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• In ≈ 70–80 % of Ui24R “dead-channel” cases the fault is confined to two sacrificial parts on the input PCB:
– the tiny 6 Ω (±1 Ω) series “fuse” resistors in line with pins 2 & 3 of the XLR;
– the first dual low-noise op-amp (most units use NJM4580, some early lots use NE5532).
• Replacing those resistors and, if necessary, the op-amp almost always restores the channel.
• If the spike was large enough, also check / replace the 6.81 kΩ phantom-feed resistors and the SOT-23 protection/TVS diodes.
• Very rarely (multiple channels blown or board discoloured) the whole preamp board is swapped.
Failure mechanism
a. Hot-plugging with +48 V engaged means the XLR shields first; the momentary short drives ≥ 150 mA through the protection network.
b. The surge heats the 6 Ω SMD resistor in <10 ms; it opens like a fuse.
c. If the resistor survives (or is bypassed by an XLR that makes pin contact in the wrong order) the spike is forced into the input pins of the NJM4580; its absolute-maximum ratings are ±15 V. A +48 V transient therefore punches the ESD cells and latches one or both input transistors → audible hiss / DC on the channel or total mute.
d. Prolonged operation in a hot rack (> 45 °C) raises the PCB temperature 20–25 K above ambient; electrolytics dry and the same 6 Ω parts drift or crack.
Circuit topology of a Ui24R channel
XLR → 6.81 kΩ (phantom feed) ↘
6 Ω → ESD/TVS → \
∑(NJM4580) → PGA/control IC → ADC
Diagnostic cues
• Open 6 Ω resistor → meter shows ∞ Ω from XLR to op-amp pin, no pre-fader metering.
• Shorted op-amp → ~Vcc/2 DC on its outputs, channel meters at clip with nothing plugged.
• Burnt 6.81 kΩ → phantom voltage measures <20 V on that channel.
Heat-related collateral damage
• SMD electrolytics on the preamp PCB rated 85 °C; after 2–3 years in hot racks ESR triples → rise in noise floor.
• The main DSP heat-spreader is often assembled with insufficient thermal compound; entire mixer reboots or Wi-Fi drops. Replacing paste or adding a 40 mm fan cures that, but is unrelated to the analogue failure except for shared temperature stress.
• Field reports (2022–2024, Gearspace, ProSoundWeb, Elektroda) consistently cite the 6 Ω resistors and NJM4580 as the parts that fail first.
• Soundcraft has started fitting higher-pulse-rated resistors (thicker film, code “R22”) in late-2023 builds.
• Some service centres now offer a retrofit kit: two 10 Ω / 150 mW pulse-proofs + OPA1652 (drop-in, lower noise, rail-to-rail).
• Growing trend to add a silent 5 V blower tied to the mixer’s USB 5 V rail; keeps interior <40 °C.
• Why 6 Ω and not 6.8 kΩ?
– 6.81 kΩ feed the phantom supply; the tiny 6 Ω parts are purely for surge-limiting the audio path, acting like PTC fuses.
• Op-amp substitution:
– NJM4580 → NE5532, OPA2134, LM4562, OPA1652 are pin-compatible. Verify supply is ±15 V before fitting rail-to-rail types.
• ESD/TVS parts: usually SM05C or PESD5V; shorted devices pull input hard to ground and mimic an “open” resistor fault.
• Opening the chassis voids warranty; be sure the mixer is out of warranty or obtain written authorization.
• Respect EU RoHS when sourcing replacement parts; avoid counterfeit ICs—buy from authorised distributors.
• Disconnect mains, wait 2 min for bulk caps to discharge; phantom rail stores ~25 mJ which is not lethal but can ruin a scope probe.
• If more than four adjacent channels are dead, suspect a blown ±15 V regulator; replacing channel parts will not help.
• Severe carbonisation around the connector = replace entire PCB; tracking leakage current on four-layer boards is unreliable.
• Study TI application note “Hot-plugging with Phantom Power” (SLAA453).
• Evaluate replacing the whole analogue front end with THAT-based digitally controlled preamp if doing a major refurb.
• Compare Ui24R thermal behaviour with competitors (Behringer XR18, Allen & Heath CQ-20) for improved enclosure design.
Most Ui24R preamp failures after a hot phantom disconnect or prolonged heat are confined to the tiny 6 Ω surge-limiting resistors and, if the spike was large, the first dual op-amp. Replace those parts, inspect the 6.81 kΩ phantom resistors and ESD diodes, and the channel is usually restored. Provide better cooling and always disable phantom before unplugging to prevent recurrence.
