Kemppi Minarc Evo 180 MMA not powering on: soft-start, relay K1, R39, auxiliary SMPS troubleshooting

User question

Kemppi Minarc Evo 180 MMA.не включается

Artificial Intelligence Response

Direct answer to the question

• Most likely causes of “doesn’t turn on” in Kemppi Minarc Evo 180 MMA: loss of mains path (cable/switch/EMI filter/fuse), failure of soft‑start (pre‑charge resistor R39 and/or relay K1), or no auxiliary (standby) supply (+24 V/+15 V/+5 V). Less often: shorted IGBT/bridge, blown MOV/NTC, broken ribbon/UI.
• Start with: verify 230 V AC at the switch, continuity through filter and bridge, measure DC bus ≈310–340 V on the main capacitors, check that the relay closes after pre‑charge, and confirm presence of +24 V and logic rails. Inspect/measure R39 (open or drifted) and the K1 relay/driver.

Detailed problem analysis

• Symptoms taxonomy

  • Absolutely dead: no LEDs, no fan, no relay click → open mains path or dead auxiliary SMPS.
  • LED in switch on but no panel/fan → aux SMPS not starting or soft‑start not completing.
  • Brief power-up then shutdown → aux SMPS marginal, undervoltage, PTC/NTC, or protection latch.

• Safety and setup

  • Unplug, wait, and forcibly discharge the DC bus through a resistor. Work with an isolation transformer if you probe the primary. Use a series safety lamp (100–150 W at 230 V) or variac for first power-up after repairs.

• Fault tree and measurements (from simple to specific) 1) Input and protection

  • Mains cord, plug, strain relief: continuity L–switch, N–filter.
  • EMI filter/switch block: mechanical switch contacts burn/oxidize; neon lamp in the rocker can glow even if contacts are bad.
  • Primary fuse/thermal link (if fitted on the filter or PCB) and MOV: a surge can open the fuse or crack the MOV.
  • Expected: 220–240 V AC on the switch output under load.

  2) Rectifier and DC bus

  • Bridge rectifier: diode-test each leg; shorts blow fuses, opens make the unit “dead”.
  • Main capacitors: visual (bulge/leak), ESR if possible. With power applied, expect ≈325 V DC (230 V mains) on the bus after 0.3–1 s.
  • If bus stays <50 V DC: soft‑start path open (see R39/K1) or upstream open circuit.

  3) Soft‑start (pre‑charge) and relay K1

  • Purpose: limit inrush via pre‑charge element (often a wirewound resistor) until K1 shorts it.
  • R39 ambiguity by PCB revision:
    • On some boards R39 is the pre‑charge (soft‑start) resistor (typically 47–100 Ω, 5–10 W). If it is open, the DC bus barely charges and the unit looks dead; it often opens due to shorted IGBT/diode or because K1 never closes and R39 overheats.
    • On other boards R39 belongs to the UDC sensing divider (100 kΩ–1 MΩ range). If it goes open/drifts high, the controller “thinks” bus is low and blocks start (no relay/fan even though the bus may be at 325 V).
  • How to tell quickly:
    • If one end of R39 goes to mains/DC bus tracks and the other to the relay path → pre‑charge part.
    • If R39 chains with several high‑value resistors to a microcontroller/optocoupler node → UDC sense.
  • Relay K1:
    • Coil typically 24 V; DC resistance ~0.8–1.5 kΩ (indicative). Expect a distinct click shortly after power-on.
    • Check coil voltage while starting. If +24 V present but K1 does not pull in → bad relay or driver transistor. If no +24 V → aux SMPS issue.

  4) Auxiliary (standby) SMPS

  • Feeds +24 V (fan/relay), +15 V (gate drivers, varies), +5/3.3 V (logic/UI).
  • Typical failures: dried start‑up resistors (hundreds of kΩ–MΩ) gone high/open; failed PWM/Power‑SoC (VIPer/LNK/ICE series), shorted secondary Schottky, opto/TL431 loop faults, cracked solder joints.
  • Checkpoints:
    • VCC on PWM IC during start: should ramp to its UVLO threshold (e.g., 12–18 V depending on IC) then regulate; if it sawtooths between ~8–12 V without ever starting, suspect start‑up path or overload on secondary.
    • Secondary rails: +24 V ±10% at fan/relay; +5 V at UI; +15 V (if used) at driver ICs.

  5) Power stage sanity checks (only after rails OK)

  • IGBT/diode short test (collector–emitter body diode symmetry, no hard short).
  • Snubbers and gate drivers (no obvious burns). A hard short here usually takes R39/fuse with it.

  6) Interconnects and UI

  • Flat ribbon between control and UI: reseat/ohm all lines; intermittent open makes panel look dead though power is present.
  • Potentiometer/encoder contamination can simulate “no response” (less relevant if unit is fully dead).

