Jump to content

Fuse Identification and Replacement

From RetroTechCollection
Glass, sand-filled and ceramic 5x20 mm fuses (Medvedev)
Three 5 × 20 mm cartridge fuses: a glass tube with its wire element visible, a glass tube filled with sand, and a sand-filled ceramic tube whose end cap is stamped H 250V (photo: Medvedev, CC BY-SA 3.0).

Fuse identification and replacement covers reading the markings and ratings of the fuses in vintage machines and their power supplies, testing whether a fuse has opened, finding out why it opened, and fitting a correct replacement. A fuse opens because something drew too much current, so a blown fuse is a symptom of a fault elsewhere.[1] Replacing the fuse is the whole repair only when the cause was outside the machine, as with Sega CD units that had been run from the wrong adaptor.[2] Otherwise, find and repair the fault first, or the new fuse will open as well.[3] The page covers glass and ceramic cartridge fuses, soldered sub-miniature (pico) fuses and resettable polymer PTC fuses.

When to use it

[edit | edit source]
  • The machine is completely dead, with no power light. The fuse in the power input is one of the first things to check, as on the SNES and the Atari 1040STF power supply.[1][4]
  • One function is dead while the rest of the machine works. Some machines fuse a port or a supply branch separately: on the Dreamcast one fuse feeds all four controller ports, on the Amiga 2000 a 4 A pico fuse feeds the mouse port, and on the VIC-20 a 1 A fuse protects the cassette motor and the user port's 9 V AC.[5][6][3]
  • A new fuse opens as soon as the power is applied. The fault is still present; go to Finding Short Circuits.[3]

If the fuse is good, follow the supply onward with Continuity Testing and Signal Tracing and check the rectifiers and regulator with Testing Diodes, Transistors and Regulators.

Safety

[edit | edit source]
  • Disconnect the power and any adaptor before removing a fuse or testing it on the meter's continuity range.[7]
  • The input side of a switch-mode supply is connected directly to the mains, and its filter capacitors hold 320 V DC or more when powered and for some time after unplugging, particularly when the supply has failed without blowing its fuse. Goldwasser warns that their stored energy can kill: discharge them and confirm the discharge with a meter before touching anything.[8]
  • For any live measurement on the mains side, use an isolation transformer. A Variac is not an isolation transformer.[8] On a machine with a CRT, follow the CRT Discharge Procedure as well.
  • Never bridge a fuse with wire, foil or a resistor, or twist its leads together. Doing so removes the protection, and Sega's Dreamcast service manual tells technicians to replace a fuse only with the same type, amperage and voltage, for protection against the risk of fire.[5]

Tools and materials

[edit | edit source]
  • A multimeter with a continuity range (it may share a dial position with resistance), and a voltage range for live checks on low-voltage boards.[7]
  • Good light and a magnifier. On a 5 × 20 mm fuse the IEC marking is on the end cap and is separate from the maker's part number.[9]
  • The machine's parts list or schematic, which gives the fuse's rating when the fuse is unmarked or unreadable.[2][6]
  • A replacement fuse of the same type, the same current rating and at least the same voltage rating.[10][5]
  • A soldering iron and desoldering tools for fuses soldered into the board; see Through-Hole Desoldering and Through-Hole Soldering.
  • For the first power-up: a bench supply with current limit for a low-voltage board,[11] or a series light bulb for a mains switch-mode supply. Goldwasser suggests starting with a 40 W bulb for small switch-mode supplies such as a VCR's or a laptop's, 100 W for small monitors and 150 to 200 W for large ones.[12]

Procedure

[edit | edit source]

Identify the fuse and its rating

[edit | edit source]
  1. Find the fuse on the schematic or in the parts list and note its reference, usually F followed by a number.
  2. Read the rating from the parts list. Where the documents disagree, fit the rating printed on your own board: for the Mitsumi SR98 supply in the Atari 1040STF, Atari's parts list gives a 250 V T1A fuse and Mitsumi's diagram gives 2 A.[4]
  3. Read the end-cap marking of a cartridge fuse. An IEC 60127 fuse is marked with its speed, rated current, breaking capacity and rated voltage, in that order; T2AL250V is a time-lag, 2 A, low-breaking-capacity, 250 V fuse.[9] The table below decodes each part.
  4. Take the rating of a pico or other soldered sub-miniature fuse from the parts list if the body is unmarked. Sega lists F1 in the Model 1 Sega CD as a 2.5 A Pico II radial fuse, part 514-5044.[2]
  5. Note which standard the fuse was made to. A UL Listed fuse must open within an hour at 135 % of its rating, while a fuse to IEC 60127-2 standard sheet 1 must not open at 150 %, so the two are different fuses even when the current rating is the same.[13]
IEC 60127 markings on a 5 × 20 mm fuse
Part of the marking Marking Meaning
Speed F Quick-acting[14]
Speed T Time-lag (time-delay or slow-blow): carries a short surge without opening[9][15]
Rated current 2A, 315mA The current the fuse carries continuously; values under 1 A are marked in mA, as in T315mAL250V[9]
Breaking capacity L Low; Schurter's glass-tube FSF and FST fuses are L types[14][15]
Breaking capacity E Enhanced[13]
Breaking capacity H High[13]
Rated voltage 250V The highest circuit voltage the fuse can safely be used in; standard ratings for small fuses are 32, 63, 125, 250 and 600 V[9][13]

