Behringer Europort Epa300 (300-Watt Portable PA System) fuse keeps blowing

[donkey]

Hi friends,

I have a power supply unit of a PA system. I am a total beginner on repairing stuff. This unit is belongs to my friend and he asked me to look at it if i can help. I accepted it because i thought that it might be a fuse problem and i could solve it with my slight superficial knowledge by replacing it with a new one  .

As you may see at the attached image, i tried to delineate the bridge rectifier and transformer section of the pcb where i would like to ask some questions

The fuse has a continuity until the 3rd pin of the rectifier and there is no continuity i could measure beyond that leg. However i do not have enough knowledge to assume, still i assumed that there is nothing to be afraid of to replace a new fuse instead of the bad one and the fuse blown again.

I am getting continuity faults with the multimeter testings for the half of the semiconducter parts.


DMM diode mode in circuit probing results for the Brigde rectifier's legs (As noted in the image):

LEG      DMM TERMINAL        LEG      DMM TERMINAL      READING (V)
1                   (+)                   2                  (-)                   0.472
2                   (-)                    1                  (+)                  0.480
3                   (-)                    4                  (+)                  0.483
4                   (-)                    3                  (+)                  0.503
                 IN BOTH WAYS FOR AC LEGS (2-3)                    0.950

There is a continuity between the 1th and the 4th ( - and the  + ) legs of the bridge rectifier and there are no any other continuity for any other combinations.


DMM probing in circuit results for the transformer's legs (As noted in the image):
In every possible combinations for the legs number 1,2,3,4,5,6,7 the resistance is 0 ohms. Every one of them has a dead short with each other.
In every possible combinations for the legs number 8,9,10,11 the resistance is 0 ohms. Every one of them has a dead short with each other.
There are no any continuity between the leg group (1,2,3,4,5,6,7) <--> (8,9,10,11).


Do any of these inspections i made in the circuit for these two components are sufficient to desolder these parts and make some further inspections on them or are these behaviours normal for in circuit testing?

I am in the dark and i need some expert suggestions what i can test, how i can test to resolve the problem.

Instruments i have is a MASTECH-MS8268 multimeter and i will borrow a MASTECH-MS5308 LCR-ESR meter in few days.

Thanks for reading!

[LateLesley]

This looks like a switched mode power supply. It takes in the AC voltage, rectifies it, then puts it through a switching circuit to the transformer at high frequency. I think with you having dead shorts at the transformer, it could be that the switching transistors/FETs have died. I think they might be located on the left heatsink in the first picture. It may be worth removing them, and testing them.

Here's a link to a page which might help you troubleshoot it.

http://www.giangrandi.ch/electronics/smpsfix/smpsfix.shtml

[donkey]

Thank you for your time and answer LateLesley.

[donkey]

Dear friends,

I removed and tested 7 components including the bridge rectifier but every component were just fine except one 11N80C3 Power Transistor. I replaced the transistor with the new one but everything still gives the exact same in circuit readings as before (It is like shorted everywhere  ). I am get tired of desoldering the semiconductors which has in circuit shorted readings and to see that all works fine out of the circuit. I decide to give it a try to find out if just that one transistor was guilty . It is very possible that i will blow the fuse again tomorrow. I want to build a Dim Bulb Tester before i will try to connect it to the main. The unit has a 320W raiting and it has a 3.15A H 250v fuse in it.

1. What wattage of a Dim Bulb Tester i need to build for this test to protect the board?
2. Will the fuse blow even if i will use the Dim Bulb Tester?

Thank you!

[eblc1388]

1. What wattage of a Dim Bulb Tester i need to build for this test to protect the board?
2. Will the fuse blow even if i will use the Dim Bulb Tester?

A 100W filament lamp will be good 95% of the times.

The fuse will definitely not blow if there is a lamp in series with the power supply. What you are looking for is a very bright lamp at initial power ON follows by half brightness or gradually decreasing brightness. If you get full brightness and it does not change, then the power supply is still faulty.

[CJay]

You've got two power supplies in one there, the eight pin chip is a PSU and then there's the main PSU

For the moment ignore the secondary (the right hand side of your pic is the secondary and concentrate on the parts on the left hand side, T2 and (looks like) T5 next to the EU 230V sticker need testing out of circuit.

You also need to take the bridge rectifier out of circuit and test that. you should get a diode drop across AC input to + output with your -probe on the + output and the + probe on the AC inputs, then a diode drop with the + probe on the - output and - probe on the AC inputs, any other readings suggest it's faulty.

With the bridge rectifier out of circuit check to see if you still get a short across + and -  on the primary side.

[donkey]

Thank you both for your answers.

Dear CJay, I removed the rectifier and  made the following testings

DMM Terminal        Rectifier Terminal        DMM Terminal        Rectifier Terminal           Diode Drop
            (+)                            (-)                            (-)                        each(AC)                       exist
            (-)                             (+)                            (+)                      each(AC)                       exist
            (+)                            (-)                            (-)                            (+)                           exist
            Any other combinations                                                                                               not exist

I think that the rectifier was OK and yes without the rectifier and without the T2 and T5 Mosfet power transistors are connected in the circuit, the rectifiers (-) and (+) paths were still shorted.

You are correct about the transistors and i have changed one of the bad Mosfet power transistor with the new one that is why tomorrow i want to check if it solves my problem.

I have desoldered the 2 transistors next to T2, T5 either. One is an NPN the other one is a PNP type transistor and both were fine. Without all of these components, all the pins of the transformer still had a 0.1 ohm reading and it was a short for the multimeter. As you can see from the bottom side of the board the 9th and the 10th pins of the transformer are shorted by design.

Thank you!