• Expected numbers (for reference)

  • DC bus UDC: 300–340 V DC at idle (230 V mains).
  • +24 V: 23–26 V; fan typically 0.15–0.25 A.
  • +5 V logic: 4.8–5.2 V; +3.3 V if used.
  • Relay coil: ~24 V on pull‑in; resistance ≈1 kΩ (order of magnitude).
  • R39:
  • If pre‑charge: 47–100 Ω, 5–10 W, wirewound/ceramic; must not run hot in normal operation (bypassed by K1).
  • If divider: 100 kΩ–1 MΩ, 0.125–0.25 W; should measure within 1–2% if precision type (drift/open causes start inhibit).

Current information and trends

• Field reports on Minarc/Minarc Evo series highlight recurring soft‑start and UDC‑sense faults (including R39), relay contact oxidation (no bypass of pre‑charge), auxiliary SMPS start‑up resistor drift, and intermittent ribbon/UI issues.
• Some units show brief power-up then shut down due to PTC/NTC behavior or undervoltage from long extension leads/generators; VRD options reduce open‑circuit voltage but do not cause a completely dead unit.
• Practically, most “dead” cases are resolved by restoring the auxiliary SMPS (start‑up path) or fixing the R39/K1/UDC‑sense chain and cold joints.

Supporting explanations and details

• Why soft‑start matters: charging large bulk capacitors from 230 V without pre‑charge creates >50 A surges. The pre‑charge resistor/NTC limits this until K1 closes; if K1 fails, the resistor overheats and opens.
• Why UDC sensing blocks start: the controller will not enable the relay or PWM if it “sees” insufficient bus voltage to protect the power stage; an open high‑value divider leg lowers the sensed value.
• Why aux SMPS dominates “dead” symptoms: no +24 V means no fan/relay/UI; many designs bootstrap VCC via HV start‑up resistors that drift with age/heat.

Ethical and legal aspects

• Lethal voltages are present even when “off”; proceed only if you are qualified. Incorrect repairs may defeat protections (VRD, thermal), violating safety norms and potentially local regulations. Warranty/service seals, if any, should be respected.

Practical guidelines

• Tools: DMM with diode/ohms, isolation transformer or differential probes, series lamp/variac, ESR meter (optional), hot air + soldering tools.
• Minimal test sequence you can run in 20–30 minutes: 1) Visual + smell + reflow suspect joints (relay, high‑watt resistors, transformer pins). 2) Verify 230 V at switch output; continuity through filter. 3) Diode‑test the bridge; quick ohm check of IGBTs for shorts. 4) Power via series lamp: measure UDC on main caps.
  • If UDC ≈325 V but no life → aux SMPS rails missing; check start‑up resistors/PWM VCC.
  • If UDC low → measure R39, check K1 action and its driver; confirm the relay ever gets 24 V. 5) Measure +24 V and +5 V rails; if missing, lift secondary diodes/caps to isolate a short. 6) Measure R39 value and its network; replace if out of tolerance and fix root cause (relay or short).
• Parts and replacements:
  • Use equal or higher wattage/voltage ratings. For R39 (pre‑charge) select wirewound, 155–200 °C rated. For divider R39 use precision metal film, 1% (or per BOM).
  • Replace relays with identical coil voltage/contact ratings; clean PCB carbonization.

Potential challenges and how to overcome them

• No schematic: trace from landmarks (mains in, bridge, bulk caps, small flyback). Photograph both sides and annotate.
• Intermittent faults: thermal/cold joints; tap‑test and hot‑air warm to provoke.
• Hidden shorts on secondaries: lift one leg of each secondary rectifier and retest VCC.

Possible disclaimers or additional notes

• Component designations (e.g., “R39”) can vary by PCB revision; confirm on your board.
• If IGBT or driver stage is shorted, collateral damage to SMPS parts is common; expect multi‑component repair.
• Calibrations (OCV/current) may be needed after control board work.

Suggestions for further research

• Obtain the exact PCB photos/BOM or service notes for your serial/revision.
• Study typical VIPer/LNK/ICE start‑up and TL431/optocoupler feedback diagnostics to speed SMPS repair.
• Consider logging bus and +24 V rails on first start after repair (with series lamp) to confirm proper relay timing.

Brief summary

• For a “dead” Kemppi Minarc Evo 180, first prove mains path and bridge, then the DC bus. If UDC ≈325 V but no fan/LEDs, repair the auxiliary SMPS (start‑up resistors/PWM). If UDC is low and no relay click, diagnose soft‑start: check R39 (either pre‑charge or UDC‑sense depending on revision) and K1/driver. Inspect the ribbon/UI last. Replace failed parts with proper ratings and verify rails and relay timing on a series lamp before full-power tests.

If you can, please report:

• Does the switch LED or any panel LED light? Do you hear a relay click or fan twitch?
• UDC across the main capacitors right after power-on.
• Presence of +24 V and +5 V.
• Measured value and location of R39, and whether K1 coil ever gets ≈24 V.

With those four data points I can give you the next exact repair step.

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