What is it with me and Switchmode Supplies?

[kolbep]

I give up.
Almost Everything I try to fix has a faulty switchmode powersupply 

And for the life of me, I cannot seem to grasp properly how to sort them out. With SeanB's help, I have managed to get a CRT tv working again (5Vsb was not stable due to an open resistor).

Now I managed to get my hands on an HDMI LCD Television to try get working for myself.
When you press the soft power button, the SMPS ticks three times, then starts giving output voltage, but as soon as it tries to turn on the Inverters, the CCFL tubes all flash on and off the whole time (about 3 times a second) You can hear the Inverters ticking during that time. There is also a Tick-Tick-Tick from the SMPS. When this is happening, the sound is fine, and you can see the menu when the backlights flash.

I have replaced the main filter cap on the SMPS, as well as all the output caps.
I then disconnected the inverters from the control board, and put some resistance on the 18v and ground lines from the SMPS to the controller, and the PSU does tick-tick-tick.

I disconnected all the CCFL tubes. Ticking from SMPS is very soft, but it is still there.

If i test with my Voltmeter on the 18v and Ground lines from the PSU, it is about 16v and fluctuates the whole time. If I try the 5v Rail, it is solid as a rock.

I have been through repairfaq small switchmode supply repair, but still am drawing a blank. If somebody knows about another resource for SMPS repair Ideas, Please let me know

Attached is the schematic of the powersupply.

Thanks for any assistance.
P

[tom66]

Check the voltage across the 450V capacitor -- if it's below around 380V, the PFC stage is not working and this will be why the outputs are unstable.

[dfmischler]

Do you have an oscilloscope?  I find the easiest tool for me to troubleshoot a SMPS is my old Tek 475A because with its linear power supply I can float it with just a 2 to 3 prong power adapter.  I guess I have to give the "DON'T TRY THIS AT HOME" warning.

Pull data sheets on those chips and read them until you understand what is supposed to be happening.

The last SMPS I fixed that had a cyclic problem like you describe had a bad opto-coupler in the feedback circuit.  And you seem to have 3 of them.  Do you have any circuit freeze spray?

[g***!]

Disconnect the power plug to the inverter panel, check all outputs for shorted rectifiers...if nothing obviously wrong,check all the 560k resistors on pin 2 of N501, also C503 1uf and R506, R505, R504.

 You may find that it will be necessary to remove the resistors from the board for testing. If you look on the power board there will be a type number printed on it somewhere....if you can post that up it may help!

Sometimes the IC itself fails, but, assuming the components on the secondary side of the transformer are ok, I would put a bet on the resistors around pin 2

 Hope this helps.

[kolbep]

Thanks for all your suggestions so far.

>> Check the voltage across the 450V capacitor -- if it's below around 380V, the PFC stage is not working and this will be why the outputs are unstable.
Voltage across that Cap is only 16V (Have checked on AC and DC Range)

>> Do you have an oscilloscope?
Just 2 days ago my father picked his one up from me. Will have to borrow it back again (Beckmann Industries Analogue Scope)

>> Do you have any circuit freeze spray?
Nope. Maybe I can try upside down deoderant?

>> Disconnect the power plug to the inverter panel, check all outputs for shorted rectifiers...if nothing obviously wrong,check all the 560k resistors...
Checked all that you mentioned. All is fine.

I forgot to mention that the main reserviour cap had very bad leaking from it's one leg. I did replace it with 2 others (my first attempt to get it going)

The PSU is a 782-L27W18-200D

Thnks

[tom66]

Voltage across that Cap is only 16V (Have checked on AC and DC Range)

Err, excuse me, but I don't believe you. It should be at least 160V if you're in a country with 115V electricity, or 320V if you're in a country with 230V electricity.

[kolbep]

Voltage across that Cap is only 16V (Have checked on AC and DC Range)

Err, excuse me, but I don't believe you. It should be at least 160V if you're in a country with 115V electricity, or 320V if you're in a country with 230V electricity.

I don't believe me either. In my medicated, zoned out state  , I thought you were referring to C532 which is from Chassis Earth to the ground line. Are you referring to the 400V 100uF C528? Which is the largest cap on the PSU???

[tom66]

Yes, I am referring to that.

[kolbep]

  Chorus of Angels singing Hallelujah.
After testing each resistor and semi on the PSU board, My eye kept on getting drawn to 2 Electro Caps (10uF and 47uF 35v). Replaced those two, and the set sprung into life.

Unfortunately, as it did this, I saw a bright flash from under the can in the Audio Section.
Popped it open, and there  are bits of TPA300 Amplifier chip all over the show.
The nice thing is that they are using a R2S15900 as the audio switcher, tone control and volume control before the Amplifier chip.
I stripped the amplifier board and transformer out of a set of of PC speakers, mounted it in the set, wired in a relay to switch on the transformer when the set has 18v, and soldered the audio signal wires directly onto the R2S15900 chip.

So the set is now working 100% on all Inputs (HDMI/VGA/Tuner/AV), and now that it closed, you cannot even see my handiwork.

Thanks 4 all your assistance. Now that SWMBO is happy, I can get onto other projects again.

[mateolasic14]

Hi guys,
I have similar problem with same power supply. My TV is sometimes working sometimes not. Sometimes it needs to be ON for a minute to work. Not working means backlight is flashing and clicking sound from PSU. I measure 354V on C528 when TV is working, but I didnt measure it when is not working (works all day). I did get unstable 14V on some output rail when not working and 24V when working. 5V rail is always stable.

[james_s]

The most common cause of this sort of thing that I see is bad electrolytic capacitors. I remember having difficulty with switchmode power supplies too, it was about the time I got an ESR meter that I started having success.