Repair of the Week: Mac mini with No Video

This week’s repair was especially frustrating. On a day with Jon and Rebecca out of the office, I was in the tech room turning screws and diagnosing machines on top of my normal duties. Rebecca had ordered a logic board to resolve a bad RAM slot issue in this Mac mini (discovered during a RAM upgrade), and it arrived the next day.

Mac minis are very simple machines to work on. Installing a new logic board takes about 10 minutes, and there are only six or seven screws to deal with. I installed the replacement board, but when I powered on the machine, there was no image on the connected external screen. I verified the screen was working by plugging in my MacBook Pro, then double-checked my work on the mini. After performing SMC and PRAM resets and ensuring my RAM was good, I marked the board dead on arrival and ordered another.

I was alone in the tech room again the next day, and installed the new logic board, only to find the same symptom. It’s not common to need to mark anything dead on arrival, let alone receive two boards with the same symptom; in fact, it has never happened to me before.

I checked out the technician guide, suspecting something might be amiss with my reassembly. After realizing I reassembled everything correctly, I began component isolation. Component isolation is the process of whittling down a computer to its most basic configuration, and adding parts back one by one until the problem component is revealed.

In a Mac mini, minimum configuration is logic board and power supply. I pulled the logic board out of the enclosure, attached the power cable and display cable, and shorted the power switch on the board itself to isolate the power button from the equation. The same behavior persisted.

It was late in the day on Friday, and I had no choice but to update our customer that it would be at least Monday until we’d be able to get his computer back. I figured I’d unplug the backup battery over the weekend so the board could get as low-level a reset as it possibly could.

Monday morning came around, and I reconnected that button. Amazingly, the computer fired right up and displayed video perfectly. Our customer was happy to have the machine back in working condition shortly after 9AM.