Task TK-940TX-DF Blowing fuse

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Yes, that is indeed the the case here - switching off at the wall

I added another bulb (60W), so that's four bulbs in total - no noticeable change from when there was three.

The PSU will not power up, when turning on at the wall, until after the un-squeek squeek is heard (which happens when the primary caps have discharged to approx 30V)
Monitoring the rail voltages, i notice that 12V rail gets to 12.2V, 5V rail gets to 5.08V and the 3.3V rail gets to only 2.7V, before the PSU shut's itself down - so maybe there is a clue, or pointer, there that suggests looking at the 3.3V snubber circuit.

The PSU not able to start until you hear the "un-squeak squeak" is probably because a short-circuit or over-power protection is latching.

Yes, the 12V and 5V rails coming up but the 3.3V rail not regulating properly is definitely a clue here.

Since this PSU derives the 3.3V rail through a mag-amp circuit, it relies on the primary side's switching action to regulate the 3.3V rail through a transistor and a saturation coil on the secondary side. If something is not quite right with the PWM switching, the 3.3V rail can get affected by that and not regulate properly. Or it could also be that something on the 3.3V rail's mag-amp circuit is bad, causing it to not regulate properly. If you trace the entire power-side of the 3.3V rail (i.e. only big traces that carry power and not small feedback/signal traces), you should find a transistor, a diode or two, and also a 431 shunt regulator with some resistors all connected to the 3.3V rail - all of these parts are for the mag-amp circuit and should be checked out.

You should also try feeding back 3.3V from another PSU into this PSU's 3.3V rail and see if anything gives or overheats (particularly the transistor responsible for regulating the 3.3V rail.)

I found the 3 snubber circuits, all 4.7 Ohm 1/2 watt sized resistors. And there are no shorted caps or open resistors
...
I saw no need to lift any component, because, according to the following logic:
Each circuit is essentially a resistor and capacitor connected together in a closed loop.
If you place the meter leads across the resistor (which is the same as "placing them across the capacitor") and get a measurement that matches the resistor, then that means that the resistor is good, and that the capacitor is not shorted. If the capacitor was shorted then the meter would read as a short because one could rightly say that it's parallel with the resistor.

That is true.

So looks like your snubber circuits are OK.

Zener Diodes

Looking back through the posts, the two zeners that i replaced (i eventually threw the originals away) are:

ZD8, next to the heatshrinked resistor, and ZD1

Ok, looking back at the circuit once again...

ZD8 is related to the 5VSB circuit only: it appears to be used as a limiter for the voltage on the Gate of the 5VSB MOSFET. Since the 5VSB seems to be working normally, I suppose everything is good there.

However, that does raise the question: when you try to power on the main PS and you hear the PSU squeak, what happens to the 5VSB? In other words, try measuring the voltage on the 5VSB when you do this experiment. We want to see if the 5VSB drops out or not. If it is, that would be a problem. To get more accurate result, first put a load on the 5VSB - something in the range of 0.25 to 0.5 Amps should do (so connect something like a 10-20 Ohm resistor to the output of the 5VSB - that 1.5 KW heating element should have about 6.5 Ohms resistance, and probably even that will do the trick.) Then plug in the PSU, but keep main PS turned OFF, then see if the 5VSB still outputs 5V. If it does, try powering on the main PS and see if the 5VSB continued to supply 5V. The reason we want to do this is because the 5VSB circuit also has a primary side auxiliary winding that feeds the PWM controller on the primary side for the main PS. If 5VSB is unstable or dropping out, the main PS will not work either.

As for ZD1.....
That one is for Gate voltage protection for the main PS MOSFETs.

With that said, both ZD1 and ZD8 should have Zener voltage ratings that are less than the maximum rated Gate voltage of the 5VSB and main PS MOSFETs.
i.e.
ZD1 Zener voltage < V_GSS of main PS MOSFETs
and
ZD8 Zener voltage < V_GSS of 5VSB MOSFET

However, the Zener voltage of these Zener diodes should also not be too low, because then the MOSFET's may not get fully driven On.

This means the Zener voltage of these Zener diodes should be close to the rated V_GSS of the MOSFETs, but just slightly lower.

