Dead Vestel 17IPS71 power board

[dick_barton]

Re: Dead Vestel 17IPS71 power board

Ignore that for now. Budm's suggestion looks good since you only have 80V on the HV pin and the information provided indicates it should be a lot higher.
If you can report back with the resistor readings and we can take it from there.

[budm]

Re: Dead Vestel 17IPS71 power board

BTW, when power is first applied to the board, the HV pin supplies the charge pump circuit to charge up the startup cap on the VDD pin until the DCV at the VDD pin reach the startup Voltage which is 16V, then the IC will start running and drive the MOSFET which in turn the VDD to keep the IC running will be generated by the AUX winding of the transformer, the charge pump is disabled when the IC starts running.
Right now it is either the HV pin is too low or the charge pump circuit is dead or shorts circuit or low resistance is present at the VDD pin to not allow the VDD to go up to 16VDC.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

ok if this makes sense
meter set at 200k
289 reads 37.9
290 reads 38.2
291 reads 38.7

VDD to GND meter set at 2K reads .192

[dick_barton]

Re: Dead Vestel 17IPS71 power board

Thanks for the explanation which I hope I'll remember.

[budm]

Re: Dead Vestel 17IPS71 power board

ok if this makes sense
meter set at 200k
289 reads 37.9
290 reads 38.2
291 reads 38.7

VDD to GND meter set at 2K reads .192

OK, need the resistance between HV and GND.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

HV and GND 72.2 meter set at 200K

[budm]

Re: Dead Vestel 17IPS71 power board

So we have the resistor divider with the top of the string of about 120K (Three 39K in series) and 72K on the bottom and the VDC feeding the VDC of about 330VDC feeding the divider but you are only getting only about 80V instead of around 120V (per spec sheet of the IC).
So something may be an active loading that caused the HV pin to be only 80V, but even at 80V I would still expect the charge pump to charge up the cap on VDD pin but it may not be able to get to 16V, but we are seeing 0V so the pump is not doing anything.
The 2K to GND on the VDD pin seems low also. We need to get some reading from the good working board to see what kind of the resistance we should have at HV and the VDD pins.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

i may be thinking utter crap i am a newbie to this but if we remove one of the three 39k would it increase the 80v HV

[budm]

Re: Dead Vestel 17IPS71 power board

The key point right now is that you are getting 0V at the VDD pin.
The Vdrops at the HV pin measured at 80V so that means the Vdrops across those three 39K resistor (in sereis) is 330V DC on main filter cap -80V on HV pin = 250V so the current flow is 2mA so it is just impossible to have 0V at the VDD pin, there has to be DCV at the VDD pin. The problem is either internal of the IC or external of the IC.
I see MLCC C296, and C33/73 lytic cap connected to the VDD pin, may be they have leakage resistance. That 2K Ohms reading on VDD pin kind of bothering me.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

just changed c33/73 will check VDD pin again now

[london-freddie]

Re: Dead Vestel 17IPS71 power board

VDD to GND .009V

[dick_barton]

Re: Dead Vestel 17IPS71 power board

Also check the diode alongside it D20 and R301 also capacitors C295, C296, C302

Edit
Just taken another look at the photo of the back of the board and it looks as though C295 and C302 are not fitted but C296 is fitted.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

R301 seems ok, however not sure of diode D20, diode mode set on meter it is reading .194 in both directions, is that right ?

[budm]

Re: Dead Vestel 17IPS71 power board

If the reading does not look right then remove and test it off the board.

[budm]

Re: Dead Vestel 17IPS71 power board

BTW, instead of removing the SMD D20, remove the 0 Ohm resistor R301 instead since it is basically a jumper and then you can test the diode on board.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

D20 is ok

[budm]

Re: Dead Vestel 17IPS71 power board

Now, are you getting about the same reading in diode mode if you measure between VDD and GND pin without the resistor in place, try it both ways with the probes?

[london-freddie]

Re: Dead Vestel 17IPS71 power board

have already soldered back R301 but diode did read in one direction only when resistor R301 was removed

[budm]

Re: Dead Vestel 17IPS71 power board

I am talking about the diode mode reading between VDD and GND pin of the IC.
At this point I suspect bad IC.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

VDD to GND same 192 in diode mode