12V rail is not on ?

[budm]

Re: 12V rail is not on ?

I've taken the four diodes of the circuit test it with LED and they are normal...must be something else....could it be PWM chip 2003 ?

Did you not look at the diagram you provide and see that it has load resistors connected in the circuit and see how the current flow using your LED tester? You are skipping the basic understanding of the electronics. What does the PWM IC have to do with the test LED setup when the IC is in the primary side and not active and isolated form the cold side. You need to understand the basic of what you are testing first.

[bianchi77]

Re: 12V rail is not on ?

Did you not look at the diagram you provide and see that it has load resistors connected in the circuit and see how the current flow using your LED tester? You are skipping the basic understanding of the electronics. What does the PWM IC have to do with the test LED setup when the IC is in the primary side and not active and isolated form the cold side. You need to understand the basic of what you are testing first.

So what do I miss here....
the primary side is ok...I've checked the diodes on the secondary side

My thinking is only SBL2040, because it's 0.17V only on diode DMM test...on datasheet it must be 0.55V.....
...??

[budm]

Re: 12V rail is not on ?

Your meter is not putting out enough forward current to get the Vf as shown in the spec sheet. look at Fig 3 Graph.

[budm]

Re: 12V rail is not on ?

https://www.badcaps.net/forum/attach...4&d=1469140301
So let me get this correctly since the thread is getting really long.
Based on that diagram you provided, you are getting the
+5VDC
-5VDC
-12VDC
+3.3VDC
But no +12VDC?

[budm]

Re: 12V rail is not on ?

https://www.badcaps.net/forum/attach...6&d=1469104616

What is the P/N of the middle diode D14? Do you have the pictures of the correct diode you have in the middle that you installed"
Base on you picture, the diode in the middle has Cathode connected to the heatsink which cannot be right if it is for the +12V power supply, none of the Common Cathode in your diagram has direct connection to the heatsink which is connected to the circuit GROUND, is that right?
Something is not making any sense based on the diagram and your pictures.
BTW, that diode assembly in the middle, it that the original the way it was before you replace them with TO-220 3-LEG Diode with common cathode?
The heatsink for the diode is connected to the cold side GROUND?

[budm]

Re: 12V rail is not on ?

BTW, looking at the pictures of the Diodes mounted on the heatsink, I do not believe the SCH matched your board.
We need GOOD CLEAR STRAIGHT shot pictures of the top side of the board and the bottom side of the board.
It is time for you to trace the board to find out how those diodes are actually being used on the board.

[bianchi77]

Re: 12V rail is not on ?

Ok, I've taken some shots, please have a look...I checked it again...
all the voltage are gone only 5V SB...
no 12V,5V,3.3V,-12V and -5V...

[bianchi77]

Re: 12V rail is not on ?

I measured the middle diodes are connected to 12V rail...
D15 = 5V ,D14 = 12V and D13 = 3.3V, there's some kind of an isolator attached between the diode body and heat sink, so it's not straight to GND,
it's originnaly like that...

[budm]

Re: 12V rail is not on ?

You can see the top legs (CATHODE of the Diodes) of the two DIODES are soldered to the bracket, right? So measure the resistance between the heatsink body and the jointed legs, it should show 0 Ohm.
There is no isolator that I can see for the middle diode assembly.

[bianchi77]

Re: 12V rail is not on ?

yes it is zero, there is an isolator on it and I haven't put it back yet when I tested it...
I put it already now...
and I've done a test with 82 ohm resistor and I reckon those two diodes left and right are faulty....
There are three diodes giving a voltage on reverse way, suppose to be an OL...
Please have a look on my observation, cheers

[budm]

Re: 12V rail is not on ?

BTW, so you doi not have any Voltages at all beside the STBY5V, but you had indicated all these time that there is no 12V but other Voltages are OK.
"Re: 12V rail is not on ?" which is completely not correct, so it means the primary cannot be working as you have indicated.

[budm]

Re: 12V rail is not on ?

yes it is zero, there is an isolator on it and I haven't put it back yet when I tested it...
I put it already now...
and I've done a test with 82 ohm resistor and I reckon those two diodes left and right are faulty....
There are three diodes giving a voltage on reverse way, suppose to be an OL...
Please have a look on my observation, cheers

What do you mean test with 82 Ohms resistor?
You better make sure there is no 0 ohm reading between the bracket and the heatsink when you put the insulator back in place.

[bianchi77]

Re: 12V rail is not on ?

There's no short on primary and the fuse is ok....so what happened to it ?...
the cause is a short accidentally between 5V to ground or 12V to ground I couldn't remember...

[budm]

Re: 12V rail is not on ?

yes it is zero, there is an isolator on it and I haven't put it back yet when I tested it...
I put it already now...
and I've done a test with 82 ohm resistor and I reckon those two diodes left and right are faulty....
There are three diodes giving a voltage on reverse way, suppose to be an OL...
Please have a look on my observation, cheers

So what were you doing? Using 82 Ohms and 6V power supply.
When in forward bias connection the Vf should be 0.2V or so, and when you reverse the diode then it should show 6V.
Show us the test setup.

[bianchi77]

Re: 12V rail is not on ?

Use 9V, 100 Ohms resistor in series with DUT (to simulate about 90mA of If) to forward bias the diode and use your meter to read the Vf of the diode under test to see what you get.
Vf will very with current flowing through the DIODE, Temperature also affect Vf.
If you test setup show LED lights in one direction only with that diode, the the diode is OK.
You can also use your variable power supply to reverse bias the diode to see if it will break down way before the Voltage break down spec. of the diode.

I got 82 ohm for testing..

[bianchi77]

Re: 12V rail is not on ?

https://www.badcaps.net/forum/showthread.php?t=56872

Post #11

[budm]

Re: 12V rail is not on ?

If the diode is bad then you should be able to use meter in Ohm mode and it should show low resistance in both direction if the diode is shorted out.

[budm]

Re: 12V rail is not on ?

I got 82 ohm for testing..

so what is that 6V you and the 82 Ohms you show in the pictures? I thought you are using 9V and 100 Ohm resistor.
"and I've done a test with 82 ohm resistor" You are saying you use 82 Ohms resistor.
You are confusing me and other people that are reading this thread.
Bottom line: if the diode is bad, shorted out, it will show low resistance in BOTH DIRECTION using Ohm meter mode.

[bianchi77]

Re: 12V rail is not on ?

yes it's an old 9V battery and I measured it for sure, it's 6.25V and I got 82 ohm resistor for testing...I don't mean to confuse everybody...I tried to debug this diode...

[bianchi77]

Re: 12V rail is not on ?

If the diode is bad then you should be able to use meter in Ohm mode and it should show low resistance in both direction if the diode is shorted out.

"low resistance" == few kilo ohm ? or few hundred ohm ?