Power Supply Problem/Repair

ak_47_boy · 1st July 2008, 05:34 AM

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.

Eva · 1st July 2008, 06:08 AM

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.

ak_47_boy · 2nd July 2008, 12:20 AM

Tested all diodes and rectifiers. They are all good. Could they only be failing to work at high frequency?

star882 · 7th July 2008, 11:36 PM

Looks like an overload. Check components on negative rails for partial shorts.

1st July 2008, 05:34 AM	#1
ak_47_boy diyAudio Member Join Date: Sep 2006	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.

1st July 2008, 06:08 AM	#2
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	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

7th July 2008, 11:36 PM	#4
star882 diyAudio Member Join Date: Mar 2007	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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2nd July 2008, 12:20 AM	#3
ak_47_boy diyAudio Member Join Date: Sep 2006	Tested all diodes and rectifiers. They are all good. Could they only be failing to work at high frequency?

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