Hello guys, hope everyone is doing well.

I am new here and looking to learn a thing or two about PCB power supply repairs that willl help with a project I am currently working on.

I believe schematics for my PCB will help with the repair, and I had a search on the forum to see where I may find it, But I did not find anything.

(Apologies if it is here)

My question, does anybody know where the best place to download a schematic for my PCB board online?

Its a power supply from a Samsung HW-E450 sound bar. The power supply is dead.

Am having trouble with a BUSS SR-5 250V 4A fuse. Instantly blowing and blowing the cap off.

Just wondering if schematics is a good place to start ?, I have changed a capacitor that looked bad, but other than that I still have the problem.

Many thanks for everybody's input. Very much appreciated. Hopefully this group will have me successfully repair my very first PCB repair. .

 

I would start by checking that larger blue disc close to the fuse. It may be a transient suppressor that is shorted (MOV). From there, check the bridge rectifier (4 legged device near capacitors/heatsink)

 

That's a switch mode psu, probably the MOSFET has gone s/c and maybe the bridge rectifier.
For what it's worth these are not worth repairing, as they're cheaper to replace.

 

Hi Sagor / Dave.

Thank you for your kind response, very much appreciated.

Dave, I have looked everywhere for this board and most companies have it OOS. The only company where it is in stock is abroad, and around £80. + shipping. (System is worth £60)

This has now become a little project, I know nothing about PCB boards and I am keen to learn a thing or two. I dont mind paying the odd couple of £ , on potential parts. At the moment I have only spent £4.00 on two fuses and a capacitor. It would be a great challenge for me if I actually got it to work considering I am very new to this.

Hi Sagor, would I need to remove these parts from the circuit to test they are working ? I have a multimeter that also measures capacitance.

How would you usually test these two parts?
The larger blue disk / The black rectangle would it be for continuity or veryfing the ohms reading ? If ohms , would each part say what it should read? I.e like a capacitors UF , that way I should be able to verify it?

Many thanks guys.

 

The blue disc you can measure with ohm setting, but you may have to lift one leg. Between that and the bridge rectifier, either one that is shorted will show a short to both if left in circuit. All you have to do is lift one leg of the blue disc item. It should read "open" or high resistance. If very low ohms, it is likely shorted. Leave it out while testing the bridge rectifier in "diode mode" with your multimeter. For the bridge rectifier, you should get readings between the AC side and the + or - side of "open" with leads in one direction, and about 0.7 (give or take some) with leads in the other direction. Check from each AC leg to both + and - outputs. You should see the open/0.7 readings on each leg. Any really low reading on the bridge rectifier will suggest it is shorted somewhere.

 

Service manual available here: https://elektrotanya.com/samsung_hw-e450_sm.pdf/download.html . Obligatory warning---- This is a switch mode power supply operating directly off the power line. As such, it creates special servicing hazards. For a 120 VAC line input, you will find voltages as high as 170 VDC, with no isolation from the line. Use extreme care.

 

Quick look at schematic, it comes back to VX801S (blue MOV) and the bridge rectifier BD801 as first suggested. Or if the MOSFET is shorted (QA801), it could be shorting the B+ supply. As Ylli says, B+ will be around +170VDC with no load, very dangerous.

 

Here are two snapshots of the primary side of the psu..

 

Got a meter with a 'Diode' range on it? Temporarily replace or jumper the fuse. Set your meter to the diode range and connect it across where the power normally plugs in. The two pin connector on the end of the board with the fuse. The reading should start out low (around 1.2) and then drift up. Let us know.

 

This is a switch mode power supply operating directly off the power line. As such, it creates special servicing hazards. For a 120 VAC line input, you will find voltages as high as 170 VDC, with no isolation from the line. Use extreme care.

At a minimum, only power the device from a GFCI socket (buy a GFCI outlet or power strip if needed) to protect you from the lethal voltages to earth ground.

 

Hello Guys. Thank you once again to everyone who has responded with their advice . I greatly appreciate your help.

Especially thankyou for the assurance of making me aware how dangerous this also is, and to take precaution.

