Chinese Power Supply kit troubleshooting :problem to ask for ...

[Ahmed Khouaja]

Hi guys
Like most people , i have problem with this kit wich is i get -0.45v when the V and A pot are fully anti-clockwise and i get only 5.1V when the V and A are fully clockwise.
I checked almost the whole circuit with the help of schematic that is made by "Netzener" .
I'm using 12-0-12 , 3A transformer...
So guys can you guess where is the probleme? I will attach a pdf file wich can help you understant the whole circuit...
The problem is this page :

between Vout- and R15 i cant get 11.4V , i only get the half of it wich is 5.2V and in the output i only get 5.2V when the pots are fully-clockwise ...
When the pots are fully anti-clockwise , the output give -0.45V !
Any suggestion ??
Thank you .

[xavier60]

Is the reference voltage correct?

[Ahmed Khouaja]

Yes, its correct !

[xavier60]

What voltages are on pins 2,3,4,6 and 7 of U2 with both Pots fully clockwise?

[m3vuv]

this looks like a hyland psu,can be made to work ok with mods,check this link out.http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

[Ahmed Khouaja]

What voltages are on pins 2,3,4,6 and 7 of U2 with both Pots fully clockwise?

PIN 2,3: 5.07V
PIN 4: -5.6 V
PIN 7: 30.1V
PIN 6: 5.2V
And i feel that the problem is in this area , maybe i'ill replace this U2 with another new TL081 ...

[Ahmed Khouaja]

this looks like a hyland psu,can be made to work ok with mods,check this link out.http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

Yeah with some modification it will defintly works, but i want it to work as it is , its a challenge for me   

[xavier60]

What voltages are on pins 2,3,4,6 and 7 of U2 with both Pots fully clockwise?

PIN 2,3: 5.07V
PIN 4: -5.6 V
PIN 7: 30.1V
PIN 6: 5.2V
And i feel that the problem is in this area , maybe i'ill replace this U2 with another new TL081 ...

There being the same voltage on both op-amp inputs usually means that it's in control of the CV loop.
Because the voltage feedback divider has a ratio of 3:1, the PSU's output should be close to 15V.
Some things are not right. Because the reference voltage is supposed to be 11.2V, the wiper of the CV Pot and pin 3 of U2 should also be 11.2V with the Pot fully clockwise.

[Ahmed Khouaja]

What voltages are on pins 2,3,4,6 and 7 of U2 with both Pots fully clockwise?

PIN 2,3: 5.07V
PIN 4: -5.6 V
PIN 7: 30.1V
PIN 6: 5.2V
And i feel that the problem is in this area , maybe i'ill replace this U2 with another new TL081 ...

There being the same voltage on both op-amp inputs usually means that it's in control of the CV loop.
Because the voltage feedback divider has a ratio of 3:1, the PSU's output should be close to 15V.
Some things are not right. Because the reference voltage is supposed to be 11.2V, the wiper of the CV Pot and pin 3 of U2 should also be 11.2V with the Pot fully clockwise.

Yeah i know thats something goes wrong in that area and i'm pretty sure that's the TL081 is  not behaving well , i will replace it with new one and recheck the results , hoping that will fix this issue because i'm really fed up , its almost one year now since i bought this piece of scrap 

[xavier60]

If you put a load on the output, does the voltage hold up or does it drop?

[Ahmed Khouaja]

If you put a load on the output, does the voltage hold up or does it drop?

Well i didn't try it but , i shorted the circuit wich gives me 3.3V approximetly as voltage and 4.25 as Ampere ( 2V across the R7 {Rshunt})...

[xavier60]

If you put a load on the output, does the voltage hold up or does it drop?

Well i didn't try it but , i shorted the circuit wich gives me 3.3V approximetly as voltage and 4.25 as Ampere ( 2V across the R7 {Rshunt})...

It would have been best with a normal load that doesn't cause current limiting so that the tests are on the CV loop only.
If a small load causes no drop of output voltage, it will mean that the op-amp is functioning properly and that one of the problems is with the feedback divider. The feedback divider is supposed to divide the output voltage by 3.

Maybe a wrong value resistor.

[Ahmed Khouaja]

If you put a load on the output, does the voltage hold up or does it drop?

Well i didn't try it but , i shorted the circuit wich gives me 3.3V approximetly as voltage and 4.25 as Ampere ( 2V across the R7 {Rshunt})...

It would have been best with a normal load that doesn't cause current limiting so that the tests are on the CV loop only.
If a small load causes no drop of output voltage, it will mean that the op-amp is functioning properly and that one of the problems is with the feedback divider. The feedback divider is supposed to divide the output voltage by 3.

Well , for the 11.2Volts in the 2 amplifiers are coming from the U1 wich in my case is right, means that the first amplifier function well , so that 11.2 Volts needs to appears in the R15 of the output circuit wich in my case is the only error, so the question is , what makes this 11.2Volts divide by 2 in the R15 wich is the bad result, i checked all the resistors in that area and theye are good that what keeps me out of my mind 

[xavier60]

The best way to diagnose a fault is by first understanding how the circuit is supposed to work.
Starting at the CV Pot, take voltage readings to find where the voltage drops down from 11.2V on its way to pin 3 on U2.

What is the voltage on both ends or R8?. then pin 3 again

[Ahmed Khouaja]

The best way to diagnose a fault is by first understanding how the circuit is supposed to work.
Starting at the CV Pot, take voltage readings to find where the voltage drops down from 11.2V on its way to pin 3 on U2.

What is the voltage on both ends or R8?. then pin 3 again

Well at R8 the voltage is equivalent to the pin6 of the op-amp wich is 5.2V
Well i have clearly image for the schematic diagram , can you please tell wich resistor i need to check and focus on it ?

[xavier60]

Take exact voltage readings on the
left side of R8,
right side of R8,
top of R9,
and pin 6 of U3.

[nali]

Take exact voltage readings on the
left side of R8,
right side of R8,
top of R9,
and pin 6 of U3.

...and Cathode of D9

[Ahmed Khouaja]

Okey, i'll check them when i replace the U2 and the 2 transistors 2SA9015 and the 2SA9014....

[Ahmed Khouaja]

Finally guys, i just made it , i fixed it.

The problem was with the U2 :TL081

The output of this IC was giving only 6V as maximum output...

Thank you guys for your replys , it really helps me ...