30V 3A Powersupply - regulating only to 8V

[worldofskin]

Hey guys! i build a 30V 3A powersupply. My friend blown it... He connected cables to the wrong connectors. I changed the op amps, shorted zener diode and a 9014. Now the powersupply works again but only to 6V. When i measured the caps they looked in spec. But after i connected 2200uF cap on the main power rail parallel to the C1 - 3300uf one - output voltage is now regulatable to 8v. Any idea where to look ? or what to do ? R1 and R2 are getting hot. The input of the transformer is 40V after rectifiers. Its a 24V transformer on 230V mains voltage.

[redwire]

I looked at the voltages and most are OK. The IC's are powered at 44V which is up there.
I would guess Q1 SS9014 replacement has the wrong pinout or installed wrong - should be EBC. Double check whatever you put in there is NPN. It is probably clamping U2 output at the 8V or so. U2 would also get quite hot.
Q1 is supposed to conduct only during power on/off to prevent output voltage spikes. You can remove it, run the PSU and see if it works past the 8V. Otherwise U2 is bad.

[SMDFlea]

Post the full PSU model number,PCB model number and pictures of both sides of the PCB

[harp]

Maybe to put some more information about your friend, what is connected (what kind of load, battery...), where and how (respect to polarity)... I supose that you check everything and any components seems good in spec?

[redwire]

I recognize the schematic as the infamous "0-30V 2mA-3A" power supply kit board. It's a chinese copy of the Practical Electronics Nov. 1978 design but not using 741's.

OP if you take voltage measurements on all the IC's pins and post them here, can figure what is wrong. Use the output(-) as multimeter(-) common, and careful not to short stuff out with the (+) probe.

Make sure the -5.1V rail is working zener D7 should have 5.1V across it. If that rail is not working U2 will get very hot.

[worldofskin]

Post the full PSU model number,PCB model number and pictures of both sides of the PCB

101-92-133

//

[worldofskin]

Maybe to put some more information about your friend, what is connected (what kind of load, battery...), where and how (respect to polarity)... I supose that you check everything and any components seems good in spec?

My friend is a guy for whom i build the PS. Its a graduate project.... What happened is he connected the 24V output from 7824 to one of the connectors for potentiometers - my guess is to the P2.(i connected external disply on the 7824 regulator instead of a fan, and the display has the same connector as the potentiometers) It has blown U2, the 9014, and the 5V zener diode.
Yes i checked i would say "all" of the components and they seem ok. I know that if they were - the PS would work ... But i am too short for this one.

[worldofskin]

I recognize the schematic as the infamous "0-30V 2mA-3A" power supply kit board. It's a chinese copy of the Practical Electronics Nov. 1978 design but not using 741's.

OP if you take voltage measurements on all the IC's pins and post them here, can figure what is wrong. Use the output(-) as multimeter(-) common, and careful not to short stuff out with the (+) probe.

Make sure the -5.1V rail is working zener D7 should have 5.1V across it. If that rail is not working U2 will get very hot.

Thank all of you for all the replies! Not one teacher of my school could help me! You helped me more than them even now!!
I replaced all of the OP amps for "LM741CN" - because of the high input voltage. Its still 1V over the max but i think it should work for a little while

Yes thats exactly right (That it is the chinese powersupply kit). i am sorry for not posting photos of the board. Idk why i didnt.
Both of the zener diodes have 5.1V across them.

Measured with the black probe (-) -> on the OUT -
U1
1- 5mV
2- 5.146 V
3- 5.142 V
4- 0
5- 5,2mV
6- 10,28V
7- 39,9V
8- 0

U2
1- -5,2V
2- regulatable from 0 to 2.2V - after 2.2V it drops back to 2.1V
3- Regulatable from 0 to 10.25V
4- -5,2V
5- -5.2V
6- regulatable from 0 to 7.26V - with the Voltage potentiometer - P1 // output voltage is 6.5V when is this pin 7.2V
7- 38,9V
8- 0

U3
1- -5.2V
2- 0V
3- regulatable from 0 to 1.4V with the Amp potentiometer - P2
4- -5.2V
5- -5.2V
6- 38.9V
7- 39V
8- 0V

[harp]

What is voltages on base q1 q2 q3 q4?
Have you check that transistors not shorted, are they heating while power on?
Output voltages from psu is... you say 6v... is this regulated, how much current can you pull out without voltage droped?

[redwire]

I looked at the voltages and most are OK. The IC's are powered at 44V which is up there.
I would guess Q1 SS9014 replacement has the wrong pinout or installed wrong - should be EBC. Double check whatever you put in there is NPN. It is probably clamping U2 output at the 8V or so. U2 would also get quite hot.
Q1 is supposed to conduct only during power on/off to prevent output voltage spikes. You can remove it, run the PSU and see if it works past the 8V. Otherwise U2 is bad.

[worldofskin]

I looked at the voltages and most are OK. The IC's are powered at 44V which is up there.
I would guess Q1 SS9014 replacement has the wrong pinout or installed wrong - should be EBC. Double check whatever you put in there is NPN. It is probably clamping U2 output at the 8V or so. U2 would also get quite hot.
Q1 is supposed to conduct only during power on/off to prevent output voltage spikes. You can remove it, run the PSU and see if it works past the 8V. Otherwise U2 is bad.

You are the KING!! Thank you so much. Replaced the 9014 and it works!! But it was soldered correctly.... Maybe just a bad transistor...
Thank all of you for the replyes! Have a great day!!