Power Supply Repair 12v/5v

[mierkism]

Hello, I am learning to repair electronics devices and I came across simple design power supply from mobile air conditioner.
Power supply takes mains AC Voltage and turns it into 12v/5v. 5v is used for device screen, while 12v is used for turning relays on/off for motors.

Things that I tried/know:
0 Visually there is no burnt components
1 Fuse and bridge rectifier is good - main caps have 300+ DC voltage
2 "low side" of power supply seems to have no shorts/problems - if i inject needed low voltages with external power supply directly into the capacitors, device works perfect
3 Measured electrolytic capacitors, resistors, diodes - nothing to be out of spec
4 Transformer seems to have continuity on each side (as expected). There is no continuity from high side to low side
5 Tried changing IC THX202H => THX203 (Aliexpress sent me wrong one)
6 Tried changing 817B from other circuit board

After all these steps power supply still has 0v at output 
Looking for some tips on how can i troubleshoot it further. Adding images of back and front of the power supply.

[DavidAlfa]

Check the waveforms at page 8 of the datasheet and compare.

[fmashockie]

Looks like you've done a good job at doing initial checks! I like to look up the datasheets of the power management ICs when I'm working on a SMPS.  Looks like you have a AC/DC converter (U1, can't tell which kind from the photo) and the PWM (U4, THX202H).  Like DavidAlfa said, the datasheet for the PWM provides a ton of waveforms to check for each pin.  Start there.  But I would also look up the AC/DC converter datasheet and check that to see if it can provide you any additional info to start testing.  You also replaced the optocoupler - your thought process is on the right track thinking it could be something wrong with the feedback circuit.  But I would be suspecting the feedback zener diode first (ZD1?). 

When the board is this small/simple, if I hit a wall trying to fix it, I would just be testing every component in circuit (resistors, diodes, capacitors).  Check all the electrolytics if you haven't already.  Having a LCR meter will help you do that in-circuit.  If you don't have one of those, you'll need to remove them to test out of circuit.

[mierkism]

Thank you for your replies!

I add image with IC names on them to make it easier to debug.

"But I would be suspecting the feedback zener diode first (ZD1?)" => per my understanding zener works good, on it i found printed '3v9' symbols, and my TC1 meter shows 3.5v and 0.8v
", I would just be testing every component in circuit (resistors, diodes, capacitors)." => i tested electrolytics with TC1 - no problems, diodes and resistors with my multimeter - no problem

[DavidAlfa]

Nah, you should first measure the IC pins and compare voltages / waveforms.

[fmashockie]

Thanks for adding the notated photo (I had the ICs marked incorrectly).  So U4 is a ULN2003 which is a darlington transistor array.  That will be used for turning your relays on/off.  I've seen these get shorted across all outputs.  But you said you injected 12V into the secondary side and it worked fine correct?  So ULN2003 should be fine.

You've checked all the passive components.  So like DavidAlfa says, your best course of action is to check the waveforms on the THX202. 

[mierkism]

I have simple DSO150 Oscilloscope.  I have not used oscilloscopes much, so I do not have much experience with them. Maybe you could help me with IC pins measuring? Can I do such thing with this device?

[mierkism]

I tried to measure IC voltages with multimeter, but had no luck understanding how it should be powered on my circuit. Datasheet example looks logical.
I add image with datasheet example on the left. On the right you can see my circuit with some red text/changes.

Thing that I am trying to understand is: VCC (Pin 2) should have ~5v DC? How this can be done via capacitors? Marked as red C.

[DavidAlfa]

But are you using the original thx202 or the 203 you received from AliExpress?
The pinouts are different, you might get over it by bending the pins and soldering wires to the correct points.

[mierkism]

Thank you for clarification about the pins!
Per my understanding, i need to change pins VCC, CT, FB (2,4,5) - is that right?
There was nothing on pin 6 for 202, so I added R3 as 1M from 203 schematic - is that right?
Still getting nothing at output or VCC pin

[DavidAlfa]

Basically:

Code: [Select]

THX202   THX203
  2        4
  4        5
  5        2

THX203 pin 6 does the same as in THX202, connect like original, don't add anything.
If it's unused, leave as-is. Current is measured internally.
THX203H can drive almost double the current, 580mA vs 270mA

However, THX203 CT pin driving current is the double of THX202's (100uA vs 50uA).
To keep the switching frequency to same spec replace the original 330pf capacitor with 680pf.
That's the capacitor connected to pin 2 in the board (Now THX203 pin 4).

[mierkism]

I will search if i have such a capacitor to change to.

But even with wrong capacitor, i have 0 output, so maybe the problem is elsewhere? For example with transformer?

[DavidAlfa]

What's the voltage between vdd and gnd pins?
Use only the multimeter, and be careful, you're measuring rectified mains, so not isolated!

[mierkism]

2.7v and dropping.

Unplugged to reset everything
Shorted main cap
Then shorted small cap that povers VCC

Plugged in
Now measuring 0v

[mierkism]

Repairs/Diagnostics for this board takes a lot of time, so I am thinking just to leave as is and power everyting from external small power supply.
Attaching image

Not the most beautiful thing, but hey, it works

[mierkism]

Plugged in updated board, can confirm everything works.

Thank you very much for your help, DavidAlfa!

Maybe another board in the future will be more successful

[DavidAlfa]

  yeahhh

[fmashockie]

Repairs/Diagnostics for this board takes a lot of time, so I am thinking just to leave as is and power everyting from external small power supply.
Attaching image

Not the most beautiful thing, but hey, it works

  I see magic smoke in your future...

[mierkism]

You are welcome to write something helpful

[fmashockie]

You are welcome to write something helpful

I'm sorry I was only joking.  I appreciate the ingenuity you used to get it working again.