Help repairing switching PS

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Help repairing switching PS

Postby edugimeno » January 27th, 2017, 5:05 pm

Hi there!
I got a broken TV in my hands. It was clear from the first time the issue was in the power supply.
There is no power at all on any secondary. Checked secondary diodes first, all tested good
No power in the secondaries
Went back to the primary side. Transformer is good (~2ohms)
Mains rectifier and HV capacitor seems to be good (storing 320 volts of DC for a long time) (it runs on 220V)
So it all seems to be pointing at the electronics between the capacitor and the primary.
I can hear a super low, "twich...twich"...very very low. Aprox 0,5 -> 1 Hz
Checked switching Mosfet (2N70), tested good with the multimeter diode "trick" that charges the internal capacitor and starts conducting
Checked the 100Kohm parallel-series resistors that feed the mosfet's source and is good. Checked all diodes around and look good
I downloaded the datasheet for the controller (LD7575, attached) and after reading everything, I can't see what's causing all this switching.
I checked RT, CS, COMP and they are all swinging up and down at the same rate (once every 1-2 seconds)
Vcc for the LD7575 is swinging from 10 to 16volts.
HV (startup) is correctly fed to pin 8.

Should I suspect bad LD7575?

Im attaching my PS schematic along with the controller IC: Link to power supply (give it some time and a download link will show up below the preview image. It works...) Link to LD7575 (PS managing IC)

Posts: 33
Joined: February 5th, 2016, 2:40 am

Re: Help repairing switching PS

Postby bandersnatch » February 9th, 2017, 3:43 am


Nobody has answered so I will throw in my 10c...

Modern switching power supplies are much more reliable than in the old days when
they used to either work or fail horribly and spray electrolytic paste all over the inside of the casing.

If no transistors or diodes are shorted, the DC primary voltage is present and the supply is making a "tick, tick, tick" noise
then this often indicates that the PSU is working correctly and is being shut down/restarted by the current limiting circuitry.
I.e. the supply is trying to start but something connected to the supply is drawing too much current.

Switching power supplies often require a minimum load to function correctly and will not function without this.
I suggest you disconnect the rest of the TV boards and connect some (approriately rated) resistors across the DC supply
outputs so that each output supplies ca. 500mA.

The PSU might then work correctly, but you will then need to isolate the faulty TV board..
I hope this helps you in the right direction.

Posts: 150
Joined: September 17th, 2014, 12:06 pm

Re: Help repairing switching PS

Postby edugimeno » February 9th, 2017, 7:37 am

Sorry I didn't update this, I posted it here, got no replies in some days, then posted it in another forum, which led me to some ideas to fix it.
I made a basic mistake in my first examination... I said "no power on secondaries, diodes test good"...Yes, but that wasn't all.

The secondary 12V output is also powering, in this power supply, a set of HV transformers for the HV backlighting. When I isolated that area from the secondary, I got 12V and 5V. So the issue was with the HV generators

At the end I found a grounded gate in one of those integrated dual MOSFET iC's "AO4606". I lifted the pins, the shortcircuit went away.
Bought a couple of them online for $2, and the TV is working now. I made a little mess in the tracks as they are tiny and ended up lifting some of them while desoldering, but I jumpered them to somewhere else on the board and it's all good now.

Thanks for your reply!!
Posts: 33
Joined: February 5th, 2016, 2:40 am

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