I worked on this circuit 16 years ago at www.electronicslab.com. Then it was a defective kit from Greece.
1) The TL081 opamp Q2 is destroyed by over-voltage as mentioned in our thread here. The TL081 maximum allowed supply is only 36V but the circuit needs a much higher voltage.
The transformer must be 28V minimum to get 30V at 3A from this project. Without a load the transformet might produce 29.5V which makes a rectified +40.3V and my negative supply is 1.4V so Q2 is powered with 41.7V which is less than its maximum allowed voltage of 44V. TWO output transistors must be used on a large heatsink to share the heat.
I used TLE2141 opamps because their maximum allowed supply is 44V and Q2 works fine with only 1.4V for the negative supply (the TL081 must have at least 4V for its negative supply).
2) I increased the value of C1 to 12000uF.
3) Like mentioned here, I used 5mA zener diodes instead of the ones in the kit that must have a much higher current.
4) I increased the power ratings of a few resistors so they don't burn up when the output of this project is 3A.

The Chinese copy is of the original old defective kit.

Dear Audioguru,
Please can you help me?
I have a Smartkit 1138 0-30 volt 3 amp power supply which I purchase as a kit about a decade ago. I am an electronic dummy and my limit is assembling a kit. I assembled this kit and it has worked flawlessly all these years for the bits that I do with it. I doubt that I have ever drawn more than 1.5 amps off it. So all went well until I decided to upgrade it with a digital voltmeter - ammeter. I connected the ammeter incorrectly and that was the end of my power supply. With my limited knowledge I removed all the transistors and found that the 2N2219 and the BC548 were both damaged. So I replaced them with no success. So I purchase 6 new TL081's and fitted them with no success. So I took out the transistors again and found that the 2N2219 was again destroyed. So I replaced that again and removed all the diodes except the rectifier ones and tested them. They all checked out good. The zeners measured 5.12 and 5.2 volts respectfully. So I reassembled and fitted my last 3 TL081's and no result. I have checked my transformer voltage (25.2 ac) and the rectifier and that seems to be OK. I was very careful with the transistors and checked the data sheets for the pinouts so I am sure that they are mounted correctly. The 2N2219 gas a finned heatsink fitted to it.
I do not know what else I can do to fix my power supply.
Would it be possible for you to spoon feed me a bit here and tell what to test and where to test it and what to replace? Please treat me like the idiot that I am and try and get my power supply working again.
I contacted the original supplier of this kit with a view to buying a new one but the tell me that it discontinued so no linger available.
If you have any suggestions as to what components I should upgrade to make this power supply a little more immune to my trying to destroy then please let me know and I will purchase and fit the parts.
Thank you in advance if you can help me and I do hope that I am not wasting your time.
Regards Patrick

Moderator Edit: New thread created from this.

 

To suggest any thing else without it being a guess, it will require a circuit diagram (schematic).
Also, my first suggestion is to undo whatever connections were done when adding the digital meters, for the reason that is when the failure happened.
In addition, be aware that many digital meters require a completely isolated power source, with no connection to the circuit being measured.
AND, much of the time, when a component is destroyed, that component was not the cause of the failure, and so simply replacing the part will not solve any problem.
My initial guess is that a wrong connection made with adding the meters is still in place. This may include a reversed polarity in some connection.

 

Quasar Electronics
https://quasarelectronics.co.uk › ...PDF
QUASAR KIT No. 1138 0-30 VDC STABILIZED POWER SUPPLY WITH .. circuit diagram..

 

To suggest any thing else without it being a guess, it will require a circuit diagram (schematic).
Also, my first suggestion is to undo whatever connections were done when adding the digital meters, for the reason that is when the failure happened.
In addition, be aware that many digital meters require a completely isolated power source, with no connection to the circuit being measured.
AND, much of the time, when a component is destroyed, that component was not the cause of the failure, and so simply replacing the part will not solve any problem.
My initial guess is that a wrong connection made with adding the meters is still in place. This may include a reversed polarity in some connection.

