Long Wei LW-K3010D resarting loop

[alien21]

Hello! I have an LW-K3010D power supply that turns on and off continuously. I've already checked and there is no short on the output. The screen turns on and off and the voltage on the output fluctuates. At first, when I was measuring voltage on the L431 and its resistances, the source turned on, but after some time it returned to the same situation. Now that doesn't happen anymore. You can give me some tips to fix it. Thanks!
.

[alien21]

This is the PCB schematics with some little differencies

[m k]

Since something happened continue from there, it can also be a bad solder joint.

Can you find anything that remains?
Other than mains.

Top right is new DC voltage.
Small transformer above diode quartet a bit left is current.
Down center is a power switcher.
Left is internal 12V etc.

[alien21]

What is happening is it's in a repeating start loop on/off. All the voltages are oscillating from mV to a few volts. I have checked for shorts and found none.I checked the optocoupler and tested good as far as I know. For the IC U5 act like this there must be something wrong.The PCB schematics is with some little differencies the same the one I send. I know there´s a member to this forum that is an expert in troubleshooting  this PCB and that member is Chris56000. I really would appreciate any help to solve this issue. Thanks.

[m k]

If internal 12V is missing it can be overloaded or faulty.

[alien21]

Hello again! In fact the problem is in the 12V power supply wich is regulated by the IC U5 TNY280. For some reason is in a loop protection mode,this is done by pin 1 of the IC that receives the feedback from the optocoupler. I don't know where to look for the problem, because I tested the components and they look OK.
I was expecting someone to give me some tips about where I should look first to troubleshoot.
Thanks in advance for any help thay maybe given.

[m k]

First you check if the pulsing is present over U3 LED.
If not then input voltage can be not steady DC and miss a reservoir capacitor, U3 or C43 can also be faulty.
C50 is a current limiter, so obviously a suspect as well.

If opto LED is pulsing you know that there is a voltage over C16 and Q11 is possibly pulling it down through R31.
Next you can check if voltages over C16 and Q11 are sane.

[alien21]

 Thank you MK for your reply! There are voltages on C16 , U3 led  and Q11  anode and cathode but they are all low and oscillating between 1 and 7 volts.Between collector and emitter of U3 there is a steady voltage of 2,5V.

[m k]

Pin
1 enable/under voltage
2 bypass/multi-function
3 N.C.
4 drain
5 source
6 source
7 source
8 source

Pin 3 and above are one too low in schematics.

Steady voltage over U3 transistor indicates that U5 switcher is acting alone.
Pulsing itself indicates that U5 internal power FET is still operational.

U5 pin 2 is internally regulated to 5.85V and maintained by C50 when power FET is operating.
If pin 2 goes under 4.9V the switcher goes off and voltage must return to 5.85V before operation continues.
Pulsing in pin 1 indicates that it is still connected to pin 2 and they together are acting rationally.

If U5 pin 2 is not going under 4.9V the fault points to faulty U5.
Pulsing 7V is nicely about half of what is needed for 12V, maybe it's just a coincidence.

Normal auto restart is 2.5 seconds.
Is your machine looping faster?

[alien21]

 U5 pin2 has steady 5,80V , pin1 2,5V steady too. the autorestart  takes about 2.2 seconds.

[Mechatrommer]

What is happening is it's in a repeating start loop on/off. All the voltages are oscillating from mV to a few volts. I have checked for shorts and found none.I checked the optocoupler and tested good as far as I know. For the IC U5 act like this there must be something wrong.The PCB schematics is with some little differencies the same the one I send. I know there´s a member to this forum that is an expert in troubleshooting  this PCB and that member is Chris56000. I really would appreciate any help to solve this issue. Thanks.

Chris56000 is not here. since appreciation is not my concern so i post anyway... i experienced a totally dead LW-K3010D. i diagnosed up to U5 (TNY280) to be not pulsing. lazy arse i am not wanting to waste more time on diagnosis... i shot blind by buying 10pcs TNY280, its just 25 cents per piece. replaced it first time and my LW-K3010D is alive again, until now, its been more than a year... and i still have 9 pieces TNY280 ready if one of my 2 pieces of LW-K3010D is acting up again... fwiw..

[m k]

U5 pin2 has steady 5,80V , pin1 2,5V steady too. the autorestart  takes about 2.2 seconds.

So internally restarted then.

Either the chip is faulty or something is really overloading.
Since U3 LED has a voltage there is a good change U5 is not so good.

[alien21]

It seems U5 is the culprit so I´m going to replace it and then I´ll let you know if it worked.

[m k]

i shot blind by buying 10pcs TNY280, its just 25 cents per piece.

Was that from a local dealer?

Today I found
TNY280PN
1.81€ + delivery, Farnell
1.49€ + chipping + VAT, Mouser
0.48€ + VAT, 5x ebay China
TNY280GN
1.41€ + delivery, Farnell
0.65€ + VAT, 1 of 5 ebay China
0.55€ + VAT, 1 of 10 ebay China

[Mechatrommer]

i shot blind by buying 10pcs TNY280, its just 25 cents per piece.

Was that from a local dealer?

Today I found
TNY280PN
1.81€ + delivery, Farnell
1.49€ + chipping + VAT, Mouser
0.48€ + VAT, 5x ebay China
TNY280GN
1.41€ + delivery, Farnell
0.65€ + VAT, 5x ebay China
0.55€ + VAT, 10x ebay China

it was TNY280PN from our local online dealer... 0.10€ from china as you quoted is cheap already. here is from aliexpress 20cents/piece

[m k]

From ebay China was per piece.
(edited now)

E,
welcome deal from your link
€0.46 / lot (10 Pieces)
€2.70 83% off

[alien21]

Hello again! After a long time waiting for the chip I replaced the old one. Unfortunately it did not solve the problem what makes me feel more lost. Any ideas where the problem may be?

[m k]

Your options are diminishing.
Since changing the chip didn't help you can assume that overcurrent is real.
So either diode pair or T1 seems to be it, my vote goes to T1.

[alien21]

 I tested all the diodes on the primary and secondary sides  of T1 and they are all OK. Are you telling me that T1 is short circuited.The primary winding has 1.4ohms ,
 the display supply 0.3 ohms and the other secondary 0.1ohms.I don't know if these readings are ok.

[Mechatrommer]

transformer is unlikely damaged esp on quite good current limited circuit. i guess you need to check any feed back resistors, capacitors diodes etc down stream on secondary side, i cant help much further, the circuit diagram is too convoluted to understand.

[m k]

T1 insulation can be damaged or sort of aged too much.
You can test that by disconnecting all secondary routes.
If it's still overloading you know the cause is not some later thing.