1502DD Lab power supply repair

[Yig666]

I am a complete noob and I need help repairing a 1502DD lab power supply.

Power supply name: Pulsivo 1502DD
Output Voltage : 0-15 A DC
Output Current : 0-2 A
PCB code:
   DC15V2A-1
   XZH20211004
   1502A-9

I damaged my lab power supply.
I wanted to mount a heatsink on the 2N3055 transistor and I forgot to connect the wire to the collector.
I connected a 1W LED to the output and turned on the power, the power supply worked fine for a few seconds, after that both segment displays started showing 0 and the overcurrent protection warning buzzer started beeping.
I disconnected the LED and restarted the power supply and nothing changed. After that I realized what an idiot I was for forgetting to connect the wire to the collector.

I removed the PCB from the case and discovered a faulty SMD transistor with code G1 marked on PCB with Q3, I replaced it and nothing changed.

*I checked the 2N3055 transistor and it seems fine.
These are the measured values:
   LCR T4 meter:
      -hFE = 78
      -Uf = 586 mV

   UNI-T multimeter diode test mode:
      Vbe = 0.568V
      Veb = open circuit
      Vbc = 0.562V
      Vcb = open circuit
      Vce = open circuit
      Vec = open circuit

   UNI-T multimeter hFE test mode 43

*I removed and checked the D5 SMD diode with code M7 and it seems fine, after checking I soldered the diode back
These are the measured values:
   LCR T4 meter:
      Vf = 681 mV
      C = 8 pF

   UNI-T multimeter diode test mode 0.608 V

*When I turn on the power supply, I noticed that the K7 SMD resistor with the code 102 and the value of 1k heats up.


*There are 3 integrated circuits on the PCB with the label erased or missing

U2 14-pin integrated circuit:
   -GND at pin 14
   -VCC at pin 1

U3 and U4 8-pin integrated circuit:
   -GND at pin 4
   -U3 IC has the label partially visible (there are 3 lines of text):
      -on the first line is 356
      -on the second line is 30 or 80 I'm not sure
      -on the third line I can't distinguish anything


I haven't been able to find the schematic on the internet, even though this is a generic Chinese lab power supply.
I found many schematics for similar power supplies, but none of them were like mine so I decided to reverse engineer the schematic, but note that I probably made some mistakes.

I want to mention that the PCB is not burned, the strange colors in the photo are caused by editing the image in Paint.

I would appreciate if someone could help me find the faulty component or identify the unmarked chips.

[MathWizard]

Hi what voltage was normally on the collector and above that D4 and 1k resistor ? What's the actual voltage across the 1k ?

The 393 IC is probably an LM393 comparator, and they can only sink current, and so they use external pullup resistors, on the output pins 1 and 7, I didn't see it on your schematics. Vcc is pin8. I'd want to check Q4 and the 393 just to be sure, they might have seen higher voltage than they should, and that 1k could be passing too much current. As long as no pins read very low ohms, the 393 would probably be ok.

The 356 is probably an LM358 op-amp.

There's hardly any caps around the problem area, besides the collector supply. So most problems should be easy to see with the board unpowered and a DMM. If I was repairing it, I would probably remove some caps to check their ESR (just for maintenance). But yeah if you're not that used to probing with a DMM, remove removing the caps makes it way less confusing. If the 3055 collector was damaged, you shouldn't get any hFE readings.

[Andy Watson]

I would guess that U2 is some type of processor that is monitoring the current and voltage to generate the signals for the display and beeper. The current is monitored via U4 and H8 - so U4 will be an op-amp (dual).  I agree with MathWizard - U4 is probably a LM358. U3 provides the voltage regulation - so it will also be linear - my guess would be that it is also another LM358 (what remains of the markings on the chip appear to be similar to U4).

How did Q3 fail? It's possible that in the process of failing, the full unregulated voltage was transfered, via it's base, and via D3 to the output of U3. My guess is that U3 has failed. Also, if active at the time, Q4 is in the firing line!

[Yig666]

-I can't check the voltage on the PCB because I removed a lot of components and I'm afraid to turn it on but i remember i measured the voltage between collector->emitter and collector->base of transistor 2N3055 and both were around 30V.
-393 on the schematic is not the markings for the IC it is the code for the H7 resistor, the IC is marked with U3, I double check the PCB and between pin 1 - 7 of IC U3 is 1k resistor followed by a Schottky diode SS14.
-I removed and checked transistor Q4 and it seems fine, according to the T4 LCR meter it has hFE = 353 and Uf = 670 mV.

UPDATE: I have discovered the problem, one of the unlabeled 8-pin ICs marked on the PCB with U3 is faulty because pins 1, 4 and 8 are shorted, but I still need to identify the IC you said that is an LM358 op-amp I think you're right, I removed both ICs and measured them, I'm attaching a photo of the results in the post.

[Yig666]

Yes, you are right, the U3 IC is defective, pins 1, 4 and 8 are shorted, I removed both ICs and measured them, I'm attaching a picture with the results in the post.