So if you used an improper Zener diode for ZD1, that could cause the MOSFET to not get fully driven On and thus not switch properly, leading to OPP/SC protection kicking in.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

I found the 3 snubber circuits, all 4.7 Ohm 1/2 watt sized resistors. And there are no shorted caps or open resistors

Since these snubber circuits are connected directly across the transformer output pins, you won't be able to check the components in circuit

I saw no need to lift any component, because, according to the following logic:
Each circuit is essentially a resistor and capacitor connected together in a closed loop.
If you place the meter leads across the resistor (which is the same as "placing them across the capacitor") and get a measurement that matches the resistor, then that means that the resistor is good, and that the capacitor is not shorted. If the capacitor was shorted then the meter would read as a short because one could rightly say that it's parallel with the resistor.

For quite some time i've being inclined question what i'm told, because from a young age i refused to eat animal meat, but, at that time, someone told me that sausages were mostly bread crumbs, so i became somewhat addicted to them, and sausage rolls, for many years; not really understanding, or considering, what i was doing - until one day not long after i started eating mince pies, i noticed a piece of artery inside one. That was the end of that 'venture', and also the beginning of being vegetarian. I still didn't know about why cows make milk (incredibly many folk don't!) ... could go on with a long story, but suffice it to say that i'm constantly amazed at how easy it can be to miss stuff, that in hindsight, is blatantly obvious after the application of minimal thought, quietude, or/and assistance

Attached is some photos of that resistors (which all tested good, at 4.7 ohms) and caps.
The first pic looks like "102" is printed on the cap, and the other two caps have "472" printed on them.
The colours on the three resistors are yellow, violet, gold; although due to lack of sunshine, the yellow looks green in the photos.
Also attached, is a photo of the back of the board, with the three snubber circuits drawn in yellow.
The snubber at the bottom has a jumper on the right hand side (vertical yellow line) which i initially missed, and so i was looking for the path back to the transformer (hence the irrelevant diode that i drew in)

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Is the squeak sound heard when simply disconnecting the PS_ON wire from ground (to turn Off the main PS) or when you unplug the PSU from the wall?

The latter (unplugging PSU from wall outlet) is actually normal for 2-transistor 5VSB circuits. I have a lot of PSUs with a 2-transistor 5VSB circuit, and at least half of them do a fairly loud chirp/squak (typically ending with a raise in pitch as time passes) as the PSU is unplugged from the wall.

Yes, that is indeed the the case here - switching off at the wall

I added another bulb (60W), so that's four bulbs in total - no noticeable change from when there was three.

The PSU will not power up, when turning on at the wall, until after the un-squeek squeek is heard (which happens when the primary caps have discharged to approx 30V)
Monitoring the rail voltages, i notice that 12V rail gets to 12.2V, 5V rail gets to 5.08V and the 3.3V rail gets to only 2.7V, before the PSU shut's itself down - so maybe there is a clue, or pointer, there that suggests looking at the 3.3V snubber circuit.

I'll investigate those snubber circuits to see if the resistors and ceramic/film caps are free of the faults that you mentioned, and then post results here when done.

--------

Zener Diodes

Looking back through the posts, the two zeners that i replaced (i eventually threw the originals away) are:

ZD8, next to the heatshrinked resistor,

the shorted Zener is probably part of the problem. But since it appears connected to the Gate drive of the main PS MOSFETs

and ZD1

I removed zener diode, ZD1, and it tests as a shorted; and the short across R23A dropped right off.

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Last night
----------

No shorts on the output rails. (3.3V 15ohm, 5V 43ohm, 12V 233 ohm, and no shorts on the blue, white, or grey wires)
So added a 100W bulb in parallel with the current 60W bulb.
Turned it on, the fan spins and keeps spinning, there is buzzing for a half second and then the bulbs light up and the buzzing increases in volume, but the voltages are now good.
Swapped the 60W with a 75W, same result as before except that the bulb takes a second to light up.
The the voltages are good from the moment the PSU is turned on, to the moment that i turn it off.

That's an interesting result.

