Chapter 51: Solder Joints and Tolerance

Stopwatch. Tick. Tick. Thirty-nine minutes and fifty-nine seconds.

Lin Chen’s fingers came down on the tabletop. Don’t touch anyone else’s equipment. Take only his own. Resistance box. The dial had heavy damping. Needed force. Wires. Red and black. The insulation had aged. The edges were whitening. Dark copper showed through. He sorted the wires first. Grouped by length. Ten centimeters. Fifteen centimeters. Twenty centimeters. Wire stripper. Dull blades. Needed finesse to strip the insulation. Must not nick the copper strands. He twisted the strands tight. Kept them from fraying.

The circuit diagram was in his head. Already broken down. Power input. Filter capacitor. Voltage regulator module. Output terminal. Load. Ground. Nodes. Loop. He laid out the components in order. No crossing. No tangling. Left room for measurement. Multimeter. Switched to continuity mode. Probe tips touched lightly. The buzzer sounded. The line was live. He nodded. Started wiring.

First wire. Positive power terminal. Connected to the capacitor’s positive lead. The pin was oxidized. Had to be polished with sandpaper. The sandpaper was coarse. Friction. Copper showed through. Tin coating. No soldering iron was provided. The test was wiring, not soldering. He switched to alligator clips. Clamped tight. Would not loosen. Second wire. Ground. Connected to the common terminal. Third wire. Zener diode. Cathode to positive. Anode to negative. Reverse it and it would short. He checked the color bands. Brown-black-orange-gold. One megaohm. Wrong. That was the current-limiting resistor. He switched it out. Red-red-brown-gold. Two hundred twenty ohms. Correct. Inserted it into the breadboard. The holes were tight. He had to push hard. The pad of his finger hurt. He did not pull back.

The lab was cold. No water ran through the radiators. The iron casing was icy. The air held the smell of old rubber, rosin, and dust. Outside the window the sky was gray-white. Snow clouds pressed low. The light was dim. Someone rifled through a drawer. Metal clanged. Sharp. Someone cursed under his breath. Miswired. Short circuit. The faint smell of scorching drifted out. Old Li stood behind the lectern. His gaze swept across the room. He did not stop anyone. He only pressed the stopwatch. Tick. Tick.

Lin Chen did not raise his head. His fingers were steady. He connected the fourth wire. Load resistor. Paralleled across the output terminal. Fifth wire. Feedback lead. Connected to the regulator module’s adjustment terminal. The hole was off by half a millimeter. He adjusted it. Did not force it in. Prevented a broken pin. The breadboard’s spring contacts had aged. Weak elasticity. He had to press firmly to seat it. He braced it with his thumb. Pressed down slowly. Click. In place.

Stopwatch. Thirty minutes. Ten minutes left.

He stopped. Did not connect the load yet. Measured the no-load voltage first. Multimeter. DC voltage mode. Twenty-volt range. The probes paralleled the output terminal. The screen lit up. Digits jumped. 4.8V. Too low. Error: -0.2V. Out of tolerance. He frowned. Did not panic. Traced the circuit. The current-limiting resistor’s value was too high. Too much voltage division. He pulled it out. Replaced it with one hundred fifty ohms. Yellow-violet-brown-gold. Measured again. 5.1V. Too high. +0.1V. Right at the limit. He fine-tuned it. Put a one-kiloohm resistor in parallel. Diverted current. Measured again. 5.0V. Stable. He wrote down the reading. Did not wipe sweat. His hands were steady. His knuckles had gone pale. Numb with cold. He rubbed them. Feeling came back. Continued.

Stopwatch. Five minutes.

He straightened the tabletop. Smoothed the wires. Returned the components to order. Reset the multimeter. Coiled the probes. No tangling. Raised his hand. “Report. Finished.”

Old Li came over. Pressed the stopwatch. Thirty-eight minutes and twelve seconds. Ahead of time.

Old Li gave no comment. He only picked up the probes. Measured again. The tips touched the test point. The screen showed 5.02V. Error: +0.02V. Passed. Old Li put a check mark on the grading sheet. Pen tip scraping paper. Rustle, rustle. “Standard wiring. No short circuit. Accurate reading. Deduction item: alligator clip loose. Minus two points. Final score: ninety-eight.”

Lin Chen nodded. “Thank you.” No argument. No pleasure. He only packed up his tools. Put them into the canvas bag. Pulled the zipper shut. Weight evenly distributed.

One after another, people handed in their work. Some ran over time. Stopwatch reset to zero. Old Li said coldly, “Zero points. Retest tomorrow.” Someone shorted a circuit. The fuse blew. A thread of black smoke rose. Old Li wrote it down. “Damaged equipment. Full compensation. Participation points deducted.” His voice was not loud. But it cut through the cold wind. The dormitory fell quiet. Only breathing remained. Heavy. Suppressed.

Lin Chen slung his bag over his back and returned to the dorm. The corridor was cold. Wind poured up from the stairwell, carrying the smell of chalk dust and old wood. He went downstairs. Steps steady. Rubber soles came down on the frozen steps. Center of gravity shifted forward. Stride length thirty centimeters. Never stepped on the edge. The anti-slip tread bit into the ice. Made fine crunching sounds. Effective. The gauze under his sole rubbed against the skin. The crack in his foot gave a faint stab of pain. He adjusted his balance. The pain dulled. Did not hinder him.

Door pushed. Locked. Canvas bag set on the bed. Movements light. Did not shake loose any dust.

