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Old 31st December 2012, 04:51 AM   #2971
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Consistent polarity isn't really necessary; it's the absolute values that are important at this stage. But, keeping the black probe connected to the ground point will give the proper polarity.
Your circuit has virtually the same voltage into the positive regulator as it has at the output of the regulator. Replace it and repeat post #2968.
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Old 31st December 2012, 05:11 AM   #2972
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Sorry, to clarify, which component should I replace?
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Old 31st December 2012, 05:55 AM   #2973
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That would be U6.
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Old 31st December 2012, 06:39 AM   #2974
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I just replaced U6 after some difficulty. The values appear to be the same. Are you sure I shouldn't replace U5 instead? I checked the BOM and U5 is the positive regulator while U6 is the negative regulator.
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Old 31st December 2012, 07:05 AM   #2975
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My apologies. It is U5, the positive regulator.
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Old 3rd January 2013, 04:40 AM   #2976
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I just replaced U5.

Right now, with the AC adapter plugged in but the amp switched off, the voltage for C5 reads 25.6V, C4 reads 26.3V, C7 reads 12.0V, but C6 apparently is 0V. I tried all ranges of my DDM and several probe positions but was unable to get a non-zero value.
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Old 3rd January 2013, 05:46 AM   #2977
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Measure the voltage right at the output pins of the regulators.
If need be, you can find the pin assignments in the datasheets.
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Old 3rd January 2013, 06:25 AM   #2978
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Okay. The negative regulator U6 measures 12V, which I think is okay. However, the positive regulator U5 measures nothing at all for both input and output voltage. So obviously something before the regulator is spoilt. How do I determine which component is down?

Thanks for the speedy response so far, sofaspud, really really appreciate it
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Old 3rd January 2013, 07:01 AM   #2979
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I'm becoming more puzzled. You replaced U5 but still got no regulated output, and now the input voltage is missing?
What is the rating for your transformer? You wrote "C5 reads 25.6V, C4 reads 26.3V" which could put a dissipation strain on the voltage regulators, though it shouldn't with the O2 turned off. Remove the batteries if they are installed just to be sure, and to help isolate the power supply circuit. Check the D3 & D4 diodes. After that, remove C2, test the regulator input and output voltages, then put it back in and repeat for C3, C4, and C5 one at a time. If you haven't already, double check that there are no solder bridges shorting the power supply. Or a PCB trace that became a (opened) fuse when you had that little shorting accident. A good inspection and test of the dozen or so components in the ps should make the problem jump out at you.
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Old 3rd January 2013, 07:57 AM   #2980
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Okay. Across D3 is 26.7V, across D4 is 26.1V. Across C3 and C5 is also 26.1V, whereas C2 and C4 are also 26.7V. I also measured the + end of C4 into the input pin of U5, the positive regulator, which also measured 26.7V. The puzzling part is that the voltage across the input and the ground pins of U5 is zero.

U6 does not have this problem. It measures 26.1V from input to ground, and 11.9V from output to ground.

I just replaced U5. Could it be that the new U5 is defective?
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