It suggests the PSU might just be OK, but just not liking the still relatively high impedance of the bulbs on the input. However, I haven't seen a PSU that doesn't work with 2-3 bulbs in parallel, so there may still be something iffy going on - in particular something pulling too much current on the secondary side that is making the primary overload. The fact that the output voltages are stable and have correct values means the secondary side feedback is OK. Given the results, though, I suggest to check the secondary side snubber circuits for each rail. These snubber circuits would be a low-value resistor (typically 2.2 to 10 Ohms and about 1/2 Watt rating) placed in series with either a ceramic or small metal film capacitor (generally in the range of 0.01 to 0.1 uF... or 103 to 104 printed on the cap.) You'll find these snubber circuits for each of the major rails (5V, 12V, and maybe 3.3V) close to either the heatsink-mounted rectifiers or close to the secondary side output of the main transformer. Typical failure mode for these snubber circuits is either the resistor will overheat and go open-circuit or the ceramic/film cap will go short-circuit. Since these snubber circuits are connected directly across the transformer output pins, you won't be able to check the components in circuit - you will need to remove either the cap or the resistor, or lift one leg on these two components from the PCB to take proper measurement.

This morning
-----------

Added another bulb holder (so now have all three bulbs in parallel) and the buzzing was still there.
Wiggled a primary cap and noticed a spark, and noticed that i hadn't soldered it in so great.

But, foolishly, instead of just soldering it up good , i removed them and put the originals back in.
The buzzing didn't happen, the bulb briefly lights up, the fan spins for a second then stops, and i hear a brief tiny squeak sound from the PSU after turning it off for about 10 seconds.
No squeak sound when turning it on.

Is the squeak sound heard when simply disconnecting the PS_ON wire from ground (to turn Off the main PS) or when you unplug the PSU from the wall?

The latter (unplugging PSU from wall outlet) is actually normal for 2-transistor 5VSB circuits. I have a lot of PSUs with a 2-transistor 5VSB circuit, and at least half of them do a fairly loud chirp/squak (typically ending with a raise in pitch as time passes) as the PSU is unplugged from the wall.

If i just use the 100W bulb alone, the bulb stays lit, i hear a much louder squeak sound when turning it on, the fan twitches, and i hear the same soft squeak (an un-squeak?) sound after about 10 seconds, as before, after turning it off.

I get the same result now, regardless of which of the three pairs of caps i use.

Well at least we have consistency now... and the bulb is no longer glowing right away when the PSU is plugged in.

P.S. When i put the components back on, i replaced two shorted zeners with ones that looked exactly the same, from the primary of another PSU

That is certainly something worth investigating. Zener diodes can have different Zener voltage ratings, despite looking all the same way. Need to know which Zener diodes this is in the circuit so hopefully we can try to guess what voltage it should be rated for - that is, if you don't have the original shorted Zener diodes, which will tell you that by reading the parts numbers written on them.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

----------
Last night
----------

No shorts on the output rails. (3.3V 15ohm, 5V 43ohm, 12V 233 ohm, and no shorts on the blue, white, or grey wires)
So added a 100W bulb in parallel with the current 60W bulb.
Turned it on, the fan spins and keeps spinning, there is buzzing for a half second and then the bulbs light up and the buzzing increases in volume, but the voltages are now good.
Swapped the 60W with a 75W, same result as before except that the bulb takes a second to light up.
The the voltages are good from the moment the PSU is turned on, to the moment that i turn it off.

-----------
This morning
-----------

Added another bulb holder (so now have all three bulbs in parallel) and the buzzing was still there.
Wiggled a primary cap and noticed a spark, and noticed that i hadn't soldered it in so great.

But, foolishly, instead of just soldering it up good , i removed them and put the originals back in.
The buzzing didn't happen, the bulb briefly lights up, the fan spins for a second then stops, and i hear a brief tiny squeak sound from the PSU after turning it off for about 10 seconds.
No squeak sound when turning it on.

If i just use the 100W bulb alone, the bulb stays lit, i hear a much louder squeak sound when turning it on, the fan twitches, and i hear the same soft squeak (an un-squeak?) sound after about 10 seconds, as before, after turning it off.

