Making a Ten-PC Lab Exam-Ready on ₹18,000
Our vocational IT lab in Cuttack has ten assembled desktop PCs. Each has an Intel i3-8100 processor, one 4 GB DDR4-2400 DIMM, a 450 W SMPS, a 500 GB SATA hard disk and an 18.5-inch monitor. The motherboard has two DIMM slots and supports up to 32 GB. Everything is powered through a 1 kVA UPS that also feeds one laser printer and one projector. Wall voltage dips noticeably between 7 pm and 9 pm. During the practical examination, PC-7 gives three short beeps at power-on and shows no display. PC-3 restarts on its own every 20–25 minutes, and more often while the printer is printing. The principal has sanctioned ₹18,000 to make the lab exam-ready and wants written recommendations. A vendor offers 8 GB DDR4-2666 modules at ₹1,600 each or 8 GB DDR4-2400 modules at ₹2,400 each; only one of these two options may be bought, and it must be the same option for all ten PCs.
1. Diagnose the faults on PC-7 and PC-3. For each, state the most likely cause, the evidence from the setup that supports it, and the checks you would perform before replacing any part.
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PC-7 - three short beeps, no display. Three short beeps at power-on is the classic AMI/Award POST code for a base memory (RAM) failure, and a blank screen fits a POST halt before video is initialised. Because beep codes differ between BIOS vendors, confirm against the motherboard manual first. Do the free checks in order: switch off and unplug, reseat the single DIMM, clean the gold contacts, blow dust out of the slot, then move the same module to the second slot. Next, substitute a known-good module. Only if a known-good module in a known-good slot still fails should you suspect the slot or the board.
PC-3 - self-restarts every 20–25 minutes. The key evidence is that the interval shortens under load (printing draws extra current and raises the room temperature). That points to a power or thermal cause, not to the operating system: dust-clogged CPU heatsink, dried thermal paste, a failing 450 W SMPS struggling during the 7–9 pm voltage dip, or a weak UPS battery that drops the load. Check CPU temperature and fan speed in BIOS, clean the heatsink, run PC-3 from a different SMPS and a different socket, and test the UPS on battery. Isolating one variable at a time is what makes the diagnosis defensible.
2. Recommend which RAM option to buy and how to fit it within the ₹18,000 sanction. Justify the choice with arithmetic and state the trade-off you are accepting.
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Buy the 10 × 8 GB DDR4-2666 modules.
- Cost: 10 × ₹1,600 = ₹16,000, leaving ₹2,000 of the sanction.
- The 2400 MHz option costs 10 × ₹2,400 = ₹24,000, which is ₹6,000 over budget, so it is not a real option even though it matches the existing speed exactly.
Fitting. Keep the existing 4 GB DIMM in slot 1 and put the new 8 GB module in the free slot 2, giving 12 GB per machine. The board has two slots and supports up to 32 GB, so 4 + 8 GB is safe.
Trade-offs to state openly. The 2666 MHz module will run at 2400 MHz because all modules run at the slowest common speed the memory controller supports (the i3-8100 officially supports DDR4-2400). That is harmless and costs only a small amount of bandwidth. Mixing a 4 GB and an 8 GB module means only part of memory runs in dual channel ('flex mode'), which is still far better than 4 GB alone for Windows plus a browser.
Before ordering: check the motherboard's qualified-vendor list and confirm the modules are unbuffered non-ECC desktop DIMMs. Spend the remaining ₹2,000 on thermal paste, dust cleaning and a surge protector for the printer rather than on one oversized stick for a single PC.
3. The UPS is rated 1 kVA. Decide what should be plugged into it and what should not, supporting your answer with an estimate of the load.
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1 kVA is apparent power; at a power factor near 0.8 the UPS can realistically deliver about 800 W, and only for a few minutes at full load.
Estimate. A desktop with an i3-8100, one hard disk and an 18.5-inch LED monitor typically draws 120–150 W. Ten such PCs therefore need roughly 1,200–1,500 W - already well above 800 W. So the whole lab cannot run on this UPS; attempting it will overload the unit and drop every PC at once, which is worse than an orderly shutdown.
Put on the UPS: the teacher's machine, the network switch and router (so marks and records remain reachable during an outage) and, if the exam is online, at most two exam PCs.
Keep off the UPS: the laser printer, whose fuser draws a 300–500 W surge on every print job, and the projector, which is not needed during a power cut. Both belong on a separate surge-protected socket. The printing-related restarts on PC-3 suggest exactly this kind of shared-load problem.
Also: test the UPS battery. A unit more than about three years old may give under a minute of runtime, and budget for a second UPS for the lab row.