Dead Vestel 17IPS71 power board

[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

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

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

HV and GND 72.2 meter set at 200K

[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.

[dick_barton]

Re: Dead Vestel 17IPS71 power board

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

[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

[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.

[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.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

what is easiest way to check Q15, watched the vid but not making much sense to me if i am honest

[budm]

Re: Dead Vestel 17IPS71 power board

Check the resistance of R289, 290, 291.
From looking at the picture, it looks like it has '393' printed on top.
Also check the resistance between VDD pin and GND.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

can not find any voltages anywhere on the cold side

[dick_barton]

Re: Dead Vestel 17IPS71 power board

Can you check Q15 with your meter set to diode mode. Set switched off.
https://www.youtube.com/watch?v=gloikp9t2dA

[london-freddie]

Re: Dead Vestel 17IPS71 power board

no voltage at VDD on start up at all

[dick_barton]

Re: Dead Vestel 17IPS71 power board

I think the Fan6300 must kick in and drive the Mosfet Q15 for the dc supplies to come up.

[ReeceyBurger123]

Re: Dead Vestel 17IPS71 power board

You should have 24v and 12 on the output side of the secondary side diodes, do you have that ? If not sounds like something on the primary is up.

[dick_barton]

Re: Dead Vestel 17IPS71 power board

With your meter set to 20V can you measure VDD again and see if there is a voltage present immediately on switch on that drops suddenly.

[london-freddie]

Re: Dead Vestel 17IPS71 power board

ok checking FAN6300
GND and HV 80V
GND and VDD zero volts

[budm]

Re: Dead Vestel 17IPS71 power board

OK, FAN6300 SMPS IC.
Check the DCV between GND pin and HV pin, GND pin and VDD pin.
Be real careful you are working in the deadly side of the circuits.
BTW, this power supply is has an always on 12VCC and 24VCC power supplies per SCH by DB.

[dick_barton]

Re: Dead Vestel 17IPS71 power board

Data sheet: