Epson L1800 Inksonic Mod Power Supply S-75-24 LED Not Lit: What It Means and How to Test with a Multimeter

Question: I replaced both power supply parts on my Inksonic-mod Epson L1800, but I'm still getting the same error code. The S-75-24 power supply arrived with slight case damage, and its green LED still doesn't light up-just like the original. Does that definitively mean the new S-75-24 is defective? Is there any testing I can do before I undo everything again, and does BCH sell an equivalent replacement that will mount properly?

Answer: 

Regarding the Inksonic-mod Epson L1800 power system: you're asking the right question. A green LED on a metal-cased supply like the S-75-24 is often a quick indicator of "the supply has power and is running," but an unlit LED does not always definitively prove the unit is defective. There are several realistic scenarios:

• The power supply is not receiving AC input (bad cord, switch, fuse, inlet, wiring, or no AC reaching it).

• The supply is receiving AC but is not turning on because of a protection state (short circuit on the DC side, overload, wiring error).

• The LED itself is damaged or disconnected (less common, but it happens-especially if the unit arrived with physical impact).

• The supply is functioning but the LED is very dim or obstructed (rare, but worth ruling out).

That said, if both the original and the replacement show the same symptom (LED not lit) in the same installation, the odds increase that the problem may be upstream (AC input) or downstream (DC short/overload) rather than "two bad supplies in a row."

The best next step is exactly what you mentioned in your proposed answer: verify output with a multimeter. This gives you a clear yes/no without guessing.

1) Safety first (important)

Testing power supplies involves live AC voltage and exposed terminals. If you're not comfortable doing live electrical testing, stop here and avoid probing while powered.

Basic precautions:

• Use a multimeter with intact probes.

• Keep one hand away from the unit when measuring live power (reduces risk path through the body).

• Avoid loose metal jewelry.

• Make sure probes can't slip and short adjacent terminals.

2) Confirm the power supply is receiving AC input

Even before checking the 24V output, confirm the unit is getting the AC it needs.

A) Visual checks

• Make sure the AC input terminals are wired correctly (Line/Neutral/Ground as labeled).

• Look for a blown input fuse (some units have an external fuse holder, some internal).

• Check that any power switch (if present) is on.

• Look for loose spade connectors or broken strands under the terminal clamp.

B) Measure AC voltage at the input (while powered on)

1. Set multimeter to AC volts (VAC).

2. Carefully probe the L and N input terminals (or equivalent).

3. You should see your wall voltage (often ~110-120VAC in the US).

Interpretation:

• No AC at the supply input: the supply cannot light the LED or produce output. That points to wiring, switch, inlet, cable, or upstream power distribution.

• AC present but LED still off: now it's time to check DC output and load conditions.

3) Check the DC output directly (the key test)

The S-75-24 is typically a 24V DC supply (exact labeling can vary). Most have V+ and V- output terminals.

A) Test "at the supply" with the printer connected (quick check)

1. Set meter to DC volts (VDC).

2. Probe V+ to V- on the supply output.

3. A healthy 24V supply should read around 24V (often adjustable slightly, like 23-25V depending on trim pot).

Interpretation:

• ~24V present: the supply is likely functioning. The LED being off could be LED failure or an unusual wiring scenario. The remaining error code may be elsewhere (mainboard, power distribution, controller, or the second PSU stage).

• 0V or very low voltage: either the PSU is dead, or it is shutting down due to short/overload.

B) Test with the load disconnected (most definitive)

If you get 0V/low voltage, do this next:

1. Power OFF and unplug.

2. Disconnect the DC output wires from V+ and V- (label them so you can reconnect correctly).

3. Power ON (carefully, with DC outputs not touching anything).

4. Measure V+ to V- again.

Interpretation:

• 24V appears with the load disconnected: the supply is probably okay, and something downstream is pulling it down (short circuit, incorrect wiring, reversed polarity, failed board, failed driver, etc.).

• Still 0V with no load: the supply is likely defective or not truly receiving AC input (recheck AC at L/N again to be sure).

4) Check for short circuit or overload on the printer side

If the PSU works with no load but collapses with the printer connected, you likely have a downstream issue.

A) Resistance test (powered off)

1. Unplug everything.

2. Set meter to resistance (ohms) or continuity mode.

3. Measure across the printer-side DC leads (the wires that normally connect to V+ and V-).

Interpretation:

• Very low resistance / continuity can indicate a short or heavy load.

• Note: Some boards have capacitors that can show low resistance briefly then rise; you're looking for a persistent near-short.

B) Common downstream causes in modded setups

With an Inksonic mod, the most common culprits when a 24V supply won't stay up include:

• Miswired polarity (V+ and V- swapped)

• A pinched cable harness or damaged insulation causing a short to chassis

• A failed DC-DC converter board (if the system steps voltage down/up for certain subsystems)

• A failed control board or driver board pulling too much current

5) About the case damage and LED: does that alone prove it's defective?

Not definitively.

• Minor denting of the metal case often doesn't affect functionality unless it crushed internal components or caused a PCB crack.

• The LED not lighting can be a strong clue, but it's not the final verdict unless confirmed by the voltage tests above.

The multimeter checks will give you a real answer in minutes:

• AC present + DC absent with no load → likely bad PSU

• DC present unloaded, absent loaded → downstream short/overload

• DC present loaded → PSU likely fine; error code probably comes from elsewhere

6) Does BCH sell an equivalent S-75-24 replacement? What about screw-hole mounting?

Because the S-75-24 style power supplies are generic/form-factor based, equivalents exist, but the mounting hole pattern and terminal layout can vary by manufacturer and revision.

If you're shopping for a compatible equivalent, focus on:

• Output: 24V DC (match the exact output voltage required)

• Wattage/current rating: S-75-24 is commonly around 75W class (confirm your label), but you can safely use a higher wattage unit as long as voltage matches

• Input voltage range: 110/220 compatibility if needed

• Physical dimensions and mounting hole pattern (critical for drop-in fit)

• Terminal style and placement (so wires reach without strain)

If your primary need is mounting hole match, the best approach is to use the specs from the label plus a quick measurement of:

• length/width/height

• hole-to-hole distances (center to center)

That will prevent buying something "electrically correct" but mechanically incompatible.

Addressing printer issues can be a complicated affair due to the hands-on nature of the problems, especially when power and electrical measurements are involved. Because of that, we're not able to provide remote troubleshooting, suggestions, or support for printer repairs. We do offer an in-person evaluation and repair service via our local diagnostic facility: BCH Technologies printer repair service (https://bchtechnologies.com/printer-repair-service). Given the high demand, we operate on a first-come, first-served basis, so it may take a few weeks before we can get your printer scheduled for drop-off. Our services are structured to repair either a whole printer or specific parts, with clear instructions on how to proceed. However, we acknowledge that our rates aren't the most economical. Thus, we highly recommend self-help via online research first. You can start by checking out YouTube or visiting BCH Technologies YouTube channel (https://youtube.com/@bchtechnologies). Look for specific videos using the search icon next to "About" on the right-hand side of the menu bar. I receive dozens of queries every day asking about videos for specific topics. Having created videos over the past nine years, it's challenging to remember every single one. Therefore, using YouTube's search function would be most efficient-and YouTube might also suggest relevant videos from other channels that could assist you.

Thank you again for reaching out, and I appreciate your patience. I hope the multimeter steps above help you confirm whether the S-75-24 itself is truly bad or whether something downstream is preventing it from turning on.