The breaking capacity is the largest current the fuse can safely interrupt at its rated voltage. Littelfuse gives 35 A for some 250 V AC 5 × 20 mm fuses; Schurter rates its glass FSF fuses at 35 to 100 A and its FST fuses at 35 to 200 A, depending on the current rating.[13][14][15]

Test the fuse

[edit | edit source]
Blown 10 A 250 V glass fuse (TheresNothingOutThere)
A 10 A 250 V glass fuse whose wire element has melted apart near the middle of the tube (photo: TheresNothingOutThere, CC BY 4.0).
  1. Disconnect the power. On a mains switch-mode supply, discharge the filter capacitors and confirm with a meter that they are discharged.[8]
  2. Look at a glass fuse. Its element is visible through the tube, as in the photograph. A fuse that opened on a heavy short can be vaporised or burst; Goldwasser describes the main fuse of a switch-mode supply with shorted primary-side parts blowing instantly in this way.[16]
  3. Take a cartridge fuse out of its holder, or lift one lead of a soldered fuse, so that nothing else in the circuit is in parallel with it.[7]
  4. Set the meter to continuity and touch the probes to the two ends of the fuse. A good fuse shows continuity and the meter beeps; an open fuse gives no beep.[7]
  5. On a low-voltage board, a soldered fuse can also be checked in place with the power on: measure the supply voltage at each end of the fuse to ground. Voltage at one end only means the fuse is open.[1]
  6. Check the fuse holder. Clean the clips and squeeze them so that they grip the fuse; a loose fuse heats its holder, lowers the supply to the board and eventually blows.[3]

Find why it blew

[edit | edit source]
  1. With the fuse out, measure the resistance to ground from the load side of the fuse holder or pads. A reading close to zero ohms is a short that has to be found before a new fuse is fitted; see Finding Short Circuits.[2][5]
  2. Test the parts directly after the fuse with the meter's diode range. A protection diode across the supply that has failed short will blow a new fuse at once, as with D1 behind F1 in the Sega CD.[2] On the VIC-20, Carlsen checks the bridge rectifier first, then the regulator, then the chips on the board.[3] See Testing Diodes, Transistors and Regulators.
  3. In a switch-mode supply, check the primary side: the mains rectifier diodes, the filter capacitors, the MOVs and the switching transistor. Goldwasser gives these as the parts that short and blow the main fuse, and notes that with a current-limiting controller such as the UC3842 a blown fuse means the switching transistor, the controller or both have failed.[16] Atari's service manual says the same of the 1040ST supply: because it has overcurrent protection, a blown fuse probably means a catastrophic failure such as a shorted primary.[4]
  4. For a fuse that feeds a port, unplug everything from the port and measure from the port side of the fuse to ground.[5] On the VIC-20, Carlsen usually finds that a blown cassette and user port fuse was caused by connecting something with the power on.[3]
  5. Check the power adaptor's voltage and polarity against the machine's. One Sega CD with a blown F1 was found with a 17 V adaptor attached, and another owner suspected a centre-positive adaptor on a centre-negative input.[2]
  6. If no short or failed part is found, compare the machine's normal current with the fuse rating. Littelfuse recommends running a fuse at no more than 75 % of its rating at 25 °C, and says that a fuse runs hotter and has a shorter life as its current approaches its rating and as the surrounding temperature rises.[13]

