Is this proper behaviour for a switching regulator?

[mvno_subscriber]

I'm trying to fix a CD player (Project CD Box RS), that accidentally got connected to a power supply with reversed polarity.

I've replaced two L5970D switching regulators, as well as removed a few non-essential components that seemed to have shorted.

Now, one of the regulators seems to have died again, and the other one exhibits some weird behaviour. However, I'm both a novice and not very well versed with switching regulators. There are two on the board, presumably one for 5V and one for 12V.

The one we're looking at is the one for 12V. Both of the regulators seem to have been implemented somewhat like shown in the datasheet:



The only changes as far as my untrained eye can see, are 100µF input capacitor (in parallel with a ceramic capacitor), as well as a beefed up output capacitor of 470µF.

However, when looking at the output waveform on my scope, I get the following (before and after the inductor):





First, this can't possibly be acceptable ripple? And second, wouldn't it be smoothed over by the ouput capacitor? The ripple frequency seems to be 125Khz, although the datasheet specifies a 250Khz.

What could be going on here?

[fourtytwo42]

Can you post pictures of your construction, top & bottom please as well as bench test setup showing input & output leads etc.

[Microdoser]

You might want to check that your inductor is still good if the signal is the same before it as it is after it.

[mvno_subscriber]

You might want to check that your inductor is still good if the signal is the same before it as it is after it.

It's not quite the same, you can see the ringing applied by the inductor -- however, I'm not sure as to what degree it's supposed to be. I'm more puzzled the capacitor doesn't seem to affect the signal, but I get the feeling there's something very rudimentary I'm missing. How should I expect the curve to look like?

[mvno_subscriber]

Can you post pictures of your construction, top & bottom please as well as bench test setup showing input & output leads etc.

I didn't construct this, and I don't have a schematic. It's a commercial product. I'm using a bench supply to provide 20VDC in. I'll see if I can post some pictures this evening.

[mvno_subscriber]

Here's the top and bottom of the board. I've mirrored the bottom, so that the two images can be overlaid.

[fourtytwo42]

What is the input voltage ?
Pity the Y axis reference of your scope is random
I would think it's probably doing instantaneous current limiting, the probable reason is the diode is short circuit, take it out of circuit & test it.

PS re your previous comment about removing some components DON'T be tempted to run it without the full compliment, every one serves a purpose and you may blow things up without some of them (including this diode).
 

[mvno_subscriber]

Input voltage is 20V. The only components I removed were a couple of buffer chips related to an output, as these seemed shorted (they instantly got quite hot as soon as I turned on, and I suspected they were the cause of the weird power output).

I also removed the other voltage regulator, as it started outputting 20V instead of 12 (I think that was my bad, probably shortening pins while measuring with my clunky dmm).

Logic board is also disconnected, to avoid any damage from the power section.