PC monitor does not turn on

[sagor1]

Monitor has to be powered on to measure those voltages. The key ones are 5V to GND and the ON/OFF to ground.
Try what Nigel says first, heat up the D901 and D902 on one leg and lift that leg out of the hole. Then test with Diode function on the meter. Lets do that first, before doing much else...

 

[rjenkinsgb]

This manual on Elektrotanya has the PSU board schematics - see page 29:

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.

elektrotanya.com

 

[JMaia]

Monitor has to be powered on to measure those voltages. The key ones are 5V to GND and the ON/OFF to ground.
Try what Nigel says first, heat up the D901 and D902 on one leg and lift that leg out of the hole. Then test with Diode function on the meter. Lets do that first, before doing much else...

I already tested the 5v to GND and ON/OFF to GND, since it's easy and quick to do. 5V to GND is between 100 and 440, and ON/OFF to ground oscillates between 010 and 045. with mode DCV20 on multimeter.

This manual on Elektrotanya has the PSU board schematics - see page 29:

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.

elektrotanya.com

I dont think thats the same monitor

Tomorrow I'll try and desolder and test the diods.

 

[sagor1]

5V readings of 100 to 440 is wrong. So, check those diodes and report back later.
That schematic has the same diodes with same designations (D901, D902) as your board. Though it may not be exactly the same, odds are the design of the power supply is based on similar designs.

 

[rjenkinsgb]

I dont think thats the same monitor

Maybe, maybe not - it does not matter.

What does matter is that it has the same PSU board, the "715G4497" as printed on your PSU PCB.

The components match. Yours may have different options but the "guts" of the PSU are the same.

 

[JMaia]

I think I may have to buy a new tip for my soldering iron. It's all black and won't even melt any solder. I tried to wipe it on a wet sponge, immerse in flux and melt some solder on it but it wont melt. I'll try to get some tips and try again.

 

[JMaia]

After all this time, I finally got the material to try and desolder the diodes so I could test them, but I f***** up and broke one of the legs:

Even after heating the joint for a long time and using braid I was unable to remove. The leg broke after I tried removing it with a plier.

On the diod with the leg removed, testing with multimeter in diode mode, reading is 362, if I reverse the leads there is no reading.

I don't know how the hell I'm going to remove these diods from the board, anyone has any tips?

 

[Nigel Goodwin]

I don't know how the hell I'm going to remove these diods from the board, anyone has any tips?

It's absolute child's play - you simply melt the solder on the bottom and pull the wire out of the top with long nose pliers. Generally I would push the wires through from the bottom first, with the soldering iron, as they often go through quite a long way. You don't need any kind of solder wick or desoldering tool.

The wires are thick though, as are the PCB tracks, so you need a fairly substantial iron, a 15W one with a small bit probably wouldn't do, and a 45/50W temperature controlled iron with a medium/large bit would be much better.

Or - simply cut one end of the diodes to test them - like the broken one, you can solder them back together on the top afterwards (the crimp in the wire stops the end falling through).

 

[sagor1]

If you can remove the broken part of the leg from the hole, that diode will still fit, it will just be a bit lower to the board (and look a bit crooked).
Those diodes seem to be 5A types, SR515
Your meter reading seems to be correct, and I think both diodes are in parallel. Try reading the other one in-circuit while the first is still disconnected.

 

[JMaia]

Try reading the other one in-circuit while the first is still disconnected.

I did, the other diod still in circuit reads 002

 

[wkrug]

When there is a short at that Power Line You'll measure ~0V across the Diodes too.
I would do what Nigel said an solder out the second Diode too an measure again.
When this Diode is OK too, remove the Connector to the other PCB an measure if the short is present then too.
A faulty Part could be the Capacitator C918 or other things in this Line.

Please let help someone ( or look for better soldering Equipment ) to solder out the Parts, to avoid damaging the PCB.

 

[JMaia]

OK, so I finally did it, had to swap to a 60 watts iron, but it still took a lot of time:

multimeter in diod mode, both diods with one leg out of the circuit:

Diod 12 reads 358, when I reverse the leads reads 1, (I don't think it gets any reading)
Diod 11 reads 002, same thing when I reverse the leads

 

[sagor1]

Diode 11 is dead then (shorted). Good diagnosis, finally. Try to find a replacement. Replace both diodes at the same time, just to be sure.
That may not be the end of it yet, check other devices downstream from those 2 diodes.

 

[JMaia]

how do I know which diodes to buy? it's written sr515 on them, should I search this online?

EDIT: OOPS sorry i totally missed this:

Those diodes seem to be 5A types, SR515

 

[sagor1]

Yes, SR515 are the original types. They are 5A Schottky Diodes, rated for 100V (many are 150V). You may be able to find substitutes as long as they are Schottky, 5A and at least 150V (200V or higher will do as well).
Depending on what sources you have, you should be able to find them on most online vendors like Digikey or Mouser. Avoid any Chinese supplier from places like Ebay or Amazon, those are usually fakes or lower rated diodes made to look like the real thing.

 

[wkrug]

How old is the Monitor?
Solder out C918 when it's soldered out You can Check it with a good Capacitance Meter.
At suspect measurement it should be changed.
When You don't have such equipment and it is older then 6Years I would change the Elko C918 too with a low ESR Type.
It could be a part of the Problem.

I don't know Your suppliers in Portugal.
In Germany i bought most of my Parts at www.reichelt.de

 

[Nigel Goodwin]

Solder out C918 when it's soldered out You can Check it with a good Capacitance Meter.
At suspect measurement it should be changed.

No point with a capacitance meter - if you want to check it you MUST use an ESR meter, as they fail low-ESR if they are going to fail. A capacitance meter would almost certainly check it as OK, even if it was faulty - unless it does ESR, in which case it's an ESR meter.

When You don't have such equipment and it is older then 6Years I would change the Elko C918 too with a low ESR Type.
It could be a part of the Problem.

VERY unlikely - the two diodes are in parallel with each other, a SERIOUS design flaw, which means they WILL fail, it's an incredibly common fault among the cheap and poorly made TV brands. It would be a nice idea to fit the required current balancing resistors while you're doing, or at least ensure that the diodes are well matched - use ones out of the same packet.

There's no concern about the exact type of diode, as long as they are of a similar speed, current and voltage - we always used to keep one type (well actually two - big ones and small ones ) at work and used them in everything. In fact, in my new job, I've repaired a few TV's, and fitted MBR340's which we use in some of our products, they work perfectly.

 

[wkrug]

No point with a capacitance meter - if you want to check it you MUST use an ESR meter

About that cause I said to use a good! Capacitance Meter.

Mine shows the Capacitance, ESR value, and the test frequency is changeable.

 

[Nigel Goodwin]

About that cause I said to use a good! Capacitance Meter.

Mine shows the Capacitance, ESR value, and the test frequency is changeable.

Most ESR meters give you the capacitance as well - but it doesn't really matter, it's the ESR that's important (but almost certainly not a concern in this case anyway).

 

[JMaia]

VERY unlikely - the two diodes are in parallel with each other, a SERIOUS design flaw, which means they WILL fail, it's an incredibly common fault among the cheap and poorly made TV brands. It would be a nice idea to fit the required current balancing resistors while you're doing, or at least ensure that the diodes are well matched - use ones out of the same packet.

Can you explain in more detail about those balancing resistors? How to install them, and which ones to buy?