The call came in at 9:47 AM on a June morning
The trader’s message was two lines: “All four monitors went black at 9:45. Desktop makes a clicking sound when I press power. Can you come to Madhapur now?” We recognised the symptom immediately — the clicking sound on power-press with no POST is the PSU’s overload protection tripping and resetting in a loop. Something had put the system into a hard fault state. The timing — 9:45 AM, peak grid load hour, late June — told us the rest of the story before we arrived.
The setup was a purpose-built trading workstation: Intel Core i9-14900K, ASUS ProArt Z790 motherboard, 64 GB DDR5-6400 ECC RAM, a dual-port Nvidia RTX 4070 Ti Super handling three 27-inch monitors, a PCIe capture card handling the fourth monitor feed from a dedicated NSE/BSE data terminal, and a 850W Corsair HX850 PSU. The whole system ran NSE live feed with algo execution latency under 2ms. This was not a casual setup. It was also connected, at the time of the incident, to a basic power strip with no surge protection.
Bay 1 diagnosis: the PSU autopsy
MOSFET smoke test on the PSU
We removed the Corsair HX850 and opened it on the bench. The primary side — the section that takes 230V AC and converts it to a high-frequency switched DC for the transformer — showed two primary switching MOSFETs (STF13NM60N, 600V-rated) with shorted gate-drain junctions. In diode-check mode both showed near-zero resistance in both directions: a classic overvoltage failure. The NTC inrush thermistor was intact, meaning the surge had arrived faster than the NTC’s slow thermal response could limit it. The varistors (MOV surge suppressors) on the input line had fired and partially absorbed the spike, but the residual voltage spike after MOV clamping was still sufficient to exceed the 600V MOSFET drain-source rating by an estimated 20–30%.
The secondary side — the 12V, 5V, and 3.3V output rails — tested completely clean. The primary-stage failure had been fast enough to protect the downstream rails. This was the good news: PSU destroyed, everything else potentially intact.
GPU output trace test
With a known-good 650W PSU used as a bench substitute, the system posted normally on three monitors. The fourth monitor, connected to the second DisplayPort output on the RTX 4070 Ti Super, showed no signal. The GPU’s primary outputs (HDMI + DP1) were functional; only DP2 was dead. We removed the GPU and ran a MOSFET smoke test on the display output stage — the small power regulation circuit that drives the DisplayPort transmitter ICs. One N-channel MOSFET in the DP2 output power path tested shorted. This had likely been hit by a brief overcurrent through the PCIe power connector during the surge event, before the PSU’s protection fired — a <50 microsecond window that was long enough to blow the output stage MOSFET.
The repair sequence
PSU replacement
The HX850 was beyond economical repair — primary MOSFET replacement in an 850W PSU requires full recalibration of the switching frequency and overcurrent protection thresholds. We recommended replacing it with an 850W unit from the same tier. A Seasonic Prime TX-850 (80+ Titanium, full modular) was sourced and fitted. Total for PSU replacement including labour: ₹9,200 (Seasonic Titanium 850W unit + fitting fee).
GPU output stage MOSFET replacement
The damaged MOSFET on the DP2 output stage was identified as a standard SOT-23 packaged N-channel part. Under the microscope, the rework station reflowed a replacement MOSFET onto the DP2 power rail. Post-repair, DP2 showed signal on a monitor connected directly. All four monitors restored. GPU MOSFET repair labour and parts: ₹4,800. Total invoice: ₹14,000.
The UPS recommendation
We left the trader with a clear recommendation: the HX850 had performed exactly as intended — it sacrificed itself to protect the downstream components. But a ₹14,000 repair bill every pre-monsoon season is not a sustainable protection strategy. An online double-conversion UPS at 1500VA would have isolated the system from mains entirely; the surge would have gone into the UPS rectifier and battery, never reaching the workstation. Cost of an APC Easy UPS Online SRV 1500VA: approximately ₹24,000. For a trading station where every minute of downtime has a direct P&L cost, the maths is straightforward. For more on protecting desktops from power damage, see our desktop repair service page and not-powering-on diagnosis guide.