ET-8550 Mainboard Fuse Blowing and 31006 Error: How to Find the Short on the High-Voltage Rail
- By Ellen Joy
- On Dec 05, 2025
- Comment 0
When an Epson ET-8550 repeatedly blows a fuse the moment you power it on-and then throws a 31006 error-you're not dealing with a "random" failure. In most cases, the board is protecting itself from a short circuit or power leak on a critical power rail. The good news is that you can approach this problem logically, confirm the fault with basic tools, and narrow the failure down to a small set of components.
Below, we walk through a practical diagnostic flow used in real-world board troubleshooting. We also have a video below to show how it's done, so you can follow the process visually and build confidence as you go.
What "Fuse Blows on Power-Up" Usually Means on the ET-8550
A fuse that burns immediately on startup is a strong sign of a dead short somewhere on the mainboard. Instead of allowing excessive current to cook traces, chips, or the power supply, the fuse sacrifices itself. That's a good thing-because it limits collateral damage.
However, the fuse is not the root cause. Therefore, replacing the fuse repeatedly without finding the short is like resetting a breaker without fixing the wiring. The printer might power briefly, but it will fail again and may damage additional parts.
On the ET-8550, a common trouble spot is the high-voltage printing rail, often associated with a ~42V line used to drive printhead-related circuitry. When that rail becomes shorted, the fuse blows quickly-sometimes even with the printhead disconnected-because the short is on the board itself.
Tools You Need (and Why They Matter)
You can diagnose most ET-8550 fuse-blowing faults with a small set of tools:
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Digital multimeter with continuity ("beep") mode
This mode helps you quickly detect a short to ground. -
Good grounding point
Any large metal shield, USB port housing, or large screw pad on the board often works. -
Variable DC bench power supply (recommended)
This lets you inject a controlled voltage and current into the suspected rail. -
Thermal method to "see" the short (very helpful)
An infrared/thermal camera works best, but you can also use careful fingertip heat checking in some cases (more on safety below).
This isn't about fancy equipment for its own sake. Instead, each tool reduces guesswork and prevents you from randomly removing parts.
How Continuity (Beep Mode) Helps You Find a Short
Continuity mode beeps when two points are electrically connected-either directly or through a very low resistance path. That makes it ideal for detecting a rail that is shorted to ground.
Here's the logic:
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Set the meter to continuity (beep) mode.
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Put the black probe on ground (a large metal area).
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Now probe the positive side of capacitors on the suspected rail.
What you should expect
Capacitors typically have:
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One side tied to ground (negative)
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One side tied to the power rail (positive)
So, if you touch the ground side, you may get continuity (that's normal). If you touch the rail side, you may get:
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A very short chirp as the capacitor charges (often normal), then silence.
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No beep once charged (normal).
What indicates a problem
If the "positive/rail" side gives a strong, continuous beep, that often indicates the rail is shorted to ground. Consequently, the fuse blows as soon as power is applied.
This quick test can separate:
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A normal rail (no sustained beep on the rail side)
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A shorted rail (sustained beep on the rail side)
Why the 42V Rail Matters (and Where It Goes)
On ET-8550 boards, the high-voltage rail isn't just "some power line." It typically travels across the board and helps feed the printhead drive section. That section commonly includes:
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Driver circuitry near the printhead connectors
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MOSFETs that switch and regulate higher power
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Supporting capacitors for voltage smoothing and stability
Because the rail feeds downstream components, a short can occur at multiple points. Therefore, once continuity testing suggests the high-voltage rail is shorted, you need a method to localize which component is actually leaking.
The Fastest Way to Localize the Leak: Inject Power and Watch What Heats Up
Continuity tells you a short exists. It doesn't tell you where. That's where a bench supply and heat detection shine.
The idea
A shorted component will convert electrical energy into heat faster than healthy components. If you inject low, controlled power into the shorted rail, the "bad" area becomes the first obvious hotspot.
A practical workflow
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Solder a wire to the positive side of a capacitor connected to the shorted rail (choose a capacitor on the high-voltage rail, not a random small one).
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Connect bench supply negative to board ground.
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Connect bench supply positive to your soldered rail wire.
Safety rules (do not skip)
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Focus on current, not voltage.
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Keep current low-stay under 1 amp. Many techs use around 0.5-0.7A to start.
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Increase slowly.
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If you go too high, more components warm up and the hotspot becomes ambiguous. Worse, you can damage the board further.
Then, with a thermal camera, you can quickly spot the "lamp-like" bright area. Without a thermal camera, you must be extremely cautious-because components may get hot enough to burn skin.
Common Culprits: MOSFET Pair and Associated Driver Chip
In many ET-8550 fuse-blowing cases on the high-voltage rail, the short often traces back to a pair of power MOSFETs in the printhead power section.
Why MOSFETs fail this way
MOSFETs handle switching loads and can short internally between drain/source when overstressed. Once shorted, they effectively connect the rail to ground (directly or through a low resistance path). As a result, the fuse blows immediately.
Why replacing one may not be enough
Even if only one MOSFET appears to be the root cause, its "partner" may have been stressed or partially degraded. Therefore, many repair workflows replace the MOSFETs as a pair to reduce the chance of a repeat failure.
The related driver IC risk
When MOSFETs blow, the upstream/downstream driver chip that commands them can also be damaged. So, if you replace MOSFETs and the rail still reads short-or the fuse still blows-then the driver IC may also need replacement.
In repair parts catalogs, these parts may be searchable by their board markings. For example, MOSFETs might be identified by a short code on the package, while the driver IC has its own marking. BCH Technologies commonly stock components and kits that bundle these related parts for this exact repair scenario.
Installation Notes That Prevent Repeat Failures
Once you find the hot component(s) and confirm they're the cause, careful installation matters just as much as diagnosis.
Watch component orientation
Some MOSFET packages are directional, and orientation matters. Typically:
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Pads correspond to drain/source/gate relationships
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The physical alignment must match the original placement
Therefore, remove and replace one at a time when possible, or take clear photos before removal. This reduces the risk of installing the MOSFET backwards.
Re-check fuses the smart way
It's easy to forget fuses during reassembly. Instead, build a habit:
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Verify F1 and F2 continuity before powering anything.
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Then verify again after soldering (cold joints happen).
Re-test the rail before full power
Before reconnecting the printer supply:
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Re-run the original continuity checks on the rail capacitors.
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Confirm you no longer get sustained beeps on the positive side of the high-voltage capacitors.
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Also check downstream capacitors tied to the same rail.
If the short disappears, you're in a much safer position to proceed.
Understanding "Bad Capacitor Readings" During a Short
Capacitors often get blamed because they are easy test points. However, a capacitor can read "bad" in-circuit when something else is shorting the rail.
Here's why:
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If the rail is shorted, the capacitor's positive side may appear shorted to ground.
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That does not automatically mean the capacitor failed.
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It may simply be connected to a rail that is shorted elsewhere-such as through a MOSFET.
So, if you remove a capacitor to test it and it measures fine out-of-circuit, that's not a waste. Instead, it confirms the short originated in another component on the same net.
Repair Options: Ship-In Board Service or DIY Parts
Not everyone enjoys board-level troubleshooting-and that's completely reasonable. Some people want the printer to work, not become an electronics technician overnight.
If you prefer help, BCH Technologies offers repair and parts support through their website. Many customers either send:
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the mainboard alone,
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the full printer,
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or specific assemblies depending on the fault and comfort level.
On the other hand, if you enjoy DIY repairs, sourcing the correct MOSFET pair and any associated driver IC can speed up the repair dramatically-especially when you match the part codes printed on the packages.
##Finishing Checks Before Calling the Board "Fixed"
After replacing the suspected parts and confirming the short is gone:
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Verify fuse continuity one final time.
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Perform a cautious power-on test (ideally through a protected setup).
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If you have access to advanced tests like low-voltage board checks or CPU checks, those can add confidence-especially on boards that suffered repeated fuse events.
Most importantly, do not rush. A board that stops blowing fuses but still has a partially damaged driver stage can fail again later. Careful verification reduces that risk.
Wrap-Up: A Reliable Path from Symptom to Root Cause
When an ET-8550 mainboard burns a fuse on startup and throws a 31006 error, you can solve it with a repeatable process. First, confirm whether the high-voltage rail is shorted using continuity checks at key capacitors. Next, localize the short by injecting controlled current and identifying the hotspot. Then replace the truly failed components-often MOSFETs and sometimes the related driver chip-followed by careful fuse checks and a final rail verification. With that approach, you move from "mystery failure" to a confident, professional repair.