PC operation and maintenance
A computer is just another piece of electronic equipment: it has a switching power supply, buses, connectors and heat dissipation. The difference is that its failures show up as “it runs slow” or “it won't boot,” and you have to translate that into a concrete measurement.
01The parts and how they fail
| Part | Typical failure | How to check |
|---|---|---|
| Switching power supply | Electrolytic capacitors with high ESR: won't start, reboots, shuts down under load | Measure the voltages under load and the ripple with an oscilloscope. It is the number one cause of “weird” failures. |
| Motherboard | Bulging capacitors, CPU regulators, dead clock battery | Visual inspection, measure the regulator voltages, replace the CR2032 if it loses the time. |
| RAM | Intermittent errors, blue screens, freezes with no pattern | Dedicated memory test, or test module by module. |
| Drive | HDD: noise, bad sectors. SSD: worn-out cells | Read the SMART data: hours of use, reallocated sectors, percentage of life remaining. |
| Cooling | Dust, dried thermal paste, seized fan | Monitor the temperature: if performance drops because of heat, it shows in the power draw and in the clock frequency. |
| Connectors | Dirty or oxidized contacts | Reseat and clean. Many “board” failures are just a badly seated module. |
With any unstable symptom —it reboots, it freezes, it doesn't always power on— the first suspect is the power supply. Measure its outputs under load, not unloaded, and look at the ripple: if it exceeds about 50 mV peak to peak on the 12 V line, there are worn-out electrolytic capacitors. Everything covered in impedance measurements about ESR applies exactly here.
02Interfaces and buses
| Interface | Speed | Used for |
|---|---|---|
| USB 2.0 | 480 Mbps | Slow peripherals. Supplies 500 mA at 5 V. |
| USB 3.x | 5 to 20 Gbps | External storage. Connector with extra contacts and up to 900 mA. |
| SATA III | 6 Gbps | Drives and optical drives. |
| NVMe over PCIe | Tens of Gbps | Modern SSDs: connected directly to the processor bus. |
| PCI Express | Per lane, scalable | Graphics cards, network cards, controllers. People speak of “x1, x4, x16” for the number of lanes. |
| HDMI / DisplayPort | Gbps | Digital video and audio. |
| Ethernet | 100 Mbps / 1 Gbps | Wired network. See the next section. |
A USB 3 cable carries gigahertz signals: it is not just “a cable.” That is why length, shielding and connector quality matter, and why a cheap cable works at 480 Mbps and fails at 5 Gbps. The same goes for HDMI: half of all picture problems are cable problems. It is impedance matching in practice.
03Systematic diagnosis
The most expensive mistake is swapping parts at random. The method is the same as for any electronic repair: split the problem in two and measure.
- Listen to the whole symptom and note when it started and what changed beforehand.
- Reproduce it. A fault that cannot be reproduced cannot be verified as repaired.
- Power: voltages and ripple under load.
- Minimal configuration: board, processor, one memory module and nothing else. If it boots, add the rest one at a time.
- Boot signals: the board's codes —beeps, LEDs or a two-digit display— tell you at which stage the POST stopped.
- Replacement, and only when the measurement points to something specific.
- Record what was done and what was replaced.
- The symptom appears under load: it points to power or temperature.
- Measure the power supply under load: 12 V with 400 mV of ripple, far above what is acceptable.
- Open it up and there are two bulging electrolytic capacitors on the output.
- Before replacing anything, check the processor temperature to rule out the other hypothesis.
Diagnosis in twenty minutes, without swapping anything at random. Replacing the graphics card —the usual reflex— would have solved nothing.
04Networking: the minimum you need to master
Any modern industrial device is on a network. The electronics technician does not need to be a network administrator, but does need to know why a device “can't be seen” and how to test it.
| Concept | What it is | Example |
|---|---|---|
| IP address | Identifies the device on the network | 192.168.1.45 |
| Subnet mask | Says which part of the IP is the network and which part is the device | 255.255.255.0 (/24): 254 possible devices |
| Default gateway | The way out of the local network | 192.168.1.1, almost always the router |
| DNS | Translates names to addresses | Without DNS “the internet doesn't work” but IPs still respond |
| DHCP | Hands out addresses automatically | Convenient for PCs, inconvenient for fixed devices: a PLC gets a static IP |
| MAC | Physical identifier of the network card | It is unique and set at the factory |
Connects devices within the same network. It learns which MAC is on each port and delivers only through the right port. It does not look at IP addresses.
Joins different networks and decides where each packet goes out. It acts as the default gateway and, in a home, is also the one that provides WiFi, DHCP and the connection to the provider.
| Category | Speed | Distance | Note |
|---|---|---|---|
| Cat 5e | 1 Gbps | 100 m | The usual installation standard. |
| Cat 6 | 1 Gbps (10 up to 55 m) | 100 m | More demanding when terminating the connector. |
| Cat 6A | 10 Gbps | 100 m | Shielded, for demanding new installations. |
- Port lights: with no link, the problem is physical (cable, connector, port).
