Where is the inverter circuit in the late 2012 iMac?
some five years after purchase my late 2012 iMac (EMC 2544) suddenly went black with smoke (!) coming out at the top. Apple here initially diagnosed a broken power supply, and later claimed some additional (unspecified) damage, returning the machine without suggestion for a fix (well, buy a new one...).
Opening up the machine I found the power supply unharmed and working, but a mechanically damaged SMD capacitor on the logic board near the two display connectors. The capacitor was shattered, powering up the Mac produced fumes and sparks that had eaten their way around the area charring the circuit board. Not a nice sight. I removed the dead capacitor, cleaned up the charcoal, epoxied some insulation onto the board and soldered a new capacitor in place (SMD soldering is fun...). To my greatest joy the Mac now starts up without fire and smoke, and the four green LEDs on the logic board light up indicating everything is fine. However: the display remains black. Second however: an external display (attached via Thunderbolt-to-HDMI converter) works perfectly fine. And the System Preferences (Displays) do show the black internal display! This makes me think that the internal display is still sound, but that the sparks killed the inverter circuit for the LCDs backlight.
At long last here is my question: where is the inverter for the display?? In the previous iMacs it used to be a separate board. In the "late 2012" model there is no inverter board to be found. Me thinks the inverter circuit is now part of the logic board. Does anybody now more about this? And maybe know how to recognize such a circuit and defects in it? Any indication and help is highly appreciated.
With best wishes for 2017
two images added (@oldturkey03 thanks for pointing out how to do this!). First picture shows the location of the defect, second picture shows the shattered capacitor plus its specs. I did remove the broken one plus the intact one just above it to be able to identify it. Then I replaced both with a single SMD capacitor of double capacitance (4.7 microF), as the original two were connected in parallel. Soldering just a single SMD component in that damaged area was more straightforward than soldering two.
@danj System Prefs on an external monitor DOES show the black internal display (in addition to the external one), which does make me think the internal display is intact, and only some auxiliary circuit is blown.
It is my understanding that LCD monitor backlighting is either provided by an Electroluminescent Panel ELP or by LEDs. ELPs require an inverter to provide AC voltage, whereas LEDs run on DC. I was unable to find details on the backlight for that particular monitor, so I assumed it would be similar to the older iMacs that had an inverter. @oldturkey03 are you positive about LED backlighting being used in this case? If so, I stop looking for an inverter.
third image added, shows damaged area after repair: milled some channels into the PCB in order to remove charred material, backfilled the channels with epoxy to insure insulation, soldered new 4.7 microF/25 V capacitor in place. According to the schematic provided by @oldturkey03 (infinite thanks!) 25 V rating is too low, should be 100 V!
fourth image added, shows damaged area after re-repair: the wrong 4.7 microF/25 V capacitor was replaced by two correct 2.2 microF/100 V capacitors (piggybacked sideways). As I couldn't find them in 1206 package, I used the slightly larger 1210 package. Display flaw unchanged: internal display stays black, but shows correct image when illuminated with a flashlight. Voltage readings across the capacitors show 0 V indicating some short circuit. As I am unable to match actual components on the board with the schematic provided by @oldturkey03 and also have no clue what to measure on any accessible test point, I'll consider this horse comatose and just use an external monitor.
after 1. repair: two original 2.2 microF/100 V replaced by single 4.7 microF/25 V capacitor
after 2. repair: two original 2.2 microF/100 V 1206 replaced by two same rating capacitors of slightly larger 1210 package
@oldturkey03 Thanks for your help! Below is two more pictures of the identical PCB area back and front, before any repair attempts (for orientation: the bolt hole in both pictures is the same one; bottom right and bottom left show the identical end corner of the PCB). I hope they cover enough area for component identification per schematic. As for the broken traces issue: so far I looked at the board as having just one copper layer on each side, with a black protective coating covering everything. Am I wrong, is there actually two copper layers on each PCB side? Also, is it true that the schematic shows several versions of particular circuit sections, meaning that not everything shown in the schematic is actually present on the PCB?
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