thanks for the answers, i will try to find online or on old circuit boards. If i find this 1n4001 or 1N4148 numbers Diode then which one i have to replace on my board, like 1st diode or the other diodes.

thanks

 

No need to replace anything . Find the diode and let me know,I am going to see that whether you know how to check diodes first properly
Tackling a SMD loaded PCB is not easy.

 

first i search online 1n4148 Diode and i found exact matching picture of Diode on old board, so i took it out, hope this is the right Diode but i don't know what part number of this Diode.

dmm switch at diode
when i touch black probe on top & red probe on bottem then reading shows 568 , opposite side or exchange probes no reading found.

dmm switch at 2000k
when i touch black probe on top (anode) & red probe on bottem (cathode) then reading keep on increasing from 1000 over (i think this consider high resistance so reverse Bios), opposite side or exchange probes no reading (i think this consider low resistance so forward Bias).

 

dmm switch at diode
when i touch black probe on top & red probe on bottem then reading shows 568 , opposite side or exchange probes no reading found.

Good, this is how you check diodes out of the circuit.
Remember that 568 is actually 0.568V and is the forward voltage drop of the diode and reverse reading will be none.
what u have there is a normal silicon diode. A high current high speed diodes and Schottky diodes have a lower Vf. Sometimes even as low as 0.1V is common, but never 0V, which indicates a short, and if a short is exists then a reverse short will also be there.

When testing in circuit, forward reading will be between 0.1V to 0.6V never 0V, and Reverse reading is the confusing part. An in circuit reverse reading sometimes is correct and some times wrong. Either way if a dead short shows then chances are that the diode is faulty.
In circuit reverse reading could be any value which is the reading of the circuit resistance. But if shows an open circuit reading like an overload it means the diode is OK.

A diode in a power supply will show a varying reading at first and the reading will stabilize after a few seconds, which is due to the capacitors changing that are connected to the diode, so in circuit testing needs a few second to complete a good reading check and should allow time to charge the caps. Once the reading is stable then what ever you get is the diode reading.
And these things need time to master.

So do this and check that diode properly that is in your supply, see the last picture to see where the output diode is.
Post ur readings

 

I am not sure which one readings you are asking, so i try 3 diffrent readings according to Post#55 Circuit Diagram on OUTPUT Schottky Diode D1 only.

WHEN NO POWER DMM SWITCH ON DIODE
Red probe on top touch & Black probe on bottem touch reading is 864.
Black probe on top touch & Red probe on bottem touch reading is 136.

WHEN POWER ON & DMM SWITCH ON 20V & Black probe touch to ground
Red probe on top touch reading is 0.50V.
Red probe on bottom touch reading is 0.00V.


WHEN POWER ON & DMM SWITCH ON DIODE & Black probe touch to ground
Red probe on top touch reading is 830.
Red probe on bottom touch reading is 0 & DMM noise.

 

It seems you haven't used a DMM before right.
There are somethings that u should keep in mind.

NEVER, NEVER, NEVER, NEVER, NEVER, NEVER, NEVER,
EVER USE DIODE RANGE OR RESISTANCE RANGE TO TEST A COMPONENT THAT IS IN THE CIRCUIT WITH THE POWER APPLIED

You will never get a correct reading and possibilities are that you will destroy your Multimeter.

 

i don't know this i will not use in futrue when power applied DIODE RANGE OR in RESISTANCE RANGE

But i check again in Diode range with out applying power on it so i got this readings

Red probe on top touch & Black probe on bottem touch reading is 864.
Black probe on top touch & Red probe on bottem touch reading is 136.

what i have to check next.

thanks for the info i will remember the above things before use dmm in future.

 

Are you referring to the diode in the picture of post 55
and if so, give me the resistance reading with black probe to ground and red probe at diode cathode

 

DMM switch set on 20k, Black probe on ground & Red probe on Cathode (top of Diode) no power apply -- start at 2.39 & stop at 5.71 k Ω

when Red probe on Anode then reading is 0.00

 

Cathode reading is in the range of Kilo ohms?
Are you sure.
Did you replaced the capacitors?

 

i already replace the new capacitors begining of this thread 2nd & 4th capacitors on board by following polarities & same part number capacitors.

 

Hmm.
You know that picture you showed.
The capacitor and the inductor is blocking most of the view, I cannot make out the traces, Is it possible for you to get a picture exactly looking above or can you take out the cap and inductor and take a pic so that I can find the load of that converter

 

thanks any way for your time and efforts in this thread, i could not get the picture of all 4 capacators more better than previous, so i split the board in 4 peces and post back pictures.

now i want to stop testing this board, because i not even get any IC's & Diodes sale on online, i search every day not see any body selling this componentes.

thanks

 

So you cannot get the replacements.
Where are you located

 

i am in toronto canada, if i found this diodes on low price i want to replace all 4 diodes wether they are good are bad.

thanks

 

I doubt that the diode is faulty, but if you want to be sure you can desolder the diode and test it, but be carefully when removing surface mount devices, if you donno then you might damage the tracks or the components.

Give power to the board and place your finger on each IC (wait for a few sec)and tell me are they too hot to touch

 

i took it out Diode D1 as in post#55 picture, then test it
dmm switch at diode black probe on top cathode & red probe on bottom anode reading is 133 , & opposite side no reading shows

then power up board and tuch all ic's one by one, none of them are hot as usual like before this board.

 

one by one i took another 4th diode then check shows reading 135 & other side none on 4th diode.

now total 2 diodes took out from board as below in pictures
then power up still same no hot nothing.

 

Like I said the diodes seldom go bad, But the problem is both 5V and 3.3V are not working which tells me the problem is common, not the dc to dc converters itself.
There is something I might have overlooked, your problem may be quite easy, since major components are checked and working.
It is very difficult for me since the bulkier components are blocking certain areas I want to see. I wish you can give me a top direct view so the caps and inductors do not cover any thing from view.

 

ok today i will take out all inductors , capacitore & Diodes then i take a picture and i will post back.

but for the time being i like to know where i can buy parts like ic, & Diodes etc.