- Configuration (
ipconfig/ip addr): if the address starts with 169.254.x.x, the DHCP server did not answer. pingto the gateway: if it responds, the local network is fine and the problem is outside.pingto a public IP and then to a name: if the IP responds and the name does not, the problem is DNS.
With those four tests, in order, you isolate the fault in a minute without touching anything.
05Power, grounding and static
Integrated circuits are damaged by discharges you cannot even feel. Before touching a board: wear an antistatic wrist strap or, at the very least, touch the grounded metal chassis. Modules are kept in their conductive bag and handled by the edges, never by the contacts.
A PC without grounding has its case floating at about 110 V because of the mains filter capacitors: you can feel it when you touch it, and it ruins ports when devices are connected to each other. The UPS protects against outages and brief dropouts, and its battery should be replaced every three or four years, even if the unit seems to work.
06Technical documentation
- Equipment identification: make, model, serial number, location, person responsible.
- Symptom as the user described it, in the user's own words.
- Measurements made and values obtained.
- What was replaced, with the part number.
- Final verification: how it was confirmed that the fault does not come back.
- Date, time spent and who did it.
Without records there is no maintenance: there are only isolated repairs. The record is what lets you see that the same unit fails three times a year and decide to replace it, or that an entire model has the same problem.
07In the lab
With a discarded ATX power supply and resistive loads, measure the 12, 5 and 3.3 V outputs unloaded and under load, and the ripple with the oscilloscope in AC coupling. Compare with the specification (±5 % and ripple of tens of millivolts). Repeat with a suspect power supply and compare the two waveforms.
Make a UTP cable with RJ45 connectors to the T568B standard at both ends. Test it with the continuity tester pair by pair. Then deliberately make one with two crossed pairs: it will probably link at 100 Mbps and fail at 1 Gbps. Measure the real speed by transferring a large file.
The teacher introduces a fault —cut cable, wrong static IP, bad gateway, invalid DNS— and the group finds it by applying the sequence of tests, noting what each one rules out. Repeat with all four different faults.
On a PC that won't boot, reduce it to a minimal configuration and add components one at a time until the fault appears. Note the board codes at each stage. It is the method that replaces the fewest parts and finds the problem fastest.
08Common mistakes
| Symptom | Usual cause |
|---|---|
| Parts are swapped until it works | No method. Measure before replacing. |
| Turns on and shuts off right away | Power supply protection tripping on a short circuit or overload, or the processor without cooling. |
| Loses the time when powered off | Dead CR2032 battery. |
| Freezes with no pattern | RAM with errors, or a power supply with excessive ripple. |
| The local network is visible but not the internet | Misconfigured gateway. |
| IPs respond but names do not | DNS misconfigured or not responding. |
| The address starts with 169.254.x.x | No response from DHCP: cable, switch or server. |
| The cable works at 100 Mbps but not at 1 Gbps | Badly terminated pairs: gigabit uses all four pairs. |
| A spark when connecting two devices | Missing grounding: the cases are at different potentials. |
| A board stopped working after being handled | Electrostatic discharge. You can't see it or feel it, and the damage can be progressive. |
09Self-assessment
For a device that reboots on its own, what do you measure first, and how?
The power supply: voltages under load and ripple with an oscilloscope in AC coupling. Measuring it unloaded is useless: many power supplies with worn-out electrolytic capacitors read fine without load.
What is the difference between a switch and a router?
The switch connects devices within the same network using MAC addresses. The router joins different networks and decides where each packet goes out: it is the default gateway.
A device has IP 169.254.3.7. What does it mean?
That it did not get an address from DHCP and assigned itself one. Check the cable, port, switch or the DHCP server itself.
Ping to 8.8.8.8 responds but not to a website name. What is failing?
DNS. Connectivity is fine; what does not work is the translation of names to addresses.
Why can a UTP cable work at 100 Mbps and fail at 1 Gbps?
Because 100 Mbps uses two pairs and 1 Gbps uses all four. A cable with badly terminated pairs or too much untwisted length links at the low speed and fails at the high one.
What is the 255.255.255.0 mask and how many devices does it allow?
It indicates that the first three octets are the network and the last one identifies the device: 256 combinations minus the network and broadcast addresses, 254 devices.
Why is DHCP not used for industrial equipment?
Because its address must be always the same: other devices and programs have it configured. With DHCP it could change after an outage and become unreachable.
What does an antistatic wrist strap protect, and from what?
It protects integrated circuits from discharges from the body, which reach thousands of volts and puncture the oxide layers even if you don't feel them. The damage can be immediate or progressive.
What minimum information must be recorded in a service call?
Equipment identification, the user's symptom, measurements and values, what was replaced, how the repair was verified, date and person responsible. Without that there is no history and repeated faults cannot be detected.
What does the SMART data of a drive tell you?
Its internal state: power-on hours, power cycles, reallocated sectors, read errors and —on an SSD— the percentage of life consumed. They let you anticipate the failure instead of waiting until the data is lost.