He spread open the ledger. Pencil moved. The tip rasped across the paper. Rustling. In the quiet dorm room, the sound was magnified.

Day 30. 14:40. Practical exam finished. Score: 98. Third in Group A. Deduction: alligator clip loose. Lesson: aged equipment needs secondary reinforcement. Spare cable ties. Electrical tape. Funds: 50.00. Deposit paid. Balance 0. Tomorrow: morning drill. 6:00. Written exam. Electromagnetism.

The pencil tip paused. He closed the ledger. Fingers tightened. The page edges curled. He shoved it to the bottom of the bag and pinned it down.

He sat on the edge of the bed. Closed his eyes. Breathing steady. Chest rising and falling. Fixed rhythm. His fingers unconsciously rubbed the zipper pull of the canvas bag. The metal was icy. Rough. Like an unpolished stone. The pain in his sole was distinct. But it did not interfere. He categorized it. Entered it into the variables. Did not magnify it. Did not ignore it.

Outside the window. The sky was dark. Clouds hung low. No snow yet. Wind passed through the gaps and gave a low howl.

The loudspeaker. Static. Zzzt. Old Li’s voice. Distorted by interference. Penetrated the walls.

“Group A. Tonight’s assignment. Load-characteristic curve of a voltage-regulator circuit. Hand-drawn. On graph paper. Hand it in tomorrow morning. Anyone who fails to hand it in will be expelled.”

A pause. A tiny burst of static.

“Also. Notice from the Municipal Education Commission. The winter camp is adding an extra practical exam. Soldering. Bring your own soldering iron. Solder wire. Rosin. If you do not bring them, you may borrow them. Deposit will be deducted. Borrowing limited to twice per day. Overtime charged by the hour.”

The current cut off. Dormitory quiet.

Lin Chen opened his eyes. His gaze fell on the side pocket of the canvas bag. Dried tangerine peel. Old newspaper. He reached into the pocket against his body. Receipt. Red stamp. Fifty yuan. Already paid. But the road ahead was not smooth. Soldering. He did not understand it. But he could learn. Graph paper. He had to draw. Curves. He had to calculate. Time. He had to squeeze it out.

He got up. Went to the washroom. Cold water. Piercing. Scrubbed his face. The towel was coarse. Scraped his skin. Left red marks. Did not hurt. Made him alert. Returned to the room. Spread out fresh graph paper. Pencil. Ruler. Drew the axes. Marked the graduations. Did not trace. Did not shade. The lines were straight. Even. He closed his eyes. In his mind there was no curve. Only solder joints. Temperature. Time. Molten solder. Solidifying. Three lines. Crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. His gaze fell on the sheet. The first stroke came down. Steady. No shake.

Coordinate axes. Horizontal axis. Load current. Vertical axis. Output voltage. Origin. Zero point. Graduations. Each square 0.1A. 0.1V. He calculated. No-load 5.0V. Full load 4.95V. Voltage drop 0.05V. Linear. Negative slope. He drew the line. No break. No bend. Pencil dust fell on the paper. He erased it lightly. Left no trace.

His foot hurt. He changed posture. Shifted his weight onto his right leg. Left leg suspended. Did not press the injury. His fingers had gone numb with cold. He breathed warm air on them. White mist dispersed. Continued drawing.

Outside the window. The wind stopped. The snow grains grew finer. They tapped against the glass with light pattering sounds. Everything was being smoothed flat. Only white remained. And cold.

Tomorrow morning. Six o’clock. Written exam. He needed sleep. But his hand was moving. Pencil tip rasping across the paper. Rustle. Like the stopwatch. Tick. Tick.

He took out the old notebook. Blank page. Pencil moved.

Soldering. Key points. 1. Soldering-iron temperature. 350°C. Too high: oxidation. Too low: cold joint. 2. Rosin. Flux. Prevents oxidation. Use sparingly. Cover the lead. 3. Solder wire. Leaded. Lower melting point. Touch the joint, not the iron. 4. Time. Three seconds. Melt. Withdraw. Solidify. 5. Inspection. Bright. Conical. No burrs.

The pencil tip paused. He closed the notebook. Fingers tightened. The page edges curled. He shoved it to the bottom of the bag and pinned it down.

He lay down. Closed his eyes. Breathing steady. Chest rising and falling. Fixed rhythm. In his mind there were no formulas. No words. Only three lines. One was the rule for estimated readings. Midpoint, estimate five. Slightly right, estimate seven. One was an icy road surface. Rubber-sole tread. Shift your center of gravity forward. Do not step on the edge. One was the steps of soldering. Temperature. Time. Molten solder. Solidifying. Three lines. Crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He did not need to guess the difficulty. He only needed to prepare the tools. Graph paper. Pencil. Ruler. Eraser. Spare leads. Three sticks. Draft paper. Two notebooks. Soldering notes. One page.

He closed his eyes. His fingers unconsciously rubbed the zipper pull of the canvas bag. The metal was icy. Rough. Like an unpolished stone.

The wind tightened. The mountain shadows in the distance darkened. Clouds pressed lower. Snow was coming again. He turned over. Faced the wall. His breathing slowed. Outside the window, the snow grains grew finer, tapping against the glass with light pattering sounds. Everything was being smoothed flat. Only white remained. And cold.

Tomorrow morning. Six o’clock. Written exam.

He closed his eyes. Slept. But his hand was still moving. His fingertips traced coordinate axes unconsciously across the bedsheet. Horizontal axis. Vertical axis. Origin. Graduations. The lines were straight. Even. No shake.

Tick. Tick.