I get the same result now, regardless of which of the three pairs of caps i use.

So i don't know if replacing the caps caused another fault.
I suspect that it fixed the buzzing, but the buzzing (which i'm assuming was a loose primary cap leg that was arcing with the surrounding solder) may have created a condition whereby the PSU could power up with good voltages. So maybe there was two problems and i fixed one, which subsequently brought an underlying one to the surface.
Or maybe the impedance of the parallel bulbs is still too high for the PSU to start?

The numbers on those caps indicates their capacitance and not their resistance.

Doh! I got tricked by the "472M" that was printed on the cap, and bypassed my logic gate.
Thanks for the additional info.

P.S. When i put the components back on, i replaced two shorted zeners with ones that looked exactly the same, from the primary of another PSU

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Buzzing for single-transistor forward PSUs typically means over-load condition.

However, since you have a series light bulb, it could also be a false over-load condition, as some PSUs simply don't like high-impedance on their input.

So I think there are three possibilities for what you are seeing here:

1) There is a short-circuit or over-current condition on the main PS secondary side, causing the primary to over-load. (Could be a faulty rectifier on one of the rails, for example, or an open feedback path.)

2) There is a component fault on the primary side of the main PS, causing the PWM IC to over-drive main PS MOSFET until its protection kicks in (open opto-coupler, open opto-coupler feedback path, faulty snubber component, or sense resistor going high resistance.)

3) PSU's main PS design might be sensitive to high-impedance on the input, which means the series light bulb could be the reason why you're seeing this behavior. (On that note, Active PFC PSUs... or PSUs with APFC... are even more sensitive, and generally cannot be tested with a series bulb less than 200-300 Watts.) If that's the case, then using several 60-100 Watt incandescent bulbs in parallel... or one 200-300 Watt heating element... should provide sufficiently lower impedance on the output to make the PSU function.

I suggest you start by assuming case #1 above and check all secondary-side components. Start with the large power-handling components first, like large rectifiers and diodes for the 3.3V, 5V, 12V, and -12V rails. To do that, check resistance between each of these rails and ground. Note that there _may_ be minimum-load resistors on the output, which could possibly make you think that a rail is shorted to ground (common mistake when someone uses continuity test on their multimeter to check if a rail is shorted to ground, since most multimeters will beep at resistance under 100 Ohms, and load resistors on the 3.3V and 5V rails typically are 100 Ohms or less.) An alternative (and somewhat easier) method to checking the rails for short-circuit is to use a 20-40 Watt, 12V, incandescent/halogen bulb and backfeed an external voltage to each rail. If you have a spare ATX PSU, simply wire the output of that PSU to the rail you want to test. For example, if you want to test if the 12V rail is shorted, connected ground on both PSUs together, then connect the 12V rail of one PSU to the 12V rail of the other PSU _through_ the indancesence/halogen bulb (i.e. similar to how you use the series light bulb on the input.) Then, plug in only the working PSU and turn it On (the PSU under test should remain unplugged from wall.) If the bulb glows steadily, there is likely a short-circuit on that rail. If not, then rail is probably OK. Just make sure to always connect the right rails together - i.e. only use 12V rail to test 12V rail on the PSU in question, or 5V rail to test the 5V rail, and 3.3V rail to test the 3.3V rail. If you accidentally mix then up (particularly inserting a higher voltage on a lower-voltage rated rail), you could blow up some caps.

If you find that none of the rails are shorted with the above tests, then we could assume case #3 for a bit, and simply add a few more incandescent bulbs in parallel to your single bulb on the input, or simply replace that with a 200-300 Watt heating element. Then try powering up the PSU and see if it buzzes again. If it does, go back to assuming case #1, and check all secondary-side components again. After this, assume case #2 and check all primary side components related to the main PS.

As far as i can hear, the light buzzing is coming from the small 472 Mohm blue cap (red line), and after i turn it off and on until the loud buzzing happens, it is coming from the adjacent side of the big white resistor (green line), possibly the other small 221 ohm small blue cap or the larger one.

