Fuse blowing and MOSFET short in Jamo Sub 200 plate amp – sourcing replacements and repair tips

User question

Fuse burning and mosfit shortage

Artificial Intelligence Response

Direct answer to the question

• Most likely you have a blown mains fuse because one or more MOSFETs in the power stage are shorted (“MOSFET short,” often misheard as “mosfit shortage”). Replace-the-fuse-and-try-again will only repeat the failure unless the root cause is found and fixed.
• If you also meant difficulty finding replacement MOSFETs (“part shortage”), select verified substitutes by matching key parameters (VDS, ID, SOA, RDS(on), Qg, package, and gate-drive level) and buy only from authorized distributors to avoid counterfeits.

Key points

• Isolate whether the short is in the power supply or the audio output stage before installing new parts.
• Check/replace the gate driver, gate resistors, snubbers, current-sense parts, and any “brown glue” leakage paths; these often fail together with the MOSFETs.
• Power up the repaired unit through a series-lamp (dim‑bulb) limiter or current‑limited source, not straight to the wall.

Detailed problem analysis

• Typical failure chain

  • Output or SMPS MOSFET shorts D–S → mains fuse opens.
  • Short often propagates backward: gate driver IC, gate resistors, zener clamps, and current‑sense resistors get damaged or go open.
  • Secondary faults that keep blowing fuses after partial repair: shorted bridge rectifier diodes, failed NTC inrush limiter, dried/bulged electrolytics (primary and secondary), and conductive aging glue creating leakage/shorts across high‑impedance nodes.

• Decide where the fault lives (linear vs SMPS, supply vs amplifier)

  1. With power disconnected, measure resistance:
     • Across the DC rails after the rectifier/bulk caps: a few kΩ is typical; near‑0Ω indicates a supply‑side short.
     • Across the speaker output to ground (with speaker disconnected): near‑0Ω suggests a shorted output stage.
  2. If the supply is a switch‑mode (common in plate amps):
     • Primary suspects: primary MOSFET(s), bridge rectifier, bulk capacitor, startup resistor, PWM/driver IC, optocoupler/TL431, snubbers/RC clamps, and the inrush NTC.
  3. If the amplifier is Class‑D (also common):
     • Suspects: half‑bridge MOSFET pair, gate driver (e.g., IRS/IR211x/IRS2092 family), bootstrap diode/capacitor, dead‑time network, gate resistors, and output LC filter components.
  4. If the amplifier is Class‑AB (some models):
     • Suspects: output devices (or MOSFET source resistors / BJT emitter resistors), driver pair, Vbe multiplier/thermal sensor, and feedback network.

• Component‑level checks (power OFF; desolder to confirm when in doubt)

  • MOSFETs: D–S < 1 Ω → shorted; G–S should not read a short; any punched‑through gate means replacement.
  • Bridge rectifier: diode‑test each leg; any short/open → replace.
  • Gate resistors (typically 5–100 Ω): should be within tolerance; open or charred → replace.
  • Gate clamps: zeners (typically 12–18 V) should not be shorted/open.
  • Current‑sense resistors (0.05–0.5 Ω): check for opens/shifts; replace in kind and wattage.
  • Electrolytics: check capacitance/ESR; replace bulged or high‑ESR parts; use 105°C, low‑ESR types.
  • “Brown/yellow glue”: scrape off any discolored glue around hot parts; it turns conductive/corrosive with age and causes phantom shorts/leakage.

• Power‑up and verification strategy

  1. Remove shorted MOSFETs and any clearly failed semis; leave the rest in place.
  2. Power the board via a series 60–100 W incandescent lamp (on 120 V mains) or a current‑limited bench supply. A lamp that goes bright and stays bright indicates a remaining short.
  3. On SMPS: confirm auxiliary VCC (typically 10–18 V) to the PWM/driver IC and that it starts/stops cleanly without the power MOSFETs installed.
  4. On Class‑D: with MOSFETs still out, scope the driver outputs (through 1–2 kΩ probes) for balanced, complementary gate drive and proper dead‑time; fix driver issues before fitting new MOSFETs.
  5. Fit new semiconductors, renew thermal pads/insulators, apply fresh thermal compound, verify no continuity from drain/tab to heatsink where isolation is required, then re‑test on the series lamp.
  6. Only after idle behavior is normal (lamp flashes then dims, rails in‑spec, no DC on output) should you return to direct mains and reconnect the speaker.

Current information and trends

• Aging adhesives and dried capacitors remain a leading field failure in plate amps from the same era as the Jamo Sub 200.
• Counterfeit power semiconductors are common in gray channels; failures at first power‑up or under modest load are typical symptoms.
• If you truly face a procurement shortage, many mainstream 60–200 V TO‑220/TO‑247 audio/SMPS MOSFETs have multiple second‑source equivalents; prioritize electrical fit over brand.

Supporting explanations and details

• Why fuses keep burning: A shorted MOSFET looks like a copper bar across the supply; replacing only the fuse recreates the fault current instantly.
• Why drivers must be checked: A damaged driver can bias both MOSFETs on (shoot‑through), instantly destroying fresh parts and the fuse.
• Series lamp rationale: The lamp’s filament adds a large, self‑heating resistance that limits fault current and gives a clear visual indicator of shorts while allowing partial operation for measurements.

Ethical and legal aspects

• Safety first: You are working with mains voltages and high‑energy capacitors. Use an isolation transformer, discharge capacitors, and follow lockout/tagout practices.
• Use only the specified fuse type and rating printed on the product/PCB; upsizing a fuse defeats safety certification and risks fire.
• Respect ESD handling for MOSFETs and drivers to avoid latent damage.

Practical guidelines

• Minimum toolset: DMM with diode/ohms, ESR meter (or LCR), oscilloscope, isolation transformer, series‑lamp limiter, hot air + soldering station.
• Replace in sets: Output MOSFET pairs, complementary drivers, and low‑value sense resistors should be replaced together to maintain balance.
• Preventive upgrades: Replace suspect electrolytics near hot zones; clean/replace glue; reflow dull or cracked solder joints; verify heatsink mounting pressure and insulating hardware.

Possible disclaimers or additional notes

• Without the exact model, schematic, and part numbers, values here are typical; always follow the device’s silk/fuse label and component markings.
• Some designs won’t tolerate being powered with output MOSFETs removed unless the driver is disabled; if unsure, lift the driver’s enable pin or remove the driver IC during PSU bring‑up.

Suggestions for further research

• Identify the amplifier topology (Class‑D vs Class‑AB) and gate driver part number; obtain the driver datasheet to verify expected gate‑drive timing and VCC.
• If a schematic is unavailable, photograph both sides of the PCB and trace critical nets (gate drive, sense, feedback) to build a block diagram before replacing parts.
• If replacements are scarce, search by parametric filters: VDS ≥ original, ID ≥ original, RDS(on) ≤ original at the available gate voltage, equal or lower Qg compatible with your driver.

Brief summary

• Repeated fuse blows almost always mean a hard short, commonly a shorted MOSFET; do not power the unit again until the short is located and fixed.
• Check the entire drive/protection chain and surrounding passive parts, remove conductive aging glue, and bring the unit up on a current‑limited source.
• For part sourcing, match electrical parameters and buy from authorized channels; many safe substitutes exist even if the exact MOSFET is hard to find.

If you can share the exact model, fuse value/location, MOSFET part numbers, and whether it’s SMPS/Class‑D or linear/Class‑AB, I’ll map out a part‑for‑part replacement list and test points for your board.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.