RCF ART 415A no audio, IRS20955S pin 14 only 3V, pin 10 -70V, +12V VCC rail fault

User question

Circuit is okay, but pin 14 is 3 v and pin 10 voltage -70 V and no audio output

Artificial Intelligence Response

Direct answer to the question

• Pin 10 = -70 V is normal for the negative power rail of the ART-4 series class-D module.
• Pin 14 at only 3 V is far too low; the IRS20955S needs ≈ 10 … 18 V between VCC (pin 14) and VSS (pin 10) to come out of undervoltage lock-out, generate gate drive and produce audio.
• Because VCC is missing, the driver is in UVLO, the output MOSFETs are kept off, and nothing reaches the loudspeaker. The fault is therefore in the +12 V low-side gate-drive supply (or the IC itself), not in the ±70 V main rails.

Detailed problem analysis

1. IC pin functions (IRS20955S, 14-lead SOIC, ref. Infineon DS-18Oct2010)
   • Pin 10 = VSS → negative rail / power ground (-70 V here).
   • Pin 14 = VCC → low-side gate-drive supply (10…18 V above VSS).
   • Pin 1 = VDD → logic supply (3.3 / 5 V referenced to system ground).
   • Pin 11/12 (VB/HO) and Pin 7/8 (VS/LO) form the high-/low-side outputs.

2. What the 3 V reading means
   • The internal UVLO threshold for VCC is ≈ 8.5 V (typ.) rising, 7.6 V falling.
   • At 3 V the IC stays latched OFF; HO and LO are tri-stated, so the class-D half-bridge never commutates → speaker relay/protection stays engaged → no sound.

3. Typical VCC generation path in the RCF ART415A module
   • +12 V is produced by a small offline auxiliary SMPS or by a linear/zener dropper from the +70 V rail.
   • It is filtered by 10 … 22 µF MLCC/ electrolytic directly across pin 14-to-10.
   • Series elements: 3R…10 R safety resistor, sometimes a miniature fuse or a BAV21/BAT54 diode for reverse isolation.

4. Probable failure modes
   a) Open series resistor / miniature fuse → only leakage reaches the IC (≈3 V).
   b) Shorted VCC decoupling capacitor (4.7 … 10 µF) → supply collapses.
   c) 12 V regulator (78L12 / TL431-based) open, short or no feed.
   d) IRS20955S itself shorting VCC to VSS internally (less common but possible after MOSFET shoot-through).

5. Corroborating checks
   • Measure VCC on the supply side of the series resistor: if you see 10-15 V there but only 3 V on pin 14, the IC is defective or the resistor is open.
   • If 10-15 V is absent altogether, trace back to the auxiliary regulator and its source from the +70 V rail.
   • Before applying power, ohm-check pin 14-to-10 with IC unplugged/board cold; a few kΩ is normal, tens of Ω indicates an internal short.

Current information and trends

Recent field repairs on the ART-4 Mk II (see 2023-2024 threads on elektroda.com and Audiofanzine) confirm that:

• 12 V rail failure is the most frequent cause of “both LEDs on – protected” with no audio.
• Infineon IRS20955S is still in production; replacements are readily available at Mouser, Digi-Key and Farnell.
• Many technicians now pre-emptively change the 12 V zener or TL431 and the 10 µF MLCC to increase long-term reliability.

Supporting explanations and details

• Analogy: Think of the ±70 V rails as the “engine”, and the 12 V VCC as the “ignition coil”. The engine can have full fuel pressure, but if the coil is dead the spark plugs never fire.
• Scope observation (with differential probe): with correct VCC you should see a 250-400 kHz PWM waveform of ~±10 V at LO and HO. Without VCC these lines stay flat.

Ethical and legal aspects

• ±70 V rails carry >100 Vpp and are lethal. Discharge the 470 µF mains caps and use an isolation transformer when probing.
• Local regulations (EN60065/62368-1) require functional testing of protection circuits after repair.

Practical guidelines

1. Safety first: isolate, discharge, wear eye protection.
2. Confirm rails: ±70 V present, +5 V present, +12 V missing at IC.
3. Locate VCC feed: follow PCB trace from pin 14 back to regulator; test components out of circuit if necessary.
4. Replace faulty parts: use 105 °C, low-ESR capacitors; if IC shorted, replace IRS20955S and all four MOSFETs as a set.
5. Power-up with series bulb or variac, monitor VCC: it should rise briskly above 10 V; audio should return and protection LED go off.
6. Final tests: THD, noise floor, thermal run-up, DSP presets, acoustic sweep.

Possible disclaimers or additional notes

• If the ADAU1701 DSP firmware is corrupted, audio can still be muted even after VCC is restored, though the protection LED will usually stay off.
• A blown MOSFET often takes the driver with it; changing only the IC may result in repeat failure.

Suggestions for further research

• Service manual of ART 415A (RCF p/n 10531184) for exact PSU schematic.
• Infineon app-note AN-1135: “Optimising auxiliary supplies for IRS2095x drivers.”
• Murata white-paper on MLCC cracking in class-D gate-drive rails.

Brief summary

The −70 V you see on pin 10 is expected, but the 3 V on pin 14 is not.
The IRS20955S needs ≈12 V between VCC (pin 14) and VSS (pin 10); since it only gets 3 V it remains in undervoltage lock-out, the output stage never switches, and the speaker stays silent.
Trace and repair the +12 V VCC rail (series resistor, regulator, decoupling cap) or replace the driver IC if it is shorted. Once VCC is restored, the amplifier should resume normal operation.

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