The numbers on those caps indicates their capacitance and not their resistance.

caps with numbers xx5 = x.x uF
caps with numbers xx4 = 0.xx uF
caps with numbers xx3 = 0.0xx uF = xx nF
caps with numbers xx2 = 0.00xx uF = x.x nF
caps with numbers xx1 = 0.000xx uF = 0.xx nF

* where xx = 2-digit number
(Also, nF = nanoFarads, uF = microFarads)

So 472 = 4.7 nF and 221 = 0.22 nF

[socketa]

Re: Task TK-940TX-DF Blowing fuse

I used a tube to locate the buzzing, and it's not coming from the primary caps area.
As far as i can hear, the light buzzing is coming from the small 472 Mohm blue cap (red line), and after i turn it off and on until the loud buzzing happens, it is coming from the adjacent side of the big white resistor (green line), possibly the other small 221 ohm small blue cap or the larger one.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Forget what i said about the caps being charged or not.
Whether the bulb flickers brighlty, or doesn't glow, seems to be more random. Switching it on after 5 second intervals of being off - sometimes it will flicker, sometimes it wont.
With same results as previously mentioned

[socketa]

Re: Task TK-940TX-DF Blowing fuse

With everything* back on, there is still 5VSB, no problem, and when i start it with a paper clip, one of two things happen:
1. If the caps are somewhat discharged: The bulb doesn't glow, the fan spins briefly, soft buzzing around the primary caps area, voltage rises and falls back to zero.
2. If the caps are fully charged: The bulb lights up and flickers rapidly, fan briefly spins less than before, loud buzzing around the primary caps area, output voltage rises and doesn't fall. (the 5V rail goes to 3V, 12V rail goes up to 7.3V)

* I haven't put the two NTC thermistors back on, because one of them was kaput - so i've used jumpers in place of them for the meantime.

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Yup, should be OK for testing.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Can i temporarily use a 0.2 ohm resistor, instead of the pink 0.1 ohm resistor that i lost?
One leg of it goes to primary caps -ve rail, and the other leg goes to one of the main MOSFETs.

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Have put the components back in, (minus the main transformer, the main PS MOSFETs, the 8-pin TLP384P IC, and the big pink resistor) and now have 5.1V ON THE 5VSB.

Alrighty... looks like you finally got the issue with the bulb resolved.

I suppose you can continue with reinstalling all the other components for the main PS (minding the orientation a little better this time )
and then see what happens.

Could the fact that the 3rd pair of caps (330uf) have substantially less capacitance than the other two pairs (820uf and 680uf) be the cause of their anomalous good result?

No.

You already have a series bulb and NTC thermistor in the PSU, so to the circuit there, it makes no difference what the capacitance is, as long as both caps have matching capacitance rating. And even outside the PSU, where there is no NTC, it still doesn't matter, because the NTC is there just to reduce large surge currents from damaging the bridge rectifier... which the series bulb provides too.

There is no logic as to why the caps would work outside of the PSU but not on the PSU itself, so it must be a wiring error or possibly something to do with the PCB. One possibility is that if either Live or Neutral is somehow getting connected to the middle series point between the two caps, that would create a voltage doubler circuit and send ~330-340V DC across each cap.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Have put the components back in, (minus the main transformer, the main PS MOSFETs, the 8-pin TLP384P IC, and the big pink resistor) and now have 5.1V ON THE 5VSB.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

In post #18, look at the orientation of the rectifier compared to the orientation in post #27.

Post #26

So you have the 5VSB MOSFET and these (four!) components removed, and the bulb still glows??

What happened is that i thought that the black square on the board meant -ve (like capacitor markings), so i suspect that from that point onward, till post #35 the rectifier was installed the wrong way around. Although i'm at a loss how/why i could have reversed the rectifier during the course of putting the components back in, as there was no reason to remove it, but somewhere between there and to

Might start putting components back in and see what happens just for fun

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Result of testing:
These first two sets of caps test good in that capacitor bulb tester circuit (with, and without, balancing resistors), but not in the PSU;
but the 3rd set of caps works well in the PSU.
Could the fact that the 3rd pair of caps (330uf) have substantially less capacitance than the other two pairs (820uf and 680uf) be the cause of their anomalous good result?