Fit the replacement

[edit | edit source]
  1. Choose a fuse of the same type (normal, fast or time-lag), the same current rating and at least the same voltage rating.[10][5]
  2. Check that its breaking capacity meets or exceeds the original's. Littelfuse states that the interrupting rating must meet or exceed the maximum fault current of the circuit.[13]
  3. Fit a cartridge fuse into its clips and check that it is held firmly.[3]
  4. Solder a pico or pigtail fuse quickly and with no more heat than the joint needs. Most fuses contain soldered joints, and Littelfuse warns that excessive heat can reflow the solder inside the fuse and change its rating.[13]
  5. Power a low-voltage board for the first time from a current-limited bench supply, and measure the rails against the machine's own figures.[17]
  6. Power a mains switch-mode supply for the first time through a series light bulb. If the bulb lights at full brightness, a major fault is still present. If it lights brightly and then settles at reduced brightness, the supply is drawing a normal current. If it pulses, the supply is starting and shutting down, either because a fault remains or because the bulb is too small.[12]
Pre-arcing times of quick-acting (F) and time-lag (T) 5 × 20 mm fuses, 0.125 to 6.3 A, from Schurter's FSF and FST datasheets
Current Quick-acting F (FSF) Time-lag T (FST)
1.5 × rated Does not open within 60 min Does not open within 60 min
2.1 × rated Opens within 30 min Opens within 120 s
2.75 × rated 50 ms to 2 s 600 ms to 10 s
4 × rated 10 ms to 300 ms 150 ms to 3 s
10 × rated Within 20 ms 20 ms to 300 ms

At ten times its rating a quick-acting fuse opens within 20 ms, while a time-lag fuse of the same rating carries that current for at least 20 ms and up to 300 ms.[14][15] A time-lag fuse is made to carry temporary surges without blowing, such as the surge when a supply's capacitors charge at switch-on.[9][16]

Resettable PTC fuses

[edit | edit source]
Radial polymer PTC resettable fuses (Shaddack)
Four radial-lead polymer PTC resettable fuses of different sizes, with a centimetre scale (photo: Shaddack, public domain).

A polymer PTC (PPTC) resettable fuse heats up when the current through it is too high, and at a critical temperature its resistance rises sharply and cuts the current. Once the overload is removed and the power switched off, it cools and its resistance returns to its starting value.[18] Its ratings are a hold current, which it carries without tripping, and a trip current. Bourns' MF-R110, for example, holds 1.10 A and trips at 2.20 A at 23 °C, trips within 6.6 s at 5.5 A, and holds only 0.67 A at 70 °C.[19]

  1. Switch off, unplug whatever is connected to the protected circuit, and leave the device to cool.[18]
  2. Measure its resistance and compare it with the datasheet. The MF-R110 measures 0.10 to 0.18 Ω new and up to 0.27 Ω one hour after it has tripped.[19]
  3. Find the overload before switching on again, in the same way as for a fuse.[5]

Restorers have fitted PTC fuses in place of single-use fuses: a 1.6 A 60 V polyfuse for the SNES's 1.5 A fuse, and a 72 V 0.4 A part for the Dreamcast's controller-port fuse.[1][5] That is a modification, and the manufacturer's part remains a fuse.[1][5] On Bourns' MF-R series the trip current is twice the hold current and a device can take seconds to trip, where a quick-acting fuse at four times its rating opens within 300 ms, so a PTC lets more current flow for longer before it acts.[19][14]

Checking the repair

[edit | edit source]
  • The new fuse shows continuity on the meter before it is fitted.[2]
  • With the power on, the supply voltage is the same at both ends of the fuse.[1]
  • The rails after the fuse are within the machine's own figures.[17]
  • On a series light bulb, a mains supply lights the bulb brightly and then settles to reduced brightness.[12]
  • The fuse holder does not run hot; heat at the holder points to loose clips.[3]
  • The fuse survives switch-on and normal use. A fuse that opens again at once means the fault is still present.[3]

Common mistakes

[edit | edit source]
  • Fitting a fuse of a higher current rating, or a wire, foil or resistor in its place. The protection is lost. Sega's Dreamcast service manual gives the risk of fire as the reason for fitting only the same type, amperage and voltage,[5] and Goldwasser makes the same point about fusible resistors: a higher-rated part does not open at the same overload, which can damage other parts of the circuit and increase the risk of fire.[20]
  • Fitting a lower current rating to be on the safe side. Goldwasser points out that the main fuse of a switch-mode supply is sized for the switch-on surge and the supply's low power factor, so a smaller fuse makes the equipment no safer and may blow in normal use.[16]
  • Fitting a quick-acting fuse in place of a time-lag one. The switch-on surge that a time-lag fuse rides through can open a quick-acting fuse of the same rating.[14][15][16]
  • Fitting a fuse whose voltage rating is lower than the circuit's voltage. The voltage rating must be equal to or greater than the circuit voltage.[13]
  • Fitting fuse after fuse without looking for the cause. Each one opens, and a shorted part such as a protection diode or bridge rectifier stays in the circuit.[2][3]
  • Overheating a soldered fuse, which can reflow the solder inside it and change its rating.[13]
  • Treating a UL fuse and an IEC fuse of the same current rating as identical. They are tested to different overload limits.[13]
  • Testing a fuse in circuit on the continuity range. Another path through the circuit can make an open fuse read as good; isolate the fuse first.[7]