Hello MisterBill2,
Thank you for your response to my cry for help.
I used a separate 12 volt supply for the voltmeter ammeter with it's own rectifier by tapping off the AC supply. It was when I connected the ammeter side that I damaged the power supply. When I realised what I had done I disconnected and removed the meter before attempting the repair. Essentially the ammeter shorted the output of the power supply (I think) I have changed the transistors twice, changed the op amps 3 times and removed and tested all the diodes. That is about all I am capable of doing but nothing has worked. Should I remove the capacitors and test them as well?
Attached is the circuit diagram as requested, If you go to the website where I obtained the circuit from there is a very detailed description of how this power supply works but it is no good to me as I don't understand it.
Thanks again, any help that you can give me will be much appreciated.
Regard Patrick

 

Is Q2 (2N2219) damaged again?
Does this happen with no load on the supply or only with a load connected?

 

Is Q2 (2N2219) damaged again?
Does this happen with no load on the supply or only with a load connected?

Hello AlbertHall,
Thank you for the reply.
After replacing the transistors and op amps the first time, the power supply put out a low voltage of 9.8 volts and this could be varied down to zero. So I connected a 5mm green led to the output and set the voltage to 3.2 and the led switched on. The current pot had no effect. The led ran for about 30 seconds and then the output voltage disapeared again. That's when I removed the transistors for the second time and found the 2N2219 was fried again. The LED was not dammaged.
Hope that answers your question. Now that I have replaced the 2N2219 for the third time the power supply has no output at all.
Regards Patrick

 

What are the values of R5 and R6 from the parts list (can you post the full parts list)?
[Edit] Set the current and voltage pots to the centre and have no load on the output.
Please measure the following voltages and report here with the black meter lead on the negative output terminal:
U2 pin 4
Q4 colector
U1 pin 6
U2 pin 6
U3 pin 6

 

What are the values of R5 and R6 from the parts list (can you post the full parts list)?
[Edit] Set the current and voltage pots to the centre and have no load on the output.
Please measure the following voltages and report here with the black meter lead on the negative output terminal:
U2 pin 4
Q4 colector
U1 pin 6
U2 pin 6
U3 pin 6

Thank you AlbertHall,
I will do those readings and report back shortly.
Do you think that replacing the TL081 with TLE2141 would be a good idea.
R5 and R6 are both 10k 1/4 watt

 

Thank you AlbertHall,
I will do those readings and report back shortly.
Do you think that replacing the TL081 with TLE2141 would be a good idea.
R5 and R6 are both 10k 1/4 watt

Here is the parts list on 2 pages

 

Here is the parts list on 2 pages

Here is the parts list on 2 pages

Hello AlbertHall,
Voltage readings as requested:
U2 to pin 4 minus 5.64
Q4 collector plus 42.7
U1 pin 6 plus 11.0
U2 pin 6 minus 3.16
U3 pin6 minus 3.22
Additional information:
AC input = 31.7 volts
After rectification = 42.8 volts
R7 and R3 are getting very hot

Hope that helps
Looking forward to hearing from you
Regards Patrick

 

Please confirm that there is nothing connected to the positive output terminal and report the output voltage.

 

Your opamps U2 and U3 have their positive supply pin 7 at +43.8V and their negative supply pin 4 at -5.64V.
Then their total supply voltage is 49.44V. But the absolute max allowed supply for the TL081 opamp is only 36V so they are way over-voltaged.
I used a 28VAC transformer (yours is too much at 31.7VAC but in post #1 you said 25.2VAC) and TLE2141 opamps that are rated at 44V and I changed the negative supply to -1.4V (the TL081 needs -5V) so that my opamps get a total supply of 39V which is fine.
I replaced the antique 2N2219 with a real little BD139 power transistor bolted to a modern heatsink.

