Antminer T21 – Hashboard Not Detected

Hard power-cycle the T21 at the breaker for 60 seconds, then power back up. Watch the dashboard for the first 3 minutes. Transient chain-detect failures clear on a clean cold boot roughly 15% of the time — a soft reboot via the UI is not enough because it does not drop the PSU rails. Record which chain was missing before the cycle so you can confirm whether it returned or whether the same one is still dark.

Check kernel log (via SSH or the miner's debug page) for the exact failure line: `check_asic_number_with_power_on: Chain[X]: find 0 asic`. Note the chain number. Also look for `fail to read pic temp for chain X` — that is a distinct PIC-communication failure mode with the same downstream symptom. You will reference these log lines in every subsequent step.

Factory-reset within the 2-10 minute post-boot window: press and hold the recessed reset button on the control board for 5 seconds. Wait 4 minutes for the auto-restart. This clears any stale configuration or chain-mapping state in the control board firmware. If the reset button was pressed outside the 2-10 minute window, Bitmain firmware ignores it — power-cycle and re-try.

Verify intake air temperature is below 35 degrees C at the T21 front grille with an IR thermometer (not in the middle of the room). Verify the intake is not blocked by dust, curtains, or furniture within 15 cm of the chassis. A thermally-marginal board can be tipped into `ERR_NO_HASHBOARD` by ambient heat in summer months; cooling the intake sometimes recovers the chain without any physical work.

Power off at the breaker. Open the chassis. Disconnect and reconnect the data ribbon and power harness on the dead hashboard only. Inspect the connector faces for bent pins, darkening, oxidation, or mechanical damage. Reconnect firmly and listen for the audible click. Loose ribbons are the single most common cause on T21s that have recently shipped or been moved between racks — solve this before anything more invasive.

Label the 3 hashboard slots with tape as 0, 1, 2. Remove the dead board and swap it into a known-good slot. Power up. If the fault follows the board, the board itself is the problem — continue to Step 10 (board-level inspection). If the fault stays in the original slot even with a known-good board in it, the control-board path (slot connector, ribbon, or control board itself) is the problem — continue to Step 7.

Swap the ribbon and power cables between a known-good slot and the failing slot. Re-run the boot. If the fault still stays in the same slot even with known-good cables, the control board itself is suspect. Inspect the slot connector on the control board for damage, oxidation, or bent header pins. This is the hand-off point from cable-fix (cheap) to control-board issue (more expensive).

Multimeter on DC, probe at the PSU-to-control-board input connector while the miner is running (carefully — the T21 draws significant current; keep probes steady). Expect the input voltage to match your PSU rating within +/- 2% under load. If voltage sags under load, the PSU or the circuit feeding it is marginal and can cause chain-boot failures. Swap PSU with a known-good unit to isolate.

Flash the latest stable Bitmain T21 firmware via SD-card recovery. Use a 16 GB or smaller Micro SD card formatted FAT32, official firmware image downloaded from `service.bitmain.com/support/download`. Insert the card, power the miner, wait for the LED sequence that indicates flash complete (usually 3-5 minutes), power off, remove the card, and boot normally. SD-card recovery repairs corrupted eMMC firmware that may have lost chain inventory.

With the dead board out of the chassis, inspect both faces under bright light with magnification. Look for burn marks, blackened components, cracked MLCCs, bulging electrolytic caps, damaged ASIC chip packages (chipped corners, lifted substrate), or solder joints that have obviously failed. Any of these = Tier 4 (ship to D-Central). No visible damage = continue to Step 11 for electrical testing.

Use a multimeter on the hashboard's power-input domain: measure resistance to ground on the main rail with no power applied. Expect a few ohms (not a hard short, not fully open). A dead short indicates a failed MOSFET, shorted MLCC, or damaged ASIC — component-level repair. A fully open indicates a broken trace or failed boost component. Both are Tier 3/4 work; record the reading for the repair ticket.

Flash DCENT_OS (D-Central's own open-source Antminer firmware — preferred; confirm T21 is on the supported model list before flashing) for per-chip HW% visibility. Alternatives: Braiins OS+, LuxOS, Vnish. Let the miner stabilize for 20 minutes after flash. If the dead chain now appears and shows per-chip data, the issue was firmware-level or a chip-level fault that chain-level stock firmware could not expose. Record any chip positions flagged red or >20% HW%.

If DCENT_OS / Braiins OS+ per-chip view isolates one or two failing chip positions: perform a reflow on those chips. Remove the heatsink, clean the old paste with IPA 99%, flux the BGA, preheat the board bottom side to ~150 degrees C, apply top-side hot air at 310-330 degrees C for ~30 seconds, let cool naturally. Reapply Arctic MX-6 or Kryonaut thermal paste in a thin uniform layer, reassemble, re-test. BM1398-generation chips tolerate a reflow cycle well.

Replace thermal paste on all chips regardless of reflow outcome. Use Arctic MX-6 or Thermal Grizzly Kryonaut. Uniform thin layer — do not glob. Pay attention to the control IC (PCH) and any voltage-domain ICs on the board; dried thermal pads there are a frequent cause of drift that eventually manifests as chain-boot failures. Old paste comes off with IPA 99% and a lint-free wipe.

If the board is still failing after reflow and thermal refresh, the PIC microcontroller is the next suspect. The PIC reflash requires Bitmain's internal hash-board code editor tool — not distributed to end users. At this point the economic call is to stop DIY and ship the board to D-Central. A PIC reflash on the bench is typically 15 minutes of work and $3 of parts, recovering a $400 hashboard.

Book a D-Central ASIC Repair slot via https://d-central.tech/services/asic-repair/. Package the hashboard in an anti-static bag, double-box with at least 5 cm of foam on every side, include a note listing: observed kernel-log lines, firmware version, which steps you tried, what changed each attempt, and your contact info. Clear documentation saves diagnostic time, which saves you repair dollars.

D-Central bench process on a T21 hashboard: visual inspection under microscope, full electrical test on programmable load, PIC reflash via hash-board code editor, per-chip isolation using official Bitmain test binaries, chip replacement with graded BM1398-family chips from D-Central's inventory, full reflow and reseal, thermal paste refresh, 24-hour burn-in at nameplate. Expected turnaround 5-10 business days.

If the board is beyond component-level repair (multiple dead chips, PCB damage, PMIC failure beyond reseat), D-Central can swap in a salvaged-grade replacement hashboard from inventory — CAD $350-$650 depending on bin. The board is re-tested and re-burned-in before ship-back so you are not rolling the dice on a second dead board. All shipments include a 30-day parts-and-labour warranty on the repair.