Power suppy TL494 waveform

[Perry Babin]

Most people never use the second channel so when you stated that they were aligned, I assumed (incorrectly) that you were looking at them one at a time (single channel) which would always make them appear in the same place on the display.

If they were aligned (using both channels), the IC failed in a way that made it change output modes. This is controlled by pin 13. When pin 13 is low (near ground), both outputs are in phase. When pin 13 is high (~5v), the outputs are out of phase. This is how the IC normally operates (outputs out of phase) for car audio amps.

I didn't punch the numbers into a calculator but you used the right method to calculate frequency.

[jol50]

I was always under the impression that the overlap of the waveforms was a very important thing to check with any PS issue, and that is the only way to see it. Plus it is easier to see any difference between them, if any. And sometimes its fun to hook more stuff up, play with more buttons and dials... If I were just browsing a working amp I might not hook both up, and suppose current draw is going to tell you if you could have a serious problem with overlap. I use the scope any time I can because I don't use it that much. It does not tell the voltage on the screen either, I have to count lines, so it is not as nice for plain voltage checking. Wish I could add a volt meter to it. I was looking at the audio section for shorts before I put the scope on it.

Unfortunately I didn't look closely at the 494 because it was working. I might have a junk board, if so I can have it in there in a few seconds. If the 5v were gone I would have caught that, though I forgot all about pin 13, and guessing a problem with it is rare. Pin 13, 14, 2 are connected and some of the protection circuits, or transistors of protection circuits. Also via resistor to pin 4 that connects via another resistor to ground. The power LED did not work either, but I assumed at 7-8v supply it might not.