Session 5: Power, Display & Heat Faults

A machine that shows no sign of life has a fault somewhere in a short chain, and you work along it from the easiest point. Is the wall socket live? Test it with something else — this is embarrassing to overlook and it happens constantly. Is the adapter working? A laptop adapter should output its rated voltage, commonly 19 or 19.5 volts, measured with a multimeter on the DC setting; zero or a wildly wrong figure means a dead adapter, which is a cheap fix. Is the cable damaged? Check the whole length, especially at the strain reliefs near the plug and the connector, where internal breaks are common and invisible.

Then the machine itself. On a desktop, check the power supply's own switch at the back, which people routinely leave off, and the 24-pin and CPU power connectors on the board, which come loose. A useful test is the paperclip test on a suspect supply — bridging the green wire to a black one makes it run without a board — though this is a diagnostic, not a repair. On a laptop, check the charging port for a loose or broken centre pin, and try booting with the battery removed and only the adapter connected, which isolates a battery that is preventing startup.

If power reaches the board and still nothing happens — no fan, no LED — the fault is usually the board itself or a short somewhere on it. That is the point at which you consider whether the repair is economic, and section five covers how to make that judgement honestly.

A multimeter does three things you need constantly. DC voltage measures what a power source is outputting — set the dial to a DC range above the expected voltage, black probe to the negative or ground, red to the positive, and read. A laptop adapter should read its rated voltage; a car battery reads about 12.6; a CMOS battery reads about 3. Continuity tests whether a path is unbroken — the meter beeps when the probes are connected through a continuous conductor. Use it to check a cable, a fuse, and whether a switch actually closes.

Resistance measures opposition to flow, and it is how you test a battery's health indirectly and check for shorts. A very low resistance between a power rail and ground suggests a short circuit, which is a serious board fault.

The habits that keep you safe: never measure resistance or continuity on a live circuit; never use a current setting when you mean voltage; and start on a higher range than you expect and work down. A multimeter is not dangerous on the low-voltage circuits inside a computer, but treating it carelessly is how people damage the meter or get a surprise from a power supply that is still connected to mains.

A bad display has three possible causes and the diagnosis distinguishes them. Connect the laptop to an external monitor first — this one test splits the problem in half. If the external display works correctly while the laptop's own screen is faulty, the fault is the panel or its cable, not the graphics hardware. If both are faulty, the problem is the GPU or the board, which is usually uneconomic to repair.

A failed panel shows as a cracked screen, lines, dead areas, or a completely black screen with a faint image visible under a torch — which indicates the backlight has failed rather than the panel. A failing display cable shows as flickering, distortion, or a display that changes when you move the lid, because the cable runs through the hinge and fatigues there. That last symptom is diagnostic: if the picture changes with lid position, it is almost certainly the cable, and it is a cheap, common repair.

A backlight failure produces a screen that looks dead but shows a very faint image under bright light. It is worth checking with a torch before condemning a panel, because the fix may be a backlight circuit rather than a whole screen. And note that after any screen replacement you must verify the resolution and connector match — a panel that fits physically but reports the wrong resolution is the wrong part.

A machine that shuts down under load, or that becomes too hot to keep on a lap, is protecting itself. The CPU has a thermal limit, commonly around 95–100°C, and when it reaches it the machine cuts power immediately to prevent damage. That is what a random shutdown under load usually is, and it is a symptom rather than a mystery.

The causes, in order of likelihood: dust-blocked cooling, which session three covers and which is by far the most common; dried thermal paste; a failed fan that spins erratically or not at all; and, rarely, a genuine heatsink mounting problem. Diagnose by measuring: run a temperature tool, watch the figure under load, and listen to the fan. A machine that idles at 85°C has a cooling problem, not a software one.

Distinguish this from battery-related shutdown. A machine that dies suddenly at a reported 30% charge has a degraded battery whose reported capacity no longer matches reality — the operating system thinks there is charge left and the cell cannot deliver it. That is a battery replacement, not a thermal fault, and the two are confused often enough that it is worth checking the battery report Windows can generate before opening the machine.

Part of being a technician is knowing when not to repair. A board-level fault on a laptop — a dead CPU, a failed GPU, a cracked board — usually costs more in parts and labour than the machine is worth, particularly on a mid-range machine that is several years old. Fitting a replacement board often approaches the price of a comparable used machine, and it carries no warranty on the rest of the hardware.

Make the judgement out loud and show your working: what the part costs, what the labour is, what the machine is worth, and what the alternatives are. Then let the customer decide with real information. This is where honesty earns its keep — a technician who says 'this is not worth repairing, and here is why, and here is what I would buy instead' is trusted for life, and that customer sends everyone they know. A technician who repairs a dead machine for a large fee gets paid once and talked about badly forever.

There is also a data-recovery dimension. A machine that is beyond repair may still hold files the customer desperately needs, and recovering them — by removing the drive and reading it in another machine or through a USB adapter — is often the real service, and it is worth more to the customer than the repair would have been. Always ask what is on the machine before you conclude anything.