+OLD CS1 Posted December 19, 2017 Share Posted December 19, 2017 On my desk is a 1541 with PCB 1540050. Neither the drive motor nor the head stepper will run. I did some testing per Ray Carlson's page and found the these values: Orange: 13.7VACBlue: 8.9VAC CR4: + 5.13V - 9.67V CR2: + 0.61V - 15.02V Compared to a good 1541 (white body, did not record the PCB number): Orange: 15.3VACBlue: 8.7VAC CR4: + 5.06V - 9.12V CR2: + 12.10V - 19.65V Given these values it looks like the 7812 12V regulator may have failed, which I can believe as the heat sink on the regulators becomes scalding hot -- as in, left a mark on my hand. I read the 2V difference in the power input from the transformer and 4V difference from the bridge rectifier as a potential result of a bad regulator. I do not recall ever having to troubleshoot the power in one of these, so I ask those who have if they would concur with my assessment? If so, I am considering this unit to be the guinea pig for replacing the old power supply with a new switching supply. Quote Link to comment Share on other sites More sharing options...
norm8332 Posted January 9, 2018 Share Posted January 9, 2018 There is the possibility of a shorted tantalum capacitor on the board. I had a drive with the same issue and it turned out to be that the short was dragging down the 12V and overheating the regulator. It was one of the "blue bead" capacitors in layman terms. See the very end of this document. http://personalpages.tds.net/~rcarlsen/cbm/1541/fix1541.txt exerpt: "TANTALUM CAPACITOR C15 3 can burn up or short and drop 12V power" Quote Link to comment Share on other sites More sharing options...
+OLD CS1 Posted January 9, 2018 Author Share Posted January 9, 2018 I will check that cap later this week and update. I have actually already completely disassembled the power circuit of the board in preparation for replacement by a switching PSU (the guts of an external hard drive power supply.) Quote Link to comment Share on other sites More sharing options...
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