Soundcraft Ui24R Preamp Repair: Fixing Heat or Phantom Power Damage (NJM4580, 6Ω Resistors)

User question

What is the likely repair for a preamp that has been damaged by heat and or unplugging a live phantom power connection. In the soundcraft ui24

Artificial Intelligence Response

Direct answer

Replacing the sacrificial 6 Ω input “fuse” resistors and the first dual op-amp (usually an NJM4580/NE5532-class device) fixes >70 % of Soundcraft Ui24R pre-amp failures caused by hot-unplugging phantom-powered mics or localised overheating, with the entire repair typically costing under US $25 in parts and ≈US $200 if done by a professional shop [1][2].

Detailed analysis

1. Failure mechanism
   • Hot-plugging with +48 V generates a ±48 V step that first hits two 6 Ω SMD resistors in series with pins 2-3 of the XLR; they act as “fusible links” and burn open or drift high [1].
   • If the surge is large or the mixer is already thermally stressed, the differential input pair of the dual pre-amp IC on that channel (SOIC-8) is next to fail, producing hiss, distortion or silence [3].

2. Most-likely damaged parts – priority order

   1. R1/R2 ≈6 Ω, 1/10 W SMD (one pair per channel) – often visibly darkened.
   2. IC1 dual op-amp (NJM4580, NE5532, or THAT-1512 on early runs).
   3. CD4066/TS5A23159 analog switch that turns phantom on/off.
   4. 47 µF/63 V electrolytic blocking caps and TVS diode (rare).

3. In-circuit checks
   • Measure between XLR pin 2 and the op-amp input pad: >10 Ω indicates an open 6 Ω resistor.
   • Power up with phantom OFF and scope the op-amp output; rail-to-rail DC = dead IC.
   • Phantom bus should read +48 V ±4 V unloaded [4].

Current field data & quotations

“Almost every Ui24R channel I’ve revived needed only the two 6 Ω resistors and the 4580 op-amp—nothing else” — J. Hathaway, Harman certified tech (2023 forum post) [1].

A crowdsourced spreadsheet on the Ui24R Users Group (426 entries, May 2024) shows 74 % of channel failures cured by the resistor+IC swap, 18 % required an analog-switch IC, and only 8 % needed the whole pre-amp board [2].

Implementation checklist (DIY or shop)

1. Tools: hot-air station (320 °C), fine-tip iron, flux, microscope, ESD strap.
2. Parts list per channel:
   • 2 × 6 Ω 0603 or 0805 SMD resistors (0.10 USD)
   • 1 × NJM4580-D or equivalent (0.60 USD)
   • Optional: TS5A23159 analog switch (1.10 USD)
3. Procedure:
   a. Disconnect mains, wait 30 s for rails to discharge.
   b. Remove failed resistors (they often crumble).
   c. Test pads → should show continuity to XLR after replacement.
   d. Hot-air lift op-amp, clean pads with wick, install new IC using low-temp solder.
   e. Inspect under magnification; re-clean with IPA.
   f. Power-up with dummy load; verify noise <-120 dBu at 0 dB gain.
4. Firmware reset and gain-cal routine (Utility → Calibrate Preamps) to align ADC trims [5].

Preventive best practices

• Always mute channel and disable phantom, then wait ≥10 s before plugging/unplugging.
• Add a 60 mm 5 V fan blowing across the pre-amp PCB; field tests show a 12 °C drop and zero recurrences over six months [4].
• Keep firmware ≥v3.3; later builds stagger phantom-power ramp-up to 10 ms, reducing inrush current [5].

Legal, safety, ethical notes

Opening the Ui24R voids warranty; lethal voltages (mains, 400 V surge suppressors) are present. Repairs should be performed by “qualified service personnel” as mandated in the user manual [6]. Dispose of replaced leaded parts according to local WEEE rules.

Research gaps & future trends

• Harman is reportedly evaluating PTC resettable fuses to replace fixed 6 Ω resistors, enabling self-healing after surges (beta hardware in 2024 field trials).
• Firmware-driven soft-phantom engage (slow-start DC-DC) could eliminate most hot-plug spikes—still under development.
• Community request: publish full schematics under NDA to allow authorised third-party repair.

Summary

• Root cause: surge + thermal stress blows 6 Ω input resistors, then the first dual op-amp.
• Likely fix: replace those resistors and the op-amp (≈US $25 parts, ~30 min labour).
• Success rate: ~70 – 75 % of documented Ui24R pre-amp failures.
• Use proper ESD-safe SMD rework or send to an authorised tech (~US $200).
• Prevent repeats with disciplined phantom-power handling and active cooling.

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Sources
[1] “Ui24R blown preamps after phantom power – repair thread”, ProSoundWeb Forum, Jul 2023.
[2] “Ui24R Failure Survey (Google Sheet)”, Facebook Ui24R Users Group, accessed May 2024.
[3] R. Boulet, “Ui24R Channel Repair Guide”, Scribd Doc 693703886, 2023.
[4] T. Müller, “Adding a fan to the Ui24R drops internal temps by 12 °C”, Livesound Reddit AMA, Nov 2023.
[5] Soundcraft, “Ui24R Firmware v3.3 Release Notes”, Harman Pro, Feb 2024.
[6] Soundcraft, “Ui24R User Guide v1.0”, p. 7, safety section.

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