Task TK-940TX-DF Blowing fuse

[socketa]

Re: Task TK-940TX-DF Blowing fuse

i found both of zeners in a small plastic bag, with (post)#24 written on it.
Both of them have "18 2" printed on them.
They didn't do a good job of protecting the 5VSB MOSFET, or one of the main PSU MOSFETs
Neither of the MOSFETS should have being short-damaged; as both of the zeners would have conducted at 18V, which is well below the rated MOSFET gate voltages of 30V

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Thanks for your clarity of input and assistance - so congrad's to you as well.
During the course of this, i've done a lot of thinking, and learned a lot, in more ways than one

No problems. Glad that's the case.

Was the creation of the bulb trick the result of your own thinking/reasoning?

The bulb trick is pretty old concept (dating back many decades ago.)

I first read about it here on badcaps.net and started trying it / using it when repairing amplifiers.

As far as the suggestions I made with adding more light bulbs in parallel or using a heating element - that is indeed my own thinkering with the bulb trick. It stemmed from the first time I encountered a problem with using the bulb trick on a beefy workstation ATX PSU with APFC circuitry. In particular, the APFC made the bulb flash On and Off very erratically for the first 2-4 seconds the PSU was forced to turn On. This prompted me to think how the APFC circuit works, what was causing the bulb flashing phenomenon, and if there was a way to avoid it.

Any thoughts as to which primary capacitor balancing resistors to use, in respect to resistance values and wattage?
I've looked at quite few PSU schematics, and noted that values range from 100k to 330K, with 150k and 330k being the most common.

Both 150k and 330k will be OK. The lower the resistances of these two resistors, the better the voltage distribution will be between the caps and the quicker they will discharge.

As for the power rating (Watts) of these resistors, you can use P = (V^2)/R to determine how much the resistors will dissipate, then multiply that x2 to get the power rating you need (it's a good "rule of thumb" to size resistor power rating to be able to handle 1.5x to 2x the anticipated nominal heat generation / power dissipation). V is going to be about 170 Volts DC across each cap in most cases.

So with 330 KOhm resistors, you'll get:
P = 170^2 / 330 000 = 0.087575 Watts
Times that 1.5 and you have 0.13136 Watts
A 1/8-Watt resistor can do 0.128 Watts, whereas a 1/4-Watt resistor can do 0.25 Watts.
For this case, probably even a 1/8 Watt resistor will work fine above, despite not quite sticking to the "rule of thumb". But it's close enough. If you want a cool-running resistor, of course, then use 1/4 Watt.

On the other hand, if you run the same calculation for the 150 KOhm resistors, you'll see that they should dissipate around 0.193 Watts each. 1/8 Watt rating will not be enough to cover that. So if you use 150 KOhm resistors, you'll need 1/4 Watt or higher-rated resistors.

Do i have to replace the damaged NTC thermistor with one of the same value, or is there some wiggle room here?

There's plenty of wiggle room there, especially for this PSU, since it only appears to be capable of 250-300 Watts output.

SCK-054 is a common part used on 200-350 Watt PSUs. Since you live in 230V country, probably even an SCK-083 will work in your case. (Hint: look up "SCK 10 series" to get some PDF datasheets that show other parts numbers.) 15SP 2R5 and SCK 2R58 are two NTC thermistors I've seen used a few times as well. Or if you have another junk / parts PSU rated for a similar power range, you could reuse the thermistor from that too.

Just for reference, SCK-054 is rated for 5 Ohms resistance when there is no current going through it and can do a maximum of 4 Amps.

Since there is no balancing resistors in this PSU, but the PSU does discharge (you could measure and hear the point when it's discharged - it aligned with the squeak sound), how do you think that could be discharging?

5VSB circuit.
It's always connected and the minimum load resistor and feedback resistors on the 5VSB will discharge the primary caps relatively quickly (usually under 30 seconds on most PSUs).