 

[donkey]

Ok friends,

I used a 95W bulb in my current limiter.

What do we have so far: I made a dim bulb current limiter today and tried the unit.

Observations

1. I am getting a relay like clicking sound somewhere in the circuit.
2. I feel like i have programmed the unit with the Arduino's blink sketch . The unit have the exact response as the Arduino's blinking sketch, only with one difference; Arduino blinks an light emitting diode but my unit blinks the dim bulb current limiter's bulb instead, almost with the same frequency.
3.The fan of the unit and the equalizers leds on the front panel turns on for a fraction of a second and off for a fraction of a second and this loop goes on with the bulb blink.

What i should look in the circuit?

What may cause this problem?

If i will desolder the (8,9,10,11) pins as i noted in the image ( it is the primary stage i suppose ) Will the unit start if every part before the transformer's primary stage is not faulty?




Any help will be very valuable.

Thanks!

[CJay]

The clicking could indicate the PSU is trying to start but is being shut down by some protection circuit.

[eblc1388]

Blinking is absolutely good news. You are nearly done.

Use two 95W bulbs in parallel.

[donkey]

Thank you!

I come across with the part below and i desoldered it. I got a 0.337V diod drop reading over the legs. Is it low for this part or is it normal?

https://www.st.com/resource/en/datasheet/cd00004895.pdf

I have already tried it eblc1388 but the bulbs was lighted up brightly then before and the blinking frequency became higher as such almost can't visable with eyes. I was afraid of to burn up the unit and i switched the unit off immediately but if you say so i may try again.
 
Thank you!

[donkey]

Things gone wild  .

I desoldered and tested the components and resoldered them in the board. However the blinking effect continues with a one 95W bulb current limiter, this time not the fan nor the equalizers leds are working anymore. With 2x95W bulb current limiter test, bulbs are not blowing now, but not the fan nor the  equalizers leds are working .

May it be possible that i have destroyed the components by testing it with 2x95W bulbs parallel? It is odd that however with 2x95W bulbs in parallel current limit test was more like a short circuit observation before (The bulbs were in maximum brightness ) but the fan was working at that moment; now i observe with the same 2x95W bulbs parallel limit test the bulbs are not lights up at all but nothing is working in the unit instead. Even the fan.

Thnak you!

[CJay]

That ST part should check as a diode in one direction only, if you get a reading when you reverse the meter leads then it's faulty.

It's also likely that the PSU won't start up properly if it detects a missing voltage on the secondary so you need to replace that diode if it is faulty

[donkey]

Thank you CJay!

The diod only conduct's in one direction. It has a 0.337V's voltage drop in multimeter which is below 0.400V - 0.600V margin, that is what i am concerned.

Update on the bulb current limiter test result

As eblc1388 stated to use two 95W bulbs in parallel, when i tested the unit with 2x95W bulb current limiter without the front panel connection, i have observed that the PSU starts working. The fan is working properly and there are no clicking sound anymore. Now the problem is some how the front panel is not working anymore 

Maybe it is because the relay stoped doing its job and everything on the front panel burned out. 

How i suggest that the PSU is working is by the observation of the working fan and the bulb current limiter's bulbs are not lights up even slightly. ( May be my consumption is totally wrong )

[CJay]

I'd say you're nearly there with the PSU, can you post a decent pic of the PSU board with the connections to the front panel and main board?

[donkey]

Thank you my friends ( CJay, eblc1388, LateLesley ) for your time and eager to help me to resolve my problem.

I have found out the problem. The problem was not the front panel connection but the short between the transistors heat sink between the radiator which is connected to the earth ground i suppose. I think someone has opened this unit before and removed all the protector plastic caps over the transistors to make a better thermal connection with the radiator. How, Dave says this kind of situations << WRONG!!! >> . I have isolated all the transistors with a paper between them and everything start working. The problem was one of the 11N80C3 Power Transistors on the primary side and the isolation between the transistors to the radiator.

The system is now working properly. I now need to find something thermally conductive and electrically not conductive to isolate the transistors from the chassis.

Do you have any suggestions?

Thank you very much to all of you again.

[CJay]

Depends how the transistors are fastened to the heatsink...

I *think* they're all TO-220 transistors and diodes so google for transistor insulating washer TO-220 and you'll get a slew of hits.

 

[donkey]

Thank you  CJay.
See you!

[LateLesley]

Yeah, CJay is right I think. here's an example of what you are looking for.

https://www.aliexpress.com/store/product/5Pcs-TO-220-TO220-1x14x20mm-Ceramic-Transistor-Triac-Thyristor-Insulator-Insulation-Protection-mica/1892449_32777214417.html?spm=2114.search0104.3.42.49816081USrrlI&ws_ab_test=searchweb0_0,searchweb201602_4_10152_10151_10065_10344_10068_10342_10325_10343_10340_10548_10341_10696_10192_10084_10083_10618_10307_10301_10135_10821_10133_10869_10868_10059_100031_10103_10624_10623_10622_10621_10620,searchweb201603_1,ppcSwitch_5&algo_expid=fb83e295-2bb7-45d2-9b54-1cb727596e46-6&algo_pvid=fb83e295-2bb7-45d2-9b54-1cb727596e46&transAbTest=ae803_2&priceBeautifyAB=0

[donkey]

Great! Thank you LateLesley.

[abraxa]

I have a power supply unit of a PA system. I am a total beginner on repairing stuff. This unit is belongs to my friend and he asked me to look at it if i can help. I accepted it because i thought that it might be a fuse problem and i could solve it with my slight superficial knowledge by replacing it with a new one  .

Let me just say that for someone who claims to be a total beginner, you did an excellent job. Keep it up!