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Apple Computing Products:
macOS - Desktop Hardware
Imac 21.5" 3.06ghz no chime/noscreen no led 3 and 4
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<blockquote data-quote="AndrewDun" data-source="post: 1846515" data-attributes="member: 404249"><p>Seven years later, I wonder whether Mikecana1 came upon anything helpful. Reading the thread, I felt reassured that there are others who expect to be able to repair most equipment themselves. I put myself in this category except where microprocessors are involved, and would never take something elsewhere for repair. What satisfaction would there be in that? (I have recently repaired three inverter welders with logic (control) board faults, not power semiconductor failures, but my Mac repairs have been limited to power supply repairs and rebuilding corroded PCB tracks.)</p><p>I have looked every so often into an iMac A1200, which displays very similar behaviour (black screen, no chime, fans full on, front LED full on, 2 (of 4) diagnostic LEDs on, hard drive and optical drive running). I have checked the power supply outputs (all correct) and various mosfets near the graphics card, all with plausible voltages. Also all connecting cables and their sockets. My reading leads me to wonder whether the graphics card is faulty, but on the other hand the fault came on completely suddenly, with perfect behaviour beforehand. Graphics card faults are usually described as gradually developing. </p><p>Any ideas? Apart from substitution, is there any way of positively checking a graphics card? And is there any way of procuring a circuit diagram (schematic), perhaps with test points and voltages or waveforms marked? And is the POST process in any way accessible? (I am a 70-year old engineer, and my fluency with electronics does not really extend far into the digital era.)</p></blockquote><p></p>
[QUOTE="AndrewDun, post: 1846515, member: 404249"] Seven years later, I wonder whether Mikecana1 came upon anything helpful. Reading the thread, I felt reassured that there are others who expect to be able to repair most equipment themselves. I put myself in this category except where microprocessors are involved, and would never take something elsewhere for repair. What satisfaction would there be in that? (I have recently repaired three inverter welders with logic (control) board faults, not power semiconductor failures, but my Mac repairs have been limited to power supply repairs and rebuilding corroded PCB tracks.) I have looked every so often into an iMac A1200, which displays very similar behaviour (black screen, no chime, fans full on, front LED full on, 2 (of 4) diagnostic LEDs on, hard drive and optical drive running). I have checked the power supply outputs (all correct) and various mosfets near the graphics card, all with plausible voltages. Also all connecting cables and their sockets. My reading leads me to wonder whether the graphics card is faulty, but on the other hand the fault came on completely suddenly, with perfect behaviour beforehand. Graphics card faults are usually described as gradually developing. Any ideas? Apart from substitution, is there any way of positively checking a graphics card? And is there any way of procuring a circuit diagram (schematic), perhaps with test points and voltages or waveforms marked? And is the POST process in any way accessible? (I am a 70-year old engineer, and my fluency with electronics does not really extend far into the digital era.) [/QUOTE]
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Apple Computing Products:
macOS - Desktop Hardware
Imac 21.5" 3.06ghz no chime/noscreen no led 3 and 4
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