This PSU is sure living up to it's name.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

But you can take the old caps and place them across your bridge rectifier circuit out of the PSU and see if the bulb still glows. If it does, then they must be bad, as we've confirmed the bridge rectifier is already good.

Not feeling confident that the issue was resolved, i just tried both pairs of caps (the original pair, and then the first replacement pair) in that capacitor tester circuit (used 2 X 330k ohm resistors) and the bulb lit then went out in both instances.
So wondered if it was due to the balancing resistors. I removed the balancing resistors, and the result was the same

Rechecked that the bulb was still going out on the PSU (that now has the 3rd pair of caps installed in it), which it is.

Now i'm going to put the original primary caps back into the PSU and see what happens:

I put the original ones back into the PSU, but the bulb didn't glow (because i put then in with the wrong polarity). Put them back in the right way around, and the bulb came on, nearly went out, but then lit back up again.

Then i put the original caps back into the capacitor tester circuit with the 330k ohm resistors and the bulb lit up and then went out. Same result happens after removing the balancing resistors.

[momaka]

Re: Task TK-940TX-DF Blowing fuse

I'm wondering what happened from hereafter (when it looked like the issue was resolved), when i put all of components back in - and if there was fault on the board that destroyed the replacement caps.

That's what I'm wondering too.
It looked all fine, then some parts were reinstalled and things went haywire.
As long as the bridge rectifier is providing DC, and the voltage across the primary caps is balanced, there shouldn't be any way they can get damaged.

On that note, discharging them with a screwdriver or other means where it produces large sparks can damage them over time. Use a hair dryer or wall-type soldering iron to discharge the caps (preferably the latter, as it will minimize sparking and large current draws.)

The lack of balancing resistors may have contributed to this?

Yes, possibly.
I'm thinking if one cap receives too much voltage from unbalance, it can short out... and then the full rectified voltage will be dumped on the other cap, making it short out too.

But you can take the old caps and place them across your bridge rectifier circuit out of the PSU and see if the bulb still glows. If it does, then they must be bad, as we've confirmed the bridge rectifier is already good.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

If you take the green X2-cap from the PSU's AC side and connect that across the bridge rectifier, then measure the voltage on the bridge rectifier, you should get around 340V DC stable.

Yep, got 331V-333V (240V/0.707 ± 2V ripple?)

I connected the Laptop power adapter across the caps
The only small spark that i saw and heard was when i bumped the wire that went into the inside of the adaptor connector.
The voltage started of at 16V and then slowly dropped, over the course of 5min, to 15V, at the rate of 0.01V/s

So even though i had two pairs of primary caps that i was using, i removed another couple of smaller primary caps (200V 330uf) from another parts PSU, reinstalled the bridge rectifier and put this third pair of caps in, and the bulb glowed for half a second (yay!) and i got 320V across the primary caps, with a nice spark/pop when i shorted them out.
Rechecked, and the same good result.

From post #32

So the last test with the new primary caps confirmed the old ones are probably fine.

I'm wondering what happened from hereafter (when it looked like the issue was resolved), when i put all of components back in - and if there was fault on the board that destroyed the replacement caps.

The lack of balancing resistors may have contributed to this? (i found them easy enough on the parts boards that i got the caps from - they are labelled as R2 and R3 / R201 and R202(add-on board).

[momaka]

Re: Task TK-940TX-DF Blowing fuse

If there is a spark / short-circuit would that mean that i'd have to replace a fuse inside the adapter?

No, any decently-built laptop adapter will have short-circuit protection.

With that said, if that adapter is a no-name (non-OEM) and designed badly, then it could blow its fuse and possibly more stuff inside it. But if it's a Dell, HP, Lenovo, or similar, it should just spark, shut-down, then restart (or worst case, you might have to unplug from wall and plug back in to reset it.)

[socketa]

Re: Task TK-940TX-DF Blowing fuse

OK, but while it is decreasing, does it still stay in the MOhm range after, say, 10-20 seconds?

Yes

...and see if it sparks / short-circuits.

If there is a spark / short-circuit would that mean that i'd have to replace a fuse inside the adapter?