On your particular PSU with it's 2-transistor 5VSB design, the caps can technically discharge down to zero because of that 2 MOhm resistor connected between (+) VDC on the primary and the 5VSB MOSFET.

What was their function?

Referring to the "102" ceramic caps by the bridge rectifier... those are just for extra "extra" EMI/RFI filtering. But they are rather redundant, because your PSU already has proper Y2 (blue disc) and X2 (square box) -class caps on the input. Most PSUs (even the really good quality ones) never bother putting such small ceramic caps across the bridge rectifier because of that. So just leave them out - extra parts for your spare parts bin.

Yes it is, if the bridge rectifier is installed the wrong way around.

Well, now, I wasn't expecting that kind of user error. (Though at some point it did cross my mind something like that must be going on.)

Would the PSU have a 5VSB critical cap, and, if so, what circuity configuration would i be looking for, in order to find it?
(maybe there's one in the attachment that i've included)

Yeah, on the attached shcematic photo, capacitor C14 (50V, 47 uF) in the lower-left corner by optocoupler IC4 would be considered the "startup" / "critical" cap.
However, looking at this picture on the left side:
https://www.badcaps.net/forum/attach...1&d=1587365635
... that would correspond to capacitor C15 on your PSU (and you can see that cap is located right by the 5VSB transformer and its optocoupler, so that's one way to tell which one is the critical cap on a 2-transistor 5VSB circuit.)
Make sure you use a good reliable low-ESR cap there, like Nichicon PW/PM/PS, Panasonic FC and similar.

And since the 5VSB voltage is not coming from any chip that needs to be protected, then drive voltage is not a concern, right?

Correct.
The Zener in that circuit is just to protect the 5VSB MOSFET Gate, in case higher voltage goes through the 2 MOhm resistor connected to (+) VDC.

So it seems to be more ideal to find another zener that's closer to the maximum MOSFET gate voltage of 30W.
Or, realistically, what's the likelihood of the MOSFET gate-source voltage going above 18V, since i measure only 1V across the zener?

Not likely, so you can leave the original 18V Zener in there.

P.S there is another 12V fan on the other half of the case and the PSU starts when that's plugged in - so no extra load is required for this PSU to start

LOL.
I guess that load is just borderline close enough to keep it running.

i found both of zeners in a small plastic bag, with (post)#24 written on it.
Both of them have "18 2" printed on them.
They didn't do a good job of protecting the 5VSB MOSFET, or one of the main PSU MOSFETs
Neither of the MOSFETS should have being short-damaged; as both of the zeners would have conducted at 18V, which is well below the rated MOSFET gate voltages of 30V

Well, in the case of the 5VSB MOSFET... the MOSFET probably shorted because the output caps on the 5VSB were bad, causing the 5VSB regulation to go haywire, which often puts a lot more stress on the driving MOSFET. This applies to both 2-transistor and IC-based 5VSB circuit. Bad caps on the output are simply not good for it.

And in the case of the main PS MOSFET... probably the same thing happened: MOSFET was overloaded due to regulation going crazy with the bad caps on the output. With that said, the Zener diode there did DO its job - it protected your UC3843 PWM controller from going bad. Otherwise, you'd have to had that replaced too by now.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Thanks - that tidy's this up nicely enough
Was an interesting, and sometimes arduous and gruelling, journey.
But well worth the persistence.
Will find a way to put the balancing resistors on the surface, and also reload the heavy traces with solder, and put some glue around the primary caps bases (now i know why the glue was there, as they did tend to loosen as the unit was being handled)

i was trying to understand voltage drop across resistors and it occurred to me to mentally collapse them into wires, and to turn the voltage into push/pull, then things became very clear.
There are parallels with other stuff in life: The more that you mindfully repeat an action or an investigative thought process the easier, or more natural (intuitive), it becomes - and sometimes it's a good idea to apply some "thinkering" (i like that) before the physical action.
i also resolved current into two components: electron flow and electron orbital state, and that cleared up the fogginess of why a load would actively decrease it's resistance, and why a lower resistance equals greater dissipation (power/work)
We create a lot of needless resistance (often as a consequence of certain mental or 'physical' habits), and that's why we often lack in power from infinite potential transcendent source.
Also, it's a very unwise idea to open oneself up to more voltage than the rated wattage of one's current (pun not intended) resistorance.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

