Epson L1800 Transistor Failures on Motherboard - Causes and Fixes

Question:
I purchased a set of transistors for an Epson L1800 motherboard because transistors 2 and 4 were bad on a used unit I bought. I swapped in another board from a different printer and got it working, though the printhead wasn't printing well. The next morning, the printer was off even though the switch was on. After cycling the power, the heater panel lit up, but the printer wouldn't turn on. This time, transistors 1 and 3 had failed instead of 2 and 4. The power supply's green LED near the 3-wire plug was dim, but it brightened when unplugged from the motherboard.

I then replaced 2 and 4 on the original motherboard, reinstalled it with a new printhead, and the printer still wouldn't turn on-now transistors 1 and 3 are bad. On another board, the printer worked temporarily with the old bad printhead but blew transistors 1 and 3 overnight. Is this a different issue than before? Do transistors 1 and 3 control something different than 2 and 4?

Answer:

1. Roles of the Transistors (1 & 3 vs. 2 & 4)

• The Epson L1800 motherboard uses paired MOSFET transistors to drive the printhead nozzles.

• Transistors 1 and 3 generally drive one set of nozzle groups (for certain color channels), while 2 and 4 drive another. This explains why sometimes 2/4 blow and other times 1/3 fail-it depends on which head circuits are shorting.

• If the printhead has an internal short, it can overload the driving pair of transistors connected to that part of the nozzle array.

2. Why 1 & 3 Failed After 2 & 4

• Since the original board had 2 and 4 blown, it suggests the head was already compromised in that section.

• When you swapped boards and continued running with the bad printhead, the head's other failing circuits likely stressed 1 and 3, leading to their failure overnight.

• Essentially, the bad head is shifting the stress to different transistor pairs until all are damaged.

3. Dim LED on Power Supply

• The dim green LED on the power supply that brightens when disconnected suggests the motherboard is pulling excessive current, which is consistent with shorted transistors or a shorted printhead drawing too much load.

• This reinforces that the issue is not just random transistor failure-it's a head-related overload feeding back into the board.

4. Risk of Installing New Transistors Without Fixing the Head

• Replacing transistors alone will not solve the issue if the printhead is still faulty. A bad head will quickly destroy new transistors, sometimes within minutes or overnight, as you've experienced.

• This is why replacing both the printhead and repairing/replacing the board is necessary at the same time.

5. What to Do Next

• Replace the printhead first with a confirmed good unit. Running with a bad head will only continue blowing transistors.

• Only after installing the new head should you replace the damaged transistors (1/3 or 2/4 as needed).

• Test the board with the new head and a multimeter check before applying power to ensure there are no immediate shorts.

• If multiple boards have been stressed already, you may find that repair isn't cost-effective compared to sourcing a fresh motherboard and head together.

Addressing motherboard and printhead issues can be complicated because they require board-level soldering, component testing, and precise matching of parts. For this reason, we cannot provide remote troubleshooting or direct repair support. However, we do offer an in-person evaluation and repair service (https://bchtechnologies.com/printer-repair-service). Our service operates on a first-come, first-served basis, and due to high demand, it may take several weeks before we can process your printer. We repair both full printers and individual components, though we acknowledge that our pricing may not be the most economical. Many customers prefer to begin with self-help, and a valuable resource is our YouTube channel (https://youtube.com/@bchtechnologies). Use the search icon next to the "About" tab to find videos on transistor replacement, motherboard testing, and Epson printhead troubleshooting.

Thank you again for your detailed question and for supporting BCH Technologies. Your persistence in diagnosing this issue helps us continue building resources that support the printer repair community.