Ticking PSU

[momaka]

Re: Ticking PSU

Great to hear you got it running again.

Yes, with self-oscillating discontinuous flyback designs, you won't see a "set" frequency nor any particular waveforms (especially sinewaves). They kind of "run wild", so to speak.

But what I like about 2-transistor designs is that you can always find parts for them, as they don't require any special ICs. On the downside, 2T designs lack protections... but that's the trade-off.

[madan1]

Re: Ticking PSU

Here are the waveforms without load ( for 12V and 5V ).
With load the fluctuation is about 15-20mv and frequency of ~25khz and ~130khz for the two V rails ( no clear sines though and lots of noise ).
Fortunately, the HDD seems to be happy with that. The mainboard also manages to filter the supply and for now everything works fine.

P.S. - no ticking sound at all!
Btw, when the "ticking" was present, the NAS was running in emergency mode with almost none functionality and refused to re-build its software.

[madan1]

Re: Ticking PSU

Thank you for all replies!
I installed new ZD 27V and C107 (22pF ) and got back the voltages on the secondary side.
Then put the correct bus capacitor ( 47/400 ) and now I do not have any visible ripple on my "scope" ( I'll attach images soon ).

[momaka]

Re: Ticking PSU

this is what bugs me most - if I will ever be able to fix the PSU without knowing what the z103 is and what should be the values of the rest of the components.
...
p.s. I checked again the bad diode and this time was able to read 27B on it ( thanks to the macro camera ).

If I am not mistaken, that part number suggests a 25V Zener diode... which more or less makes sense. In this particular case, the 2SK3567 MOSFET has a absolute maximum Gate-Source voltage rating of 30V. And we can see that this MOSFET's Source is connected to primary-side ground and its Gate is positively biased from the +170V/340V DC bus (the positive leg of the primary cap) though resistor R101. Therefore, to make sure the MOSFET doesn't get damaged, ZD103 must have a rating of less than 30V. 25V is reasonable, I'd say. But it would probably work with a lower-rated Zener diode too, in case you can't find a 25V Zener exactly.

Also, I just noticed, but are you sure ZD102 is drawn correctly on your diagram? If it is, it would seem to limit the Gate voltage to about 0.7V, which doesn't make much sense.

As for C107... most likely, that should be a few hundred picoFarads (pF) up to maybe 1 or 2 nanoFarads (nF). I don't see any SMD components on your PSU, so unless the cap is blown into bits and pieces, you should be able to read the capacitance off of its case.

[Per Hansson]

Re: Ticking PSU

The Zener diode might be ok, the transistor tester can only output a low voltage, because it's powered by a 9v battery.
Try using a bench power supply to test it, plenty of tutorials available on Youtube for it...
E.g this one: https://www.youtube.com/watch?v=5OcoF0nkgng

P.S: You don't need to set the voltage like he does, just give it 30v and if the Zener works it will show the voltage.
Obviously the current needs to be limited else the magic smoke will escape.

[madan1]

Re: Ticking PSU

All components from the above diagram were tested out of the board on a "M328 transistor tester" and DMM.
Both transistors are recognized properly. C107 and Z103 are read as "unrecognized"/no capacitance and shorted.

" These 2-transistor designs don't like it at all when any parts go bad. Thus, if you ever need to fix one, know that you have to check everything." - this is what bugs me most - if I will ever be able to fix the PSU without knowing what the z103 is and what should be the values of the rest of the components.
Thank you for the diagrams!

p.s. I checked again the bad diode and this time was able to read 27B on it ( thanks to the macro camera ).

[momaka]

Re: Ticking PSU

this is what I was able to get as a diagram from the actual board

https://www.badcaps.net/forum/attach...2&d=1543774648
( see previous page, last post for details )

Your diagram appears correct for a 2-transistor circuit, more or less.

Z103 is likely used as protection on the Gate of the 2SK3567 MOSFET.

Did you check C107 out of circuit, by the way? It is connected between Drain and Source of the 2SK3567 MOSFET. So if you got that reading in circuit, that could indicate the MOSFET is bad instead.

These 2-transistor designs don't like it at all when any parts go bad. Thus, if you ever need to fix one, know that you have to check everything.

That said, in case you are interested, here are some 2-transistor circuit diagrams I drew of some ATX PSUs in my collection:

One from a PowMax ATX PSU...
https://www.badcaps.net/forum/attach...0&d=1451693563
This one is probably the closest to what you have in your power adapter:

And below are some CWT (Channel Well Tech.) designs... with different revisions from CWT throughout the years, hence the variation. They seem a bit more elaborate, but they still work the same.
https://www.badcaps.net/forum/attach...4&d=1407035562
https://www.badcaps.net/forum/attach...1&d=1406952554
https://www.badcaps.net/forum/attach...9&d=1443215464

[R_J]

Re: Ticking PSU

That could explain the waveform, most smps that use a dedicated ic employ some kind of zero cross switching. without it the power supply switches the primary anplace in the sign wave.