I not sure about the safety measure I am taking but I am always stood 1.5 meters away from the board ( at the wall outlet which I believe will be GFCI in the United Kingdom before I attempt to turn it on at the socket)

I am sure the socket will be GFCI, they come as standard in health and safety regulations here? Also when the fuse blown first time, the main breaker switch in the house had triggered and I had to manually re-enable the sockets. That said it should be GFCI?

Following advise from Sagor,

I actually removed VX801s from the board, (blue disc) and I am getting no reading at all.

No continuity.
No Ohms.
No resistance.

Just OL - Open Load ?

Does that mean their is a problem with it ?
Should I replace it?

It also looks burnt ? Not sure if that was caused from the fuse exploding.

Many thanks

Craig

 

Since it shows open, my first though was that VX801S is probably still good. But that charring is not directly in line with the fuse, and it is the kind of damage you see on a varistor when it has broken down. Just for safety sake, it would be a good idea to replace it. Mouser has it https://www.mouser.com/ProductDetail/Bourns/MOV-10D561K?qs=lgttKnAd%2B2ArrqGQgWwr6w== for $0.17 plus the normal ridiculous price for shipping. You may be able to find it somewhere else, (Bourns 10D561K), but as this is a safety related part, do get it from a reputable vendor.

Set your DMM to diode mode. Connect the leads to the point where VX801S was (polarity irrelevant). Note the reading. Hold them there for a bit and monitor the reading. Is it steady or is it increasing? Wait a couple minutes to allow caps on the board to discharge. Now reverse the meter leads and redo the measurements. What do you get?

 

Reference to post#8,
Put your meter on Ohms and measure the resistance across the Capacitors CA801/2, ( output of the bridge rectifier) also measure the resistance of the position where the the VX801 came from.

 

Are you testing the power supply while adding a 40W incandescent lamp in series with the mains?
This is a good trick to limit the current to "non smoke" levels, and the power supply will usually operate ok, without a load that is, enough for you to see what is going on. If the lamp lights bright, there is a fault. But if it flashes then glows dully, there is no short anymore.
Doing this has saved me many exploding parts.
The lamp can be connected externally or just in place of the fuse. I have a purpose built power lead with a lamp socket fitted for just this use.
But be careful, as has been stressed above.
And if you have one, use an isolating transformer, or make one with 2 identical power transformers connected back to back.

 

Not going to get into a long explaination of why, but using a DBT can be problematic with a switch mode supply. Remember that a SMPS will draw more current when the input voltage is reduced.

 

Hi guys. Thanks for everybodys input. Some really useful information to keep in mind.

Having waited on parts to get delivered. I Got my varistor delivered yesterday and a new fuse. Made sure the circuit looked good.

Unfortunately it blew again. After changing the varistor and new fuse.

This is out of my depth, but I could be wrong with this theory. I was checking the ohms between the fuse which I think Is correct for close to 0, as their should be no resistance? Up until the end point of the NTC Thermistor (next component)

NTC 3D-9 the resistance between this was showing 6.8ohms and so forth to the other components in line...

After blowing the fuse, it now reads 36 ohms. Even desoldered and took it out and it still reads 36 Ohms.

After reading up on the NTC Thermistor, this should only allow 3 Ohms. 4Amp

I think this may be the problem or one of the problems.

1) Am I correct in saying this Thermistor should be reading 3 ohms at room temperature.?

One more thing,

2) if it is an issue with the NTC, I am really struggling to find a replacement of the exactly rating. Closest I can find is an NTC 3D-15.

Can I test it with this ?

Many thanks all.
Hope everyone is well.

 

Repeating my suggestion from post #12: "Set your DMM to diode mode. Connect the leads to the point where VX801S was (polarity irrelevant). Note the reading. Hold them there for a bit and monitor the reading. Is it steady or is it increasing? Wait a couple minutes to allow caps on the board to discharge. Now reverse the meter leads and redo the measurements. What do you get?"

 

Unfortunately it blew again.

Look at my post #14.
That would have saved the fuse.
You need to limit the input current while testing.