With only the two fans as a load the -5v and 3v rails are about 0.8V below spec, but they come up to spec when a hard drive is connected.
Added two 330k balancing resistors in unused holes next to one of the primary caps.
Sleeved a 0.2 ohm current sensing resistor (that should really be 0.1 ohm, that i lost). Spent ages reading and trying to figure out how it's used to sense current and why increasing the value of the resistor would cause the protection to kick in at a lower current. Looking at that schematic that i attached earlier on, increasing the resistor resistance means that more current will be forced through (or maybe more voltage pressure onto) the ISENSE terminal of the chip.
This PSU has traces/pads that come away from the board a lot easier than some other boards
I wrongly threw away a solder pad that i lifted, so used a small section of wire instead - Use an over-length wire, and then cut it off after soldering; otherwise it can easily be lost if you drop it as a pre-cut length.
Removed the rust, painted the case, and washed the fans. Put it back together and the top wouldn't go down due to the fact that i didn't use the original primary caps - they were a bit too long and touching the top fan, so i took it apart and put the original primary caps back on.
After fiddling around with the hot soldering iron inside the case, in hindsight, i realized that it would have being so much easier to pull out the on/off switch and then desolder the wire.
Then looked at the photos and realized that i should have filled in the empty holes in the traces, so that current can flow better (rather than around the holes)
But i'll do that at the same time when i come across a 0.1 ohm current sensing "shunt" resistor

What's the power rating of the shunt resistor?
Here is my calculation: since most of the power would be dissipated via the main transformer windings at maximum load wattage of 400 watts.
R=V^2/P = 230^2/400 = 132 ohms effective resistance
I=V/R = 1.74A
Voltage across transformer winding (ignoring the comparatively negligible resistance of the shunt resistor) = 132 X 1.74 = 229.6V
Voltage across shunt resistor = 0.1 X 1.74 = 0.174V
Power dissipated by shunt resistor = V X I = 0.3A
So a half watt resistor would suffice?

Also noticed that the printing on the label of this PSU says that it's "approved by Intel and AMD" - so i guess that the presence of balancing resistors are not part of the requirements

[momaka]

Re: Task TK-940TX-DF Blowing fuse

With only the two fans as a load the -5v and 3v rails are about 0.8V below spec, but they come up to spec when a hard drive is connected.

Sounds normal. Some PSUs do that.

Sleeved a 0.2 ohm current sensing resistor (that should really be 0.1 ohm, that i lost). Spent ages reading and trying to figure out how it's used to sense current and why increasing the value of the resistor would cause the protection to kick in at a lower current. Looking at that schematic that i attached earlier on, increasing the resistor resistance means that more current will be forced through (or maybe more voltage pressure onto) the ISENSE terminal of the chip.

Yes, higher resistance sense resistor means it will have higher voltage across it when current flows through the primary of the transformer and main PS switching MOSFET. The higher voltage across the sense resistor means higher voltage going to the ISENSE pin, which means it will trigger earlier... which may not be a bad thing for this PSU, TBH.

Also, if you find another 0.2 Ohm resistor, you can just run the two in parallel, which will give you the same 0.1 Ohms resistance.

This PSU has traces/pads that come away from the board a lot easier than some other boards

Grrr. I hate those... especially the ones that crackle and pop when I run my soldering iron over them, followed by a blistered trace. Though sometimes, it's just the PCB absorbing moisture over time, so not much can be done for that.

Removed the rust, painted the case, and washed the fans.

Good job, it looks nice!

