epson systems. When the printer receives the power-on command, the PSU output being pulled up to around 18-22V means the power supply is attempting to start the printer and provide the operating voltage.

The important clue is what happens next: the voltage drops back down to around 1.8V, the screen goes black, and the printer shuts off. That usually means the printer begins its startup sequence but fails during the boot process. The PSU is not completely dead. It is being activated, but the system cannot continue running.

I would be cautious about blaming a downstream MOSFET first. A bad MOSFET can certainly cause power problems, but the symptoms are usually different depending on where the MOSFET is located and what circuit it controls. If a major MOSFET were fully shorted on a main power rail, the printer often would not start at all, or the PSU might immediately go into protection mode with no meaningful boot attempt. If the MOSFET failure were in a printhead-related drive circuit, the printer might power on but fail to print correctly, print blank pages, trigger a printhead-related error, or damage the head circuit. In your case, the white LCD screen followed by shutdown suggests the printer is trying to initialize but the logic system is not completing the startup sequence.

The first area I would suspect is the mainboard logic side, especially the CPU or firmware memory area. When the ET-8550 starts, the board has to initialize the CPU, read firmware data, communicate with the LCD panel, check system status, and continue into the normal boot sequence. If the CPU is malfunctioning, the printer may begin to wake up but fail when it reaches the processing stage. This can cause a white screen, freezing, shutdown, or repeated failed startup attempts.

Another possible cause is corrupted or failed firmware memory. On this type of Epson mainboard, the firmware and configuration data are stored in EEPROM or flash memory chips. If those chips are corrupted, unreadable, physically damaged, or loaded with bad firmware data, the CPU may not be able to process the startup logic correctly. The result can look very similar to what you described: the printer starts to power up, the display lights or turns white, then the system fails and powers down.

For this reason, one practical diagnostic step would be to replace or reprogram the three EEPROM chips on the bottom side of the mainboard using known-good firmware data. If you have access to a known-good donor board or properly programmed replacement EEPROM chips for the ET-8550, swapping those chips can help confirm whether the issue is firmware-related. This is often a better first direction than chasing MOSFETs blindly, because your PSU behavior already shows that the power supply is not simply failing to start.

Before replacing chips, I would still recommend doing basic board-level checks. Check the mainboard for liquid damage, corrosion, burnt components, cracked solder joints, damaged FFC connectors, and any obvious shorted capacitors near the power input and voltage regulator areas. Measure resistance to ground on the major rails, especially around the 3.3V, 5V, and motor/head power sections if you can identify them. If one rail is nearly shorted to ground, then a shorted capacitor, regulator, IC, or MOSFET may still be involved. However, if the rails do not show a hard short, the CPU/firmware direction becomes more likely.

The LCD behavior is also meaningful. A white screen usually means the backlight or LCD power is present, but the display is not receiving proper image data or initialization commands from the mainboard. That points more toward a logic-board startup failure than a simple PSU failure. The display is turning on, but the printer's processor is not completing the communication and startup sequence.

So, my diagnosis would be:

The PSU is probably working because it enters standby and then responds to the power-on command by raising the output to 18-22V.

A shorted MOSFET is possible but not my first suspicion based on these symptoms.

The more likely causes are a mainboard CPU malfunction or corrupted/failed firmware EEPROM chips.

Replacing the three EEPROM chips on the bottom of the mainboard with known-good programmed chips would be a reasonable next diagnostic step.

Also inspect the mainboard for shorts, liquid damage, failed capacitors, damaged regulators, and connector problems before doing chip-level replacement.

This type of failure is not the most common everyday ET-8550 issue, but it is a known style of failure on modern Epson logic boards. When the printer shows a white screen briefly and then shuts off, it often means the machine is failing during early boot rather than failing at the power supply alone. The printer is trying to start, but the mainboard cannot complete the logic sequence.

Addressing printer issues can be a complicated affair because many of these problems require hands-on testing, voltage measurement, board inspection, and sometimes component-level repair. Because of that, we are not able to provide remote troubleshooting, detailed repair supervision, or step-by-step repair support for individual printers. We do offer an in-person evaluation and repair service through our local diagnostic facility: BCH Technologies Printer Repair Service [https://bchtechnologies.com/printer-repair-service]. Due to high demand, repairs are handled on a first-come, first-served basis, so it may take a few weeks before your printer can be dropped off. Our services are structured to repair either the whole printer or specific parts, with clear instructions on how to proceed. However, we also understand that our rates may not be the most economical option for every situation. For that reason, we strongly recommend self-help through online research whenever possible. A good starting point is YouTube, including our channel homepage: BCH Technologies on YouTube [https://youtube.com/@bchtechnologies]. You can use the search icon next to "About" on the right-hand side of the menu bar to search for specific repair topics. I receive dozens of questions every day asking whether we have videos on particular subjects. Since we have been creating videos for many years, it is difficult to remember every single one, so YouTube's search function is usually the fastest way to locate a relevant video. YouTube may also recommend helpful videos from other repair channels.

Thank you again for reaching out and for supporting BCH Technologies. I hope this gives you a clearer direction for diagnosing the ET-8550 white-screen shutdown issue and helps you decide whether to focus on the firmware EEPROM chips, the CPU/mainboard logic section, or a possible short on one of the power rails.