PCB v2 Rework Guide

Three issues found during datasheet-vs-PCB audit require physical rework on existing v2 boards. All fixes use standard soldering tools and 0805 SMD components.

Tools needed: soldering iron (fine tip), flux, 30AWG kynar/bodge wire, solder wick, magnifying glass/microscope.

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Fix 1: SD Card Power (CRITICAL)

Problem: SD card slot (U6) pins 4 (VDD) and 6 (GND) have no copper connection. The card receives no power.

U6 is on the BACK side (B.Cu). All work done on the back of the board.

Pin locations (back side, looking at component)

         Signal pads (top edge of slot)
    ┌──────────────────────────────────┐
    │  1  2  3  4  5  6  7  8  9      │
    │         CS    VDD    GND   MISO  │
    │              ^^^    ^^^          │  ← FIX THESE
    │          SD CARD SLOT            │
    │                                  │
    └──────────────────────────────────┘
     Pin 13                      Pin 10
     (shield)                  (shield)

Rework steps

1. +3V3 to Pin 4 (VDD):

   • Locate pin 4 on the TF-01A slot (4th pad from left on the signal row)
   • Find the nearest +3V3 source: the AMS1117 output cap (C2) or any pad/via labeled +3V3
   • Solder a 30AWG wire from pin 4 to the nearest +3V3 via or pad
   • Alternative: drill a 0.3mm hole next to pin 4 and insert a via wire to reach the In2.Cu +3V3 plane

2. GND to Pin 6 (VSS):

   • Locate pin 6 (6th pad from left)
   • Find the nearest GND source: any via, mounting hole pad, or ground pour
   • Solder a 30AWG wire from pin 6 to nearest GND point
   • The shield pads (pins 10-13) have large exposed copper — if any connects to GND, use that

3. GND to Shield pads (pins 10, 12) — optional but recommended:

   • Solder wires from 2 opposite shield pads to GND for EMI shielding

Verification

• Multimeter continuity: pin 4 to any +3V3 pad → should beep
• Multimeter continuity: pin 6 to any GND pad → should beep
• Insert SD card, power on → SD should be detected by firmware

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Fix 2: PAM8403 Audio Passives (CRITICAL)

Problem: The PAM8403 Class-D amplifier is missing all passive components from the datasheet application circuit. No DC-blocking, no decoupling, no bias resistors.

U5 is on the BACK side (B.Cu). All work done on the back of the board.

U5 pin layout (back side view, rotated 90°)

        Top row (y≈26.8)              Bottom row (y≈32.2)
    Pin 1  +OUT_L  ┐   ┌ Pin 16 +OUT_R
    Pin 2  PGND    │   │ Pin 15 PGND
    Pin 3  -OUT_L  │   │ Pin 14 -OUT_R
    Pin 4  PVDD    │ U5│ Pin 13 PVDD
    Pin 5  MUTE    │   │ Pin 12 SHDN
    Pin 6  VDD     │   │ Pin 11 GND
    Pin 7  INL     │   │ Pin 10 INR
    Pin 8  VREF    ┘   └ Pin 9  NC

Components to add (all 0805 SMD)

#	Value	Purpose	Connect between
C_a	0.47uF	DC-blocking	I2S_DOUT trace → INR (pin 10)
R_a	20k	INL bias	Pin 7 (INL) → GND
R_b	20k	INR bias	Pin 10 (INR) → GND
C_b	100nF	VREF bypass	Pin 8 (VREF) → GND
C_c	1uF	VDD decoupling	Pin 6 (VDD) → GND
C_d	1uF	PVDD decoupling	Pin 4 (PVDD) → GND
C_e	1uF	PVDD decoupling	Pin 13 (PVDD) → GND

Rework steps (priority order)

Step A — DC-blocking cap (most important):

1. Locate the I2S_DOUT trace arriving at pin 10 (INR) — it's a vertical B.Cu trace at x≈33.2mm
2. Cut the trace between the via and pin 10 using a hobby knife
3. Solder a 0.47uF 0805 cap across the cut (one pad to via side, other to pin 10 side)
4. This blocks DC offset from the ESP32 DAC

Step B — Bias resistors:

1. Solder a 20k 0805 resistor from pin 7 (INL) to nearest GND point
2. Solder a 20k 0805 resistor from pin 10 (INR) to nearest GND point
3. These set the audio input bias point

Step C — VREF bypass:

1. Solder a 100nF 0805 cap from pin 8 (VREF) to pin 2 (PGND) — same row, 7.6mm apart
2. Use a short bodge wire if the cap can't bridge the distance

Step D — Decoupling caps:

1. Solder a 1uF 0805 cap from pin 6 (VDD) to pin 11 (GND) — opposite rows, bridging across
2. Solder a 1uF 0805 cap from pin 4 (PVDD) to pin 2 (PGND) — same row, 2.54mm apart (1 cap can bridge)
3. Solder a 1uF 0805 cap from pin 13 (PVDD) to pin 15 (PGND) — same row, 2.54mm apart

Verification

• Power on without SD card
• Connect speaker to SPK+/SPK- pads
• Play a test tone from firmware → should hear clean audio without buzzing
• No DC offset: measure DC voltage across speaker terminals → should be < 20mV

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Fix 3: USB-C Shield Pads (HIGH)

Problem: Front shield pads are 1.1mm wide instead of datasheet 1.7mm. Reduces mechanical retention.

Rework steps

1. Add extra solder to all 4 shield THT pads (pins 13, 14 front; 13b, 14b rear)
2. Build up a generous solder fillet around each tab to increase contact area
3. Optionally apply UV-cure adhesive (Bondic or similar) at the connector base for extra mechanical strength
4. Test: firmly pull/push USB-C cable — connector should not move

This is the simplest fix — just add more solder to compensate for the undersized pads.

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Fix Priority

Fix	Effort	Impact	Required?
1. SD Card power	10 min	Board-breaking — SD won't work at all	YES — mandatory
2. PAM8403 passives	35 min	Audio damage risk + noise	Only if using speaker
3. USB-C pads	5 min	Mechanical durability	Optional, recommended

Minimum rework: ~10 minutes (fix #1 only, no audio). Full rework: ~50 minutes (all fixes).

No speaker? Skip fix #2 entirely

If you are not connecting a speaker, the PAM8403 rework is unnecessary. Without a load, the Class-D amp draws ~10mA quiescent but causes no harm. All missing passives (DC-blocking, decoupling, bias) only matter when driving a speaker. You can do fix #2 later when you want audio.

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Parts Shopping List (for rework)

Qty	Value	Package	Use
1	0.47uF ceramic	0805	PAM8403 DC-blocking
3	1uF ceramic	0805	PAM8403 decoupling (VDD + 2x PVDD)
1	100nF ceramic	0805	PAM8403 VREF bypass
2	20k resistor	0805	PAM8403 input bias
1m	30AWG kynar wire	—	Bodge wires for all fixes