EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Axk on May 06, 2020, 05:59:38 pm
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I admit I didn't do much research on the topic.
A small 0.6A 9V TP-LINK switch power supply stopped working and is measuring 0V on the output.
The electrolytics are not bulged visually, the capacitance values are fine (I don't have a meter to measure the ESRs)
The solder joints look fine to me.
The connection between the AC plug and the board is fine, the connection between the board and the 9V barrel connector is also fine.
I do have an oscilloscope, but I don't have high voltage probes for it, so I'm afraid to use in this case.
What should I look at first?
Is there anything I can measure with a multimeter?
Should I just go in ad re-solder all the solder joints first and see if it helps?
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The two electrolytics below L1 are visibly bulging. You can check the fuse, diodes and caps with a DMM. What are the rating of the two bulging caps? That will tell us if they on the hv or lv side of the unit.
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These are on the high side - 400V 105C
Look fine to me, the split tops look flat (or nearly flat), they are in focus on the first photo.
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It must be distortion in the photo. Dunno.
Did you check the fuse or ntc thermistor? Cannot tell what it is because it’s inside the black heat shrink where the mains connects. It should be a low ohmic device, whatever it is.
PS: I don’t see any reason to resolder anything. Check the actives and passive devices in both the hv and lv power loops for opens and shorts.
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Can you check the fuse, rectifying diodes and L1 with a multimeter?
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I think you will probably find that the fuse, or fusible resistor - inside that loop of heatshrink sleeving, has blown. If so, the question is why?
If you are very lucky, you may find that one of those four diodes that make up the bridge rectifier on the primary side has failed shorted (easy check with your DMM). If so, it is an easy repair. If not, then more detailed diagnosis will be needed.
You are wise to worry about using your scope. Keep it well away from this.
Edit: Ha, three very similar replies.
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I would guess the primary side of the transformer is open?
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1)The black heat-shrunk thing is marked as "10ohm 2W" the board and measures 9.7Ohms.
2) L1 measures 5.8Ohms (I don't have an LCR meter to measure the inductance)
3)The blue round epoxy blob thingy marked CY1 (I suppose it's a capacitor) measures 0.9nF
4) Cannot measure the other ceramic cap C2 for some reason, it's not short (467kOhm) but the multimeter would keep autoranging with a blank screen, it may be because it's connected in parallel with something I suppose
5) The diodes all look fine (the 4 rectifier diodes 0.56V, 2 other diodes on the primary side (0.56V and 0.49V) and a diode on the secondary side (0.22V)).
6) The transistor is 0.63V B-C, 0.41V B-E, Closed C-E, 0.55V E-C, 0.6V E-B. It may be because I'm measuring it in circuit, I should de-solder it and measure on its own I suppose (https://pdf1.alldatasheet.com/datasheet-pdf/view/449110/WINSEMI/SBR13003D.html)
7) Not sure now to measure the transformer because it has 5 connections on the primary side and I'm not sure which are the primary winding.
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And what about FB1 (or is it F81)?
If you don't mind powering it, you could measure the voltage across C1 and C2,
it should be around 330Vdc (assuming you have 230Vac).
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A couple of other possibilities then...
- Check the small electrolytic capacitor next to the big 1612 on the silkscreen. If this is the reservoir cap of the switching IC supply and has dried out, it can affect startup.
- It might be the photo angle, but the solder joints on the transformer pins, particularly near R13, don't look good.
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I would look at the switcher semiconductor. That is very often what fails.
Historically, the switching component, a transistor of some type, will fail due to hot spot overheating on the die. This has been a major problem for power supply people because it's almost black magic to determine the safe region for operation. As a result, suitability of a particular part is often determined at test. If it doesn't fail for a week, it must be good, but we can't do this with every circuit.
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The inductor is fine so long as there is continuity. At this point, it’s not looking like a job for Captain Obvious so here’s just one more thing I can think of:
Depending upon usage, one sometimes seen failure on wall warts are wire fractures at the dc plug and strain relief where the cord enters the plastic housing. I’d ohm out the cord from the pc board to the dc plug inner and outer connections.
Oh, and one other thought after rereading your last post, re: the transistor. What does C-E “closed” mean? Open and shorted, great, but closed?
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@WattsThat, I meant open )
Thanks everyone for your help! will continue testing the remaining suggestions tomorrow.
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I suspect C4 needs changing
However look closely at my photo the joint, it looks like a cold colder joint, photos don’t always do justice there may be more.
[attachimg=1]
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Not only looks like a failed solder joint, it looks as though it
was actually arcing!! So now it may indeed be a failed connection.
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This must be a photo/lighting issue. The joints look fine in person.
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Any updates regarding this?
I've gone through this post, just plug it in and measure voltage across your caps.
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The caps measured 290V, but now I accidentally shorted it to something else with on of the probes and blew the fuse (
Will de-solder and measure the transistor next.
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Looking for a replacement for the resistor fuse, appears to be uncommon.
Cannot find anything of this type from my local supplier and Aliexpress.
They have what they call thermal fuse, but these are rated in terms of max temperature and not resistance/wattage like this one.
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Hi,
You can buy the whole pcb on Aliexpress or Banggood.
It is the same board, but a different revision.
Search "TP-LINK 9V 1A Board"
[attachimg=1]
[attachimg=2]
Regards,
Jay_Diddy_B
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Thanks!
I've already bought a replacement, just trying to repair it out of curiosity.
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1) The transistor looks fine measured out of circuit.
Out of circuit the NPN (marked BR 13003D on the case) measures as follows:
B-C = 0.55V
B-E = 0.6V
C-B, E-B, C-E = open
E-C = 0.64V
Considering that it has a diode across E-C in one of the datasheets (https://pdf1.alldatasheet.com/datasheet-pdf/view/449110/WINSEMI/SBR13003D.html) I suppose the trasistor is fine.
2) The smallest electrolytic near the chip measures fine (4.4uF, marked as 4.7uF) and has no visible bulging.
So I suppose I'm out of luck at least for now.
Maybe will do additional live measurements when and if I get a replacement for the fuse resistor that I've blown.
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Hi,
What kind of bench power supplies do you have?
It is possible that this is circuit may work at a lower voltage. If it won't run, you may see signs of life at around 40V.
Do you know what the 8 pin chip is? or have the numbers been removed?
Regards,
Jay_Diddy_B
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I don't have a bench power supply (
The marking on the chip reads ..3706M E1 5485H9
Not sure what the first digit before the 3 is. Looks like they tried to scrub the number off or something.
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The replacement supply (exactly the same model) that I bough is also dead )
I just plugged it right into the switch and it didn't turn on and it also measures 0V.
I wonder if it could be the switch that's killing the supplies or just a supply from old stock with the same issue. I didn't measure the output before connecting to the switch (
Now I have a second one to repair )
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I hope you have an isolation transformer if you are going to snoop around with a scope probe
or any other test gear that is earth grounded!!! If you try to snoop on the line voltage side
you are going to have a surprising amount of smoke and flame.
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What voltages have you got and where? Knowing where you lose voltage may help you find the fault.
Edit:
First things I would check, provided no obvious damage to components or traces would be voltage in and voltage out of the transformer, rectifier and regulator. Should give you an idea of which part of the circuit is faulty.