[Yig666]

UPDATE:

I removed the IC from the PCB and measured each pin with the T4 LCR meter and took a picture of the results

I think one of the IC marked U3 is faulty because pins 1, 4 and 8 are shorted.

I think both ICs are identical, but I'm not sure.

I have attached a picture with the measured values.

[Yig666]

I decided that I would replace the U3 IC with an LM358, as MathWizard and Andy Watson indicated, but I need to order it first, I'll come back with an update after I replace it.

I would appreciate if anyone is willing to double check the measurement I took for U3 and U4 IC and confirm that it is LM358

Thank you for everyone who helped me.

[Swake]

Not convinced this allows to confirm these are the same chips or not. Checked some with 2 different multimeters and this gave similar results. Anyhow, here you go:

Found 2 LM358 in DIL package. 20+ years new old stock. Both are of different manufacture.

Transcribed your measurements in the table too.

Code: [Select]

Diode test												
pin  A	pin B	U3	U3		U4	U4		Ux	Ux		Uy	Uy
		AK	KA		AK	KA		AK	KA		AK	KA
1	2	1,92			1,37			1,49	Open		0,93	Open
1	3	1,03			1,38			1,46	Open		1,00	Open
1	4	2,17			2,15			2,18	0,74		2,09	0,69
1	5	1,12			1,49			1,52	Open		2,75	Open
1	6	1,12			1,62			2,68	Open		2,72	Open
1	7	1,06	1,45		Open			2,69	2,7		2,47	1,44
1	8	1,44 Ohm			0,979		0,82	3,01		0,77	2,33
2	3	Open			Open			Open	Open		Open	Open
2	4		1,11			1,1		Open	0,80		Open	0,79
2	5	Open			Open			Open	Open		Open	Open
2	6	Open			Open			Open	Open		Open	Open
2	7		2,32			1,63		Open	2,69		Open	2,71
2	8		1,12		Open			Open	2,86		Open	2,41
3	4		1,03			1,02		Open	0,79		Open	0,79
3	5	Open			Open			Open	Open		Open	Open
3	6	Open			Open			Open	Open		Open	Open
3	7		2,28			1,49		Open	1,52		Open	2,74
3	8		1,04		Open			Open	2,46		Open	2,39
4	5	1,01			1,02			0,79	Open		0,79	Open
4	6	1,11			1,11			0,79	Open		0,79	Open
4	7	1,06	1,45		1,04	2,16		0,74	2,19		0,69	2,08
4	8	2,36 Ohm			0,781		0,66	3,02		0,64	1,98
5	6	Open			Open			Open	Open		Open	Open
5	7		1,62			1,38		Open	1,46		Open	1,03
5	8		1,02		Open			Open	2,48		Open	2,39
6	7		1,42			1,38		Open	1,49		Open	0,93
6	8		1,12		Open			Open	2,87		Open	2,41
7	8	1,45	1,06		0,985			0,82	3,02		0,78	2,32

[max.wwwang]

This power supply is on sale here.

https://es.aliexpress.com/item/32390252855.html?gatewayAdapt=glo2esp

[Jimmy Electron]

 Never heard of Pulsivo, however here is a very basic 1502DD schematic which would suggest that the 14 pin IC 'U2' might be an LM723, of which the datasheet included below.

 I have a 1502DD++ here, and the 8 pin IC is an LM358.
 It has two 14 pins. an LM723 and a 74HC164

Looking at your photo it seems you also have a 74HC592 or 595?

 
*Some of these 1502DD's also used the 14 pin LM7223



 Good luck

[Swake]

Nah, the 14 pin chip is not a 723:
- pin 8 on the 723 does nothing, while on the schematic it is powering a green LED.
- On the 723 pin 12 is for Vcc while here it is likely an analog input as there is the current potentiometer WR3

Small error in the schematic: pin 1 (input) en pin 3 (output) of the 78M05 are reversed.

[Yig666]

Thank you for help

As Swake pointed out, there is a schematic error: pin 1 (input) and pin 3 (output) of the 78M05 are reversed
I know about the 1502DD schematic which uses the 14-pin LM723, but I check my PCB and as Swake pointed out, the 14-pin IC "U2" has different pinout.
The 14 pin IC "U2" does not have the label removed like "U3" and "U4", it looks like it comes from the factory without a label.
Anyway, if after replacing the LM358 integrated circuit it still doesn't work, I'll give up and go buy another power supply.

[max.wwwang]

No don’t give up though go for a new one if needed. 

Posting the new supply on sale was just so everyone knows what this thing looks like (as I would normally want).

[Yig666]

Update:
I replaced the LM358 IC and now the power supply works.

All the effort I put into reverse engineering the schematic and figuring out what the problem was wasn't worth it for a cheap power supply, but I learned to use the hot air station and to use KiCad and the fact that I gained experience is more important.

When I soldered the IC, the pcb started to swell (blistering), I don't know why, the temperature of the hot air station was around 250-300 degrees Celsius, I tried to lower the temperature but the solder didn't melt, at the end of the day it doesn't matter as long as the power supply works.

Thanks to everyone who helped me.

[Swake]

 

[max.wwwang]

Well done. Congrats!