Bestec ATX-250 12E going bad - waveform analysis

[everell]

I have worked with Bestec ATX-250 12E power supplies that were working OK, and I have worked on/fixed Bestec ATX-250 12E power supplies that were "blown". At last, I have one that is in between.

The critical capacitor in the 5vsb circuit is labeled C1 (see attachment). Its value is 10Mfd/50 volts. This capacitor is NOT in the 5vsb line, but in the feedback side of the 5vsb transformer. It is part of the regulation circuit. To determine if this capacitor is giving a problem, you can connect an oscilloscope across the capacitor. Two pictures are attached, a picture of a good capacitor with a nice straight line. The picture of the bad one shows a peak voltage of 16.4 volts and lots of signal. The large signal is what is confusing the regulator and causing the problem. In this case the 5vsb was measuring 8.4 volts. The critical capacitor measured ESR = 43.

Warning: when using an oscilloscope to measure this capacitor, you must have an isolated ground. If you hook the scope probe across this capacitor without proper isolation, it will blow your scope!

[xchequer]

Re: Bestec ATX-250 12E going bad - waveform analysis

Great scope pics of bad capacitor. Is that noise or oscillation? Try changing the scope time base and see what kinda results you get.

[everell]

Re: Bestec ATX-250 12E going bad - waveform analysis

Great idea. Here are some scope pictures of the same signal using different time bases on the scope. Looks like lots of oscillations on different frequencies instead of just noise. So what do you think?

[xchequer]

Re: Bestec ATX-250 12E going bad - waveform analysis

It looks more like oscillations than noise. I guess if you need an oscillator you can grab a screwed up capacitor! If you can get a hold of a spectrum analyzer it would be interesting to see a fft analysis.

[everell]

Re: Bestec ATX-250 12E going bad - waveform analysis

My USB Stringray scope does have fft, so here is a picture of the signal in fft style. Not sure as to exactly what it means, but it is neat to play with the scope and experiment with all the knobs. Perhaps some of our other experts can tell us more about what we are looking at!