[madan1]

Re: Ticking PSU

I guess it just uses a fet and few components to make it switch, I've seen similar power supplies in dvd & older vcr's

this is what I was able to get as a diagram from the actual board


( see previous page, last post for details )

[R_J]

Re: Ticking PSU

I guess it just uses a fet and few components to make it switch, I've seen similar power supplies in dvd & older vcr's

[madan1]

Re: Ticking PSU

There is no IC on the board. I'm not familiar with PSU designs and definitely hate working with high voltages, but I guess there is some kind of an oscillation occurring in the highV part which is making it to switch.

[R_J]

Re: Ticking PSU

Without knowing what the smps IC is, it could just be in a low power mode since its not under load. I would not worry about it myself.
It can't be from the actual switch circuit or anything on the secondary as the power supply runs at around 30-50 plus kilohertz.

[madan1]

Re: Ticking PSU

Are those waveforms in post #10 with a load on the power supply or without a load?

The photos are without load, but I'm able to trace the ripples down to the connected pcb.

I see the blue caps are Ltec brand. In my experience, these are very unreliable caps. They seem to go bad quite often in Delta, HiPro, and Lite-ON ATX PSUs. In fact, I just posted a thread about one of these PSUs two weeks ago, showing how badly these caps can fail without any visual signs at all:

Therefore, I suggest you to not trust these Ltec caps and replace them preemptively (especially if doing this for a customer - high chances it will come back within a year or two down the road).

All caps were measured out of the board and only one was found with 30-40% lower capacity.. and of course the 47/400 one which seems the have internal failure not related to its capacity.
The board is for a NAS which I'm going to use at home, so it is not a problem to keep an eye on the PSU and its caps




Unfortunately, I "killed" the PSU by discharging the 100/400 cap with a srewdriver.. most likely the arc jumped to something else and now I do not see anything on the low-V side.
Here is what I was able to draw as a schematic for the hi-V side ( see attached, the rectifier part is skipped ).
The bad components which I found are ZD103 and C107. I was not able to find the same design, but in others they show it as "17-18V"(?). Also, ZD102 on the board is labeled as "ZD", but the actual component is detected just as a diode.
The big transistor is K3567, the small one - C945

At this moment I 'm starting to think that it would be better just to get one of those https://www.aliexpress.com/item/12V1...310683501.html instead of fixing it.

[budm]

Re: Ticking PSU

"It's actually a Y2-rated cap between secondary-side ground and primary-side negative (-) bus (i.e. connected to the negative lead of the big input cap.) I believe this is done for attenuating EMI/RFI to and from the PSU." That is correct, I use X/Y rated cap for that purpose to pass EMI compliance testing, but since one side is connected to primary GND circuit (not safety GND), that means the cold side circuit GND which will have some AC current flows dues to Xc at 50Hz (small level) through scope safety GND, thus the 50Hz showing on the scope.
It sure does look like -2Transistor SMPS setup.
BTW, if op measure the AC Voltage between cold side GND and the safety GND, he will see AC Voltage present.

[momaka]

Re: Ticking PSU

Faint ticking without any load is normal for some discontinuous flyback power adapters, but not all of them, and certainly not with a load. So most likely that failed primary cap was the issue. Other times, this can happen if the small "start-up" cap on the primary fails... though I don't see one on your power adapter. So that and the lack of an PWM controller tells me this is likely a 2-transistor self-oscillating design with feedback. And for this design, ticking is not normal.

I desoldered and measured all 6 electr. caps and 5 of them were fine. The faulty one was the green one ( right next to the 2 big ones on the low V side ).

I see the blue caps are Ltec brand. In my experience, these are very unreliable caps. They seem to go bad quite often in Delta, HiPro, and Lite-ON ATX PSUs. In fact, I just posted a thread about one of these PSUs two weeks ago, showing how badly these caps can fail without any visual signs at all:

Therefore, I suggest you to not trust these Ltec caps and replace them preemptively (especially if doing this for a customer - high chances it will come back within a year or two down the road).

I just look at the board and I can see X cap (blue disk cap in black shrink tubing) connected between hot side and the GND cold side so I believe that is the reason for seeing the 50Hz.

This.

It's actually a Y2-rated cap between secondary-side ground and primary-side negative (-) bus (i.e. connected to the negative lead of the big input cap.) I believe this is done for attenuating EMI/RFI to and from the PSU.

[budm]

Re: Ticking PSU

I just look at the board and I can see X cap (blue disk cap in black shrink tubing) connected between hot side and the GND cold side so I believe that is the reason for seeing the 50Hz.

[R_J]

Re: Ticking PSU

Are those waveforms in post #10 with a load on the power supply or without a load?

[stj]

Re: Ticking PSU

leaky diode in the primary rectifier maybe??
also, connect the supply negative input to mains, but not the live wire - see if you get the 50Hz

[madan1]

Re: Ticking PSU

I am curious, If you do not have the power supply connected to the AC outlet, what do you see on the scope (leave the probes hooked up the same way)?

There you go.
Leads hooked to the same wires ( black and orange ), no AC.

[budm]

Re: Ticking PSU

I am curious, If you do not have the power supply connected to the AC outlet, what do you see on the scope (leave the probes hooked up the same way)?