Worked examples on this wiki

[edit | edit source]
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Super Nintendo Entertainment System Power Circuit Repair, on this wiki: a fuse opens because something drew too much current; with the power on, adapter voltage at one end of F1 only means the fuse is open; F1 is a 1.5 A axial fuse in Nintendo's parts lists; Adam Wilson replaced it with a 1.6 A 60 V self-resetting polyfuse, a modification rather than Nintendo's part.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Sega CD (Model 1) Power Fuse Replacement, on this wiki, citing Sega's sub board parts list and schematic and two owners' repairs: F1 is a 2.5 A Pico II radial fuse, part 514-5044; two owners revived dead units by replacing F1, one found with a 17 V adaptor attached and one suspecting a centre-positive adaptor on the centre-negative input; a shorted D1 across the supply after F1 blows a new fuse; measure the supply line after F1 to ground before fitting a new fuse; check the new fuse for continuity before reassembly.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Commodore VIC-20 Power Supply and Regulator Repair, on this wiki, citing Ray Carlsen and Tynemouth Software: the fuse blows again as soon as a new one is fitted while a short remains; look for it in the bridge rectifier, then the regulator, then the chips; a loose fuse heats its holder, lowers the supply and eventually blows; the 1 A fuse for the cassette motor and user port 9 V AC is usually blown by connecting something with the power on.
  4. ↑ 4.0 4.1 4.2 Atari 1040STF Power Supply Repair, on this wiki, citing Atari's 520ST/1040ST Computer Field Service Manual (because the supply has overcurrent protection, a blown fuse probably means a catastrophic failure such as a shorted primary) and the SR98 parts list and schematics (250 V T1A in Atari's parts list, 2 A on Mitsumi's diagram).
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 Sega Dreamcast Controller Port Fuse Replacement, on this wiki, citing Sega's Service Manual: Dreamcast EU ("For protection against risk of fire, replace only with the same type, amperage and voltage fuse") and dreamcast.nu (do not bridge F1 or replace it with a resistor; a 72 V 0.4 A resettable PTC fuse fitted): F1 feeds all four controller ports; with nothing plugged in, measure from the port-side pad of F1 to ground before fitting a new part.
  6. ↑ 6.0 6.1 Amiga 2000 Troubleshooting Guide, on this wiki: F1, a 4 A pico fuse (Rev. 6 and later), feeds the mouse port; F3 and F4 (4 A pico) feed the drives.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Fluke Corporation, "How to Test Continuity with a Digital Multimeter": with the circuit de-energised, connect the leads across the component, isolated from other components in the circuit; the meter beeps on a complete path and does not beep on an open one; continuity mode is often combined with resistance; a good fuse should show continuity. Retrieved 25 October 2026.
  8. ↑ 8.0 8.1 8.2 Goldwasser, "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies", section "Safety": the input side is connected directly to the AC line with large electrolytic capacitors at 320 V DC or more when powered and for some time after unplugging; their energy can kill, so discharge and confirm before touching; use an isolation transformer for line-connected circuits; a Variac is not an isolation transformer. Retrieved 25 October 2026.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 DigiKey TechForum, "IEC Standards for 5x20mm Fuse Part Markings", 7 October 2024: the IEC marking, normally on the end cap, is not the maker's part number; T2AL250V decoded as T time-delay (slow-blow, handles temporary surges), 2 A, L low breaking capacity, 250 V; examples such as T315mAL250V. Retrieved 25 October 2026.
  10. ↑ 10.0 10.1 Goldwasser, "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies", substitution guidelines: "Fuses - same type (usually normal or fast blow), exact same current rating and at least equal voltage rating". Retrieved 25 October 2026.
  11. ↑ Recommended Tools, on this wiki: a bench power supply with current limit.
  12. ↑ 12.0 12.1 12.2 Goldwasser, "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies", section "The series light bulb trick": full brightness means a major fault is probably still present; initially bright then settling at reduced brightness is expected of normal operation; pulsating means the supply is shutting down on overcurrent or overvoltage, or the bulb is too small; suggested starting bulbs 40 W for VCR or laptop switch-mode supplies, 100 W for small monitors or TVs, 150 to 200 W for large monitors. Retrieved 25 October 2026.
