Troubleshoot l4973d3.3 based power supply subsystem

[handle]

Hello,
I'm new on this board, in fact, this is my first thread. Hoping to not be too rude, I'm going to explain why I'm here asking you for help.
I own a Polycom Dect system which suddenly stopped working, my wish, is to be able to fix it up.
This system takes its power from a PoE subsystem which seams to work properly.
The Ethernet switch which suppliws the power, reports it have negotiated the PD with it, which among other things, it means that remote PoE device is working.
Still she system does not start, and a switching regulator which is on the board, becomes incredibly hot.
Supposing the switching regulator was faulty (ST Micro L4973d3.3) and it was the root of my problems, I tried to replace it.
During the replacement process, bu after I powered up the device at least a couple of times, I also noticed that a diode marked (top mark code PJ85 - SK56) is shorted on both directions, so I comed to think that maybe this, was the root of my problems.
The diode I'm speaking of is shown in the l4973d3.3 reference design (datasheet) as D1, and seems to have the function of Transient Voltage Suppression, but I'm not sure even of this.

As you may have noticed, I'm approaching the troubleshooting in not professional way, after all I'm not a professional.
Please feel free to give me advices also on this matter, I would appreciated this too.

Here the requests I want to pose you:
* Which probability is that the L4973d3.3 was effectively broken, and which is the probability I broke also the one I used to replace the previous after started the system?
* Is there an easy way to state if any of these regulators are faulty?
* Is it correct to state the diode is bad, just by if I testing it with multimeter if it shows shortcut in both directions? Does this test sufficient to state it is broken? If I need to replace it, where can I lookup top mark code PJ85 to purchase a new one?
* Should I look for other broken components in my board on such situation?

Thank you for any of your answers.
Regards.
Alessandro

[handle]

Re: Troubleshoot l4973d3.3 based power supply subsystem

Hello,
I think I managed to answer at least one of the question I posed in previous post, maybe the easier to answer, the name of the diode I suspect to be bad. In my question I used PJ85 as top mark code. It is written indeed on the package, but it is not its top mark. There is also another code which is SK56, that is its top Mark.
Evidences I found seems to converge on "PANJIT semiconductor" SURFACE MOUNT SCHOTTKY BARRIER RECTIFIER VOLTAGE 60V 5.0A.
The easier is answered but all the rest is still there, I would really apreciate some help.

[R_J]

Re: Troubleshoot l4973d3.3 based power supply subsystem

Post some good quality pictures of the board (both sides) and any schematic if you have it.
You may want to include any voltages, for example Vin and also OUT, If there is no V OUT you may have a short somewhere else. check the resistance of VOUT to ground.

[handle]

Re: Troubleshoot l4973d3.3 based power supply subsystem

Hello R_J,
Thank you for your precious advice.
I'm anything but skilled in electronic circuits troubleshooting, so I really appreciated the advice to measure resistance between Vout and ground, very clever.
It is about 5k ohm, I would say not other shorts are out there, but what if a broken component opens the circuit rather than short it?
Here a picture of the board, only the upper side. I do not have pictures of the bottom, however bottom has not components, not even a resistor. If needed I'll find a way to take a picture of the bottom.
https://photos.app.goo.gl/V41dRADCGPHFmpMi9
My attention is on the two 220uF electrolytic capacitors on the reference circuit of the L4973D3.3 which on the picture I shared with you only one appears. If you look at the top of the metal package you may see some signs not appearing on the other electrolytic capacitors. On the picture, only one appears, but both have those signs. I learned that through hole electrolytic capacitors, if they have inflated top, may be faulty. Don't know it it is a rule, but from the bottom of my experience, it seems to be a fact.
Is there anything else you suggest me to do?

[handle]

Re: Troubleshoot l4973d3.3 based power supply subsystem

According to what I read on this page(https://sciencing.com/test-schottky-diode-6920100.html), and what I comprehend from it, my schottky diode is faulty.

"Reverse the test leads of the multimeter by placing the positive test lead to the cathode and the common test lead to the anode of the diode. Observe whether the multimeter emits a tone. If the multimeter does not sound a tone, the Schottky diode is functioning correctly."

So to recap my situation:

• I still need to determine if the switching regulator (L4973d3.3) is in good conditions or it is faulty. An indication on how I can test it easily would be very apreciated.
• I still need to understand If other components on the board are faulty. For example some capacitors have signs on their top. Is that an evidence they have been shocked and not usable or faulty?
• At a given time, I will have the need to power this thing up again, is there any precaution I could use before I do it again to avoid other damages to the board components?

[R_J]

Re: Troubleshoot l4973d3.3 based power supply subsystem

The ic does have overload protection so maybe it is only the diode that is bad.
Replace the diode and see if you have output voltage, The circuit is very simple so not much can go wrong.

[handle]

Re: Troubleshoot l4973d3.3 based power supply subsystem

Thank you for the feedback.
I really appreciated that.
I need some time to recover the hardware needed, the diodes of that kind are not that common.
In the mean time, I'd like to read your opinion on the capacitor issue I mentioned in my previous posts. Is it normal for capacitors in good conditions, to have signs on their top?
The idea that also capacitors can be faulty is bugging me and I'm bugging you. :$

[R_J]

Re: Troubleshoot l4973d3.3 based power supply subsystem

In the mean time, I'd like to read your opinion on the capacitor issue I mentioned in my previous posts. Is it normal for capacitors in good conditions, to have signs on their top?

If you mean the (K) mark, that is normal, if the cap goes bad it will break along the cut lines and vent. those two caps look ok, but you can never tell just by looking at them unless they actually look like they bulged or vented.

[handle]

Re: Troubleshoot l4973d3.3 based power supply subsystem

Thank you again fo your answer.
Yes I meant the "K" mark on top of the capacitors.
If you look at the picture I shared with you, you may see that not all the capacitors have a "K" mark on their top, infact, just a couple has marks, others do not have any, not a "K", not any other. My guess were that at the beginning no capacitor had, or to say it better they all have cuts on their top, but they were not observable until they inflate because a malfunction.
I were probably wrong, I trust you because you are far more experienced than me.
Now I'm waiting for the replace components shipment.
I'll write again on this thread as soon as I replaced the components.
Thank you again for your help.
Regards

[handle]

Re: Troubleshoot l4973d3.3 based power supply subsystem

Hello,
I managed replace both components: the switching regulator and the Zener diode, just to be sure one of those two was faulty.
I struggled one night fighting with the soldering flux residue which appeared to be conductive.
It is the second time in my experience that soldering flux residue makes my work look faulty.
Do you have a special technique to remove residues after your work?
I used alcohol, but after that I had to use hair drier to dry all the board.
However my dect base station works again, thanks also to your suggestions.
Thank you for your help.