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Old 1st July 2008, 04:34 AM   #1
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Default Power Supply Problem/Repair

I have an oscilloscope with a bad switching power supply. I have it out of the scope and set up with the correct dummy loads for testing. I see no visible burnt components or capacitor leakage. The switching transistor gets very hot, i would say about 80C (180F). It should output:
+12, +12, -12, -5.2, +3.5, +5.2
I measure:
+12, +12, -2.6, -2.6, +3.5, +5.2

Old electrolytic caps? Bad regulator? It is a double sided board and quite compact so i am having a lot of trouble following traces. I think i am going to start desoldering and testing all the components individually.
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Old 1st July 2008, 05:08 AM   #2
Eva is offline Eva  Spain
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Find the diode that feds the negative rails, it may be the same for both, being the -5.2V one derived through a linear regulator. Check that it's not shorted.
I use to feel like the small child in The Emperor's New Clothes tale
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Old 1st July 2008, 11:20 PM   #3
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Tested all diodes and rectifiers. They are all good. Could they only be failing to work at high frequency?
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Old 7th July 2008, 10:36 PM   #4
star882 is offline star882  United States
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Looks like an overload. Check components on negative rails for partial shorts.
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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