  13. ↑ 13.00 13.01 13.02 13.03 13.04 13.05 13.06 13.07 13.08 13.09 13.10 Littelfuse, Inc., Fuseology Design Guide, 2022: a fuse's current rating is typically derated 25 % for operation at 25 °C and fuses should last indefinitely at no more than 75 % of rating; the hotter the ambient, the hotter the fuse runs and the shorter its life; the voltage rating must be equal to or greater than the circuit voltage, with standard ratings of 32, 63, 125, 250 and 600 V; the interrupting rating (breaking capacity) is the maximum current the fuse can safely interrupt at rated voltage, from 35 A for some 250 V AC 5 × 20 mm fuses, and must meet or exceed the circuit's maximum fault current; most fuses contain internal soldered joints that excessive soldering heat can reflow, changing the rating; IEC 60127-2 breaking capacity Low (standard sheets 2, 3, 4), Enhanced (6, 7, 8) and High (1, 5, 9, 10); UL Listed fuses must open within 1 hour at 135 % of rating, while IEC 60127-2 standard sheet 1 fuses must not open at 150 %, so a UL Listed fuse cannot meet IEC 60127. Retrieved 25 October 2026.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Schurter AG, FSF 5x20 Miniature Fuse, 5 x 20 mm, Quick-Acting F, L, 250 VAC datasheet, 8 March 2022: IEC 60127-2, L = low breaking capacity (glass tube), 0.032 to 10 A, breaking capacity 35 to 100 A; marked with rated current, rated voltage, characteristic, breaking capacity and certification marks; pre-arcing times for 0.125 to 6.3 A: 1.5 × In at least 60 min, 2.1 × In at most 30 min, 2.75 × In 50 ms to 2 s, 4 × In 10 ms to 300 ms, 10 × In at most 20 ms. PDF (copy hosted by CERN). Retrieved 25 October 2026.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 Schurter AG, FST 5x20 Miniature Fuse, 5 x 20 mm, Time-Lag T, L, 250 VAC datasheet, 4 January 2022: IEC 60127-2, L = low breaking capacity (glass tube), breaking capacity 35 to 200 A; pre-arcing times for 0.125 to 6.3 A: 1.5 × In at least 60 min, 2.1 × In at most 120 s, 2.75 × In 600 ms to 10 s, 4 × In 150 ms to 3 s, 10 × In 20 ms to 300 ms. PDF (copy hosted by CERN). Retrieved 25 October 2026.
  16. ↑ 16.0 16.1 16.2 16.3 16.4 Samuel M. Goldwasser, "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies", version 2.86, 28 January 2014, sections on common failures and "Why is the main fuse rating so big?": shorted primary-side rectifiers, filter capacitors, MOVs or switching transistor make the main fuse blow instantly (vaporise or explode); with controllers such as the UC3840 or UC3842 a blown fuse indicates failure of the switching transistor, the controller or both; the fuse is larger than the nameplate current because of the switch-on surge from charging the capacitors and the low power factor, and a smaller fuse gives no extra safety but may blow in normal use. Retrieved 25 October 2026.
  17. ↑ 17.0 17.1 Battery Explosion, Capacitor or Corrosion Damage, on this wiki, section "Checking the repair": power from a current-limited bench supply first and measure the rails against the machine's own figures.
  18. ↑ 18.0 18.1 DigiKey TechForum, "How does PPTC prevent overcurrent through resetting mechanism and typical application scenarios?", 11 June 2025: overcurrent heats the PPTC; at its critical temperature its resistance rises rapidly and limits the current; once the overcurrent is relieved and the applied voltage removed, it cools and its resistance returns to the initial value. Retrieved 25 October 2026.
  19. ↑ 19.0 19.1 19.2 Bourns, Inc., MF-R Series PTC Resettable Fuses datasheet: MF-R110 Vmax 30 V, Imax 40 A, Ihold 1.10 A, Itrip 2.20 A at 23 °C, initial resistance 0.10 to 0.18 Ω, one-hour post-trip resistance 0.27 Ω maximum, maximum time to trip 6.6 s at 5.5 A; thermal derating to 0.67 A hold and 1.34 A trip at 70 °C; trip current twice hold current across the series. PDF. Retrieved 25 October 2026.
  20. ↑ Goldwasser, "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies", section on fusible (flameproof) resistors: they serve as fuses and should not be replaced with higher wattage types unless the manufacturer allows it, because these would not blow at the same level of overload, possibly damaging other parts of the circuitry and increasing the risk of fire. Retrieved 25 October 2026.
Retrieved from "https://retrotechwiki.com/index.php?title=Fuse_Identification_and_Replacement&oldid=30990"