Task TK-940TX-DF Blowing fuse

[socketa]

Re: Task TK-940TX-DF Blowing fuse

Well, that was uneventful
The element was drawing 1.5kW during the period of six minutes, and the DC voltage across the caps (the rectifier DC terminals) only rose from 1V to 3V.
No fireworks either

[socketa]

Re: Task TK-940TX-DF Blowing fuse

The rectifier tests good after that - so doesn't that suggest that the fault is either in the green X Capacitor or the 1Mohm resistor?

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Well, that was uneventful
The element was drawing 1.5kW during the period of six minutes, and the DC voltage across the caps (the rectifier DC terminals) only rose from 1V to 3V.
No fireworks either

Wow, something must be shorted "hard" for that much current to pass and nothing burn out.
Heck, I'm surprised the bridge rectifier survived. 1.5 KW @ 230V line is ~6.5 Amps of current. Probably was running it right at the limits.

3V across the caps? That's a little higher than 2 diode voltage drops added together (probably originating from the bridge rectifier when loaded with such current.)

That brings another item to check: is your EMI/RFI coil/choke oriented properly - i.e. AC coming in on one side and exiting the other through the other to the bridge rectifier. If the leads happen to be flipped by 90 degrees on the PCB, you'll be shorting out the AC input... though that would have showed up as s short-circuit when you checked the input of the PSU. So that may not be the issue here... but still check.

The rectifier tests good after that - so doesn't that suggest that the fault is either in the green X Capacitor or the 1Mohm resistor?

Perhaps it could be the green X-cap indeed. Though if the bulb doesn't glow when the bridge rectifier is removed, then the short has to be on the DC-side.

Try it and see so you can verify. The green X-cap is only there for EMI/RFI suppression, so you don't really need it for anything.

The 1 MOhn resistor can't be the issue, because, again, resistors always go open-circuit or high-resistance when they go bad. By Ohm's law, the input voltage at the wall would need to be 6500 KV (or 6,500,000V if you like) to drive 6.5 Amps through that 1 MOhm resistor.
And just for fun, the power dissipated from that resistor with that voltage and current would be 42250000 Watts (42.25 Mega-Watts!!!)

[socketa]

Re: Task TK-940TX-DF Blowing fuse

i removed the rectifier and the bulb stopped glowing - so the AC side seems to be good.
Then i connected 230V across the AC legs of the removed rectifier (with the bulb in series) and the bulb didn't glow - the voltage across the DC legs is 211V DC and 96V AC - is that good?
Tested across the DC holes in the board and get 35M ohm that slowly decreases.
The bridge rectifier still tests good, according to that test that was done earlier on.
Surely we must be getting close to finding the culprit by now!?

[ChaosLegionnaire]

Re: Task TK-940TX-DF Blowing fuse

And just for fun, the power dissipated from that resistor with that voltage and current would be 42250000 Watts (42.25 Mega-Watts!!!)

yea, i'd luv to have a 42.25 megawatt power plant! dont need to pay no stinkin' power bills no more!

[momaka]

Re: Task TK-940TX-DF Blowing fuse

Then i connected 230V across the AC legs of the removed rectifier (with the bulb in series) and the bulb didn't glow - the voltage across the DC legs is 211V DC and 96V AC - is that good?

Looks like it.
Without a capacitor on the output of the bridge rectifier (i.e. between + and -), you get sinwave pulsed DC, which will read somewhat close to the RMS value, but not quite (will vary between multimeters)... so 211V DC doesn't seem bad. 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. AC measurement would be irrelevant. Just make sure the X2 cap is rated for 250-275V AC / 400-450V DC (usually for X2 class, that is the minimum rating anyways, but always worth checking first before plugging things in. )

Tested across the DC holes in the board and get 35M ohm that slowly decreases.

OK, but while it is decreasing, does it still stay in the MOhm or high KOhm range after, say, 10-20 seconds? You can also try hooking that 16V laptop DC power adapter across the two caps (mind the proper polarity, of course) and see if it sparks / short-circuits. If yes, the short is still on the DC side somewhere.

Surely we must be getting close to finding the culprit by now!?

Close? We should have had the PSU fixed by now.
I still can't fanthom what is causing an apparently almost dead short-circuit with power applied, yet nothing when checked with a multimeter. There are not only no active (transistor) devices, but no high-current paths available on the primary DC side (aside from the caps being damaged somehow... but I doubt it.)

Bad comes to worse, you can take the two high voltage electrolytic caps, wire them in series with proper discharge/balancing resistors, then hook the bridge rectifier to them and test with the bulb. The caps should charge to around 340 DC total (about 160-170V across each cap) depending on line voltage and bulb shouldn't glow. If it does, either the caps are bad or you wired the circuit incorrectly.

If you do build that circuit, please stay safe!
Linking an Electroboom video here to show you what could go wrong.
https://www.youtube.com/watch?v=sI5Ftm1-jik

yea, i'd luv to have a 42.25 megawatt power plant! dont need to pay no stinkin' power bills no more!

Just pray it's not a nuclear power plant close by.
Then again, coal wouldn't be that much better either. *cough cough*

[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?

[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

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

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

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.

[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

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

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.

[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

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

Yup, should be OK for testing.

[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.

[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

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.