What do you mean by washed fans, though? I hope you didn't run them through running water. Otherwise the iron on the stator core will start rusting over time and might make the fan seize.
I usually just brush the dust off the fans with a brush, then wipe with moist paper towel (often times with window cleaner.) If the fans are really full of dust, I sometimes take the rotor part off and then clean more in-depth.

Put it back together and the top wouldn't go down due to the fact that i didn't use the original primary caps - they were a bit too long and touching the top fan, so i took it apart and put the original primary caps back on.

Been there, done that.

What I don't understand is why some manufacturers do this and add a second fan on the top that is the same size as the rear exhaust fan. I mean, if there is a rear exhaust fan, then that fan on the top is kind of redundant. Moreover, whenever there is a back exhaust fan, most studies suggest that a front-to-back flow is typically more optimal for cooling. But I guess this was considered OK practice in the early 2000's, which is probably the era this PSU was made in.

But i'll do that at the same time when i come across a 0.1 ohm current sensing "shunt" resistor

What's the power rating of the shunt resistor?
Here is my calculation: since most of the power would be dissipated via the main transformer windings at maximum load wattage of 400 watts.
R=V^2/P = 230^2/400 = 132 ohms effective resistance
I=V/R = 1.74A
Voltage across transformer winding (ignoring the comparatively negligible resistance of the shunt resistor) = 132 X 1.74 = 229.6V
Voltage across shunt resistor = 0.1 X 1.74 = 0.174V
Power dissipated by shunt resistor = V X I = 0.3A
So a half watt resistor would suffice?

Well, those calculations for SMPS don't exactly work like that... though you do have the right idea that if a certain average power flows through the primary side at a certain average voltage, then you can guesstimate more or less what the average current and resistance may be like. And in that context, the above calculations do make sense more or less. It's just that we don't know which component(s) would be the most lossy or cause loss of efficiency the most. So with that said, the above power you calculated should only be used as a very rough guesstimate. Also, I think 300 Watts is a more realistic rating for this PSU. But either way, 1/2 Watt resistor will probably get you by OK. However, at least from what I've seen in most PSUs, 1 Watt is a more typical value.

On that note, also make sure your resistor is flame-proof or at least flame resistant. IIRC, that would be either Metal Oxide or Metal Film resistors... but specifically check the part datasheet to determine that. Avoid carbon film or wirewound. The former is not flame-resistant and the latter may be too inductive.

Also noticed that the printing on the label of this PSU says that it's "approved by Intel and AMD" - so i guess that the presence of balancing resistors are not part of the requirements

I really wouldn't take that text to mean anything useful, nor do I think all PSU manufacturers used it with Intel's or AMD's approval. Some just slapped it on there to give the PSU more "merit". If I remember, that label came around the early Athlon XP and Pentium 4 era to signify that the PSU can handle both older 5V-heavy systems (Pentium 3, Athlon, and early Athlon XP) and newer 12V-heavy systems (Pentium 4 and later Athlon XP.) Prior to that, it was standard for most ATX PSUs to carry only 8 Amps or less on the 12V rail (and hence lack a 4-pin 12V CPU connector), which meant you couldn't use those old PSUs on a new (at the time) Athlon XP or Pentium 4 PC. So I think they came up with that label to make it easier for people to distinguish which PSUs can and cannot be used with a newer PC. But that label really meant nothing in terms of how the PSU would perform when cross-loaded one way or the other... and some didn't do too well.

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Thanks for the confirmation, and additional info

What do you mean by washed fans, though? I hope you didn't run them through running water. Otherwise the iron on the stator core will start rusting over time and might make the fan seize.

Yes, i washed the fans under running water and shook them out and sun dried them - it's quite warm here, so the water will evaporate away to align with the current atmospheric equilibrium.
Wont do that again though.

Runs very quietly
Still does a little squeak when it turns off - which i don't mind, if it's a benign quirk of the PSU