Epson L1800 Power Supply Burnout After Mainboard Replacement - Causes and Troubleshooting Steps

Question:
I recently replaced both the mainboard and power supply on my Epson L1800 printer. It printed about a quarter of a page and then shut off. When I opened the printer, the power supply had a burnt electrical smell, so I replaced it with a new one. However, the new power supply won't power on at all. Now I'm out two power supplies and one motherboard. The printer worked fine before installing the new components. Is there a way to test the power board or motherboard to see what's causing the issue?

Answer:

Now, regarding your issue with the Epson L1800 power supply and mainboard, it's quite rare for these components to burn out repeatedly unless there's an underlying electrical or grounding issue. Under normal circumstances, both the EPS-135 power supply and the L1800 mainboard are designed with built-in feedback safety circuits to prevent catastrophic failure. The mainboard only activates the 24V output once the printer has safely initialized-so if the power supply smoked or shorted, something likely bypassed or overloaded these protections.

Here are the most probable causes and how you can check each one:

1. Short Circuit Between Mainboard and Metal Frame:
   The most common reason for repeated burnouts is when part of the mainboard or its solder joints touches the metal casing, creating a direct short to ground. Even a tiny contact point-such as a misplaced screw, exposed trace, or pin-can cause instant current overload when powered on. Carefully inspect all mounting points and ensure the board is properly seated with insulating washers or spacers. Pay special attention to the corners and the area behind the USB port.

2. Faulty Grounding or Loose Screws:
   You mentioned suspecting a grounding issue earlier, and that's a very good observation. The Epson L1800's chassis relies on specific grounding screws to safely discharge static and balance voltage across circuits. If one of these is missing or loose, the printer's electrical system can behave unpredictably, leading to power surges or PSU overload. Make sure every grounding screw is tight, especially those securing the metal shielding to the printer's body.

3. Damaged Power Supply Circuit (EPS-135):
   Even though the EPS-135 unit includes overcurrent protection, repeated stress or reverse voltage can permanently damage it. You can test the PSU by disconnecting it from the printer and measuring output voltages using a multimeter.

   • The 5V rail should read approximately 5.1V when active.

   • The 24V rail should only activate once it receives a "Power On" signal from the mainboard.
     If you detect no output at all, the power supply's internal fuse or regulator IC has likely failed. Unfortunately, these are not easily replaceable components, so a new unit is usually required.

4. Testing the Mainboard:
   Testing a mainboard outside the printer requires care. You can perform a basic continuity check to ensure there are no shorts between the 24V and ground lines. If you see very low resistance (under 5 ohms), the mainboard might have shorted internally. Also, inspect the voltage regulators and MOSFETs on the board-burn marks or discolored components often indicate where the fault occurred.

5. Cable and Connector Orientation:
   Reversed or misaligned connectors between the PSU and mainboard can also cause immediate failure. Double-check the white JST and ribbon connectors to ensure they match the original orientation and are not offset by a pin. Even being off by one pin can lead to a blown fuse on the power board.

If both your mainboard and power supply now fail to power on, it's possible that one damaged the other during the last power cycle. A common sequence is:

• The mainboard shorted → causing PSU overload → PSU burned → new PSU installed → instantly overloaded again due to remaining short.

In that case, the best course of action is to test both components individually or replace them as a pair after confirming that the metal frame is fully insulated and grounded correctly.

Addressing printer issues like these can be a complex process, especially because many faults are hardware-related and require hands-on inspection. So, we're not able to provide remote troubleshooting or repair guidance. However, we offer an in-person evaluation and repair service through our local diagnostic facility (https://bchtechnologies.com/printer-repair-service). Due to high demand, we operate on a first-come, first-served basis, and it may take several weeks before your printer can be assessed. Our repair services can handle either full printer diagnostics or individual component repair, with clear step-by-step instructions for drop-off. Although our rates may not be the lowest, we always encourage users to pursue self-help first. You can find helpful videos by visiting our BCH Technologies YouTube channel (https://youtube.com/@bchtechnologies) and using the search icon near the "About" tab to look up model-specific guides.

Thank you again for your patience and support. Your detailed report helps not only us but also others who might encounter similar problems with their Epson L1800. We truly appreciate your engagement and your commitment to learning and repairing.