 

Your opamps U2 and U3 have their positive supply pin 7 at +43.8V and their negative supply pin 4 at -5.64V.
Then their total supply voltage is 49.44V. But the absolute max allowed supply for the TL081 opamp is only 36V so they are way over-voltaged.
I used a 28VAC transformer (yours is too much at 31.7VAC but in post #1 you said 25.2VAC) and TLE2141 opamps that are rated at 44V and I changed the negative supply to -1.4V (the TL081 needs -5V) so that my opamps get a total supply of 39V which is fine.
I replaced the antique 2N2219 with a real little BD139 power transistor bolted to a modern heatsink.

Hello Audioguru again.
Thank you for your response. I have read many of your posts over the years and I was hoping that you would get involved with my post. It is obvious to me that you are a real expert on this particular power supply.
Firstly and this is probably irrelevant, the transformer identification tag rates it at 25 volts 90 va but I have never measured the output until now. My power supply has operated with this transformer for the past 6 years and I was surprised when I measured the output and realized that it was too much for the TL081's
Are you saying that I should replace the TL081 with TLE2141? If so then I will try and source them. I am based in South Africa so we can't always get what we need locally so it may take some time to get them from overseas if I have to.
Should I change my negative supply to 1.4 volts? If so how do I do that? Is it a matter of just changing a component?
I will source the BD139 now and replace the 2N2219. I have some very nice heat sinks that it will fit on so that is no problem.
Thanks again for your help so far.
Regards Patrick.

 

Please confirm that there is nothing connected to the positive output terminal and report the output voltage.

Hello AlbertHall
There is nothing connected to the output terminal during my measurements and the output voltage is zero.

 

Please measure and report the voltage across R7.
Check C7 and D11 for short circuit.

 

Please measure and report the voltage across R7.
Check C7 and D11 for short circuit.

Hi AlbertHall,
All capacitors have been tested and checked. I have changed C1 from 3300uf 50volt to 10000uf 60 volt as a precaution. There is no short across D11. I have removed D11 from the board and tested it again.
I will measure the voltage across R7 shortly and report back. R7 and R3 are getting very hot.
Thanks again for showing an interest in my problem.
Regards Patrick

 

I finished fixing and improving this defective power supply circuit 8 years ago. Since then, a few Chinese companies have been selling the original defective circuit as very cheeeeep kits.

When using TLE2141 high voltage opamps, a few resistors also need to be changed for them.

Here is my final parts list and schematic:

 

I finished fixing and improving this defective power supply circuit 8 years ago. Since then, a few Chinese copnanies have been selling the original defective circuit as very cheeeeep kits.

When using TLE2141 high voltage opamps, a few resistors also need to be changed for them.

Here is my final parts list and schematic:

Hello AudioGuru again.
To go to the trouble and effort of posting that parts list and circuit for me is truly amazing of you so a thousand thank you's.
I have found the Chinese power supply kit that I think that you are referring to on AliExpress and Banggood and I have placed an order for 2 of them. (The are unbelievably cheap) I have come across 2 YouTube video's of people assembling them and testing them and they seem to work fine with limitations that you have also pointed out in your postings so without being a pessimist it seems likely that I won't get my supply running again so I am going to source all the parts in your list and apply the upgrades to my Chinese kits. Hopefully one of them will work.
In the meantime I have purchased a 5 amp 28 volt transformer which I will change for my existing one.
One more question if you don't mind, I have been unable to source the TLE2141 locally and when I order from overseas suppliers it take up to 3 months to arrive if it ever arrives at all so would an OPA604 op amp be a suitable equivalent?
Again, many many thanks
Regards Patrick

 

OPA604 opamps are surface-mounted only without pins for a through-holes pcb. They will not work in the circuit designed for TLE2141 opamps but can be replacements in the original circuit that used TL081 opamps.

For the OPA604 then R10 should be a piece of wire and RV1 should be 100k ohms.

 

OPA604 opamps are surface-mounted only without pins for a through-holes pcb. They will not work in the circuit designed for TLE2141 opamps but can be replacements in the original circuit that used TL081 opamps.

For the OPA604 then R10 should be a piece of wire and RV1 should be 100k ohms.