DSP-167DP LOW VCC at IC901

[TJM2005]

Hello,
I am trying to repair a power supply for a Vizio M492i-B2.
Before I started:
Fusible Resistor R915 was open.
ZD909 was shorted.
D950B was shorted.

I have replaced the above parts along with a new IC901 / LD7913J
I now have:
165V on the main cap
IC901 Drain pins: 165V
IC901 VCC: 1.1V
IC901 COMP: 1.1V
IC901 BNO: 0.3V

I pulled the Q800 Mosfet and it checked good.
I also tested the two smaller caps C907 and C904B, they both tested close to the rated value.

Does anyone know of anything that could be causing this, or where I could check next?
Thanks for your time.

[R_J]

Are you sure ZD909 was shorted, or did it check shorted in circuit? ZD909 is across source resistors R907A & R907B which are low values, likely under 1Ω
IC901 has an internal startup cell that creates a startup vcc voltage of around 12vdc to charge c904B so you should replace C904B and C907 as well.

[CapLeaker]

165VDC on the main cap? When that thing is turned on you should have closer to 390VDC there. So… either it isn’t turned on or your APFC isn’t working. Plus IC901 is in UVLO with VCC at 1.1V.

[R_J]

it won't turn ON when ic901 is the standby ic which ALSO provides VCC for the PFC drive ic, NO standby = NO pfc vcc, no optocoupler voltages etc.

[TJM2005]

Are you sure ZD909 was shorted, or did it check shorted in circuit? ZD909 is across source resistors R907A & R907B which are low values, likely under 1Ω
IC901 has an internal startup cell that creates a startup vcc voltage of around 12vdc to charge c904B so you should replace C904B and C907 as well.

Yes, ZD909 tested shorted even when removed from the circuit.
I put an order in for the C904B and C907 and should have them in a day or so.
I missed that the IC generates its own VCC voltage, is this only during startup then the VCC power comes from the secondary side of the transformer ones it starts switching? Will it power up even if the optocoupler on the COMP pin is bad?
I will plan to replace the caps then retest the VCC, I also ordered another LD7913J as the first one came from e-bay, and I am not sure of an easy way to test that is fully operational.

165VDC on the main cap? When that thing is turned on you should have closer to 390VDC there. So… either it isn’t turned on or your APFC isn’t working. Plus IC901 is in UVLO with VCC at 1.1V.

I am on 120V 60Hz, shouldn't this be about the correct voltage?
Vpeak ≈ 120V × 1.414 ≈ 169.7V

[R_J]

Did you check the source resistors R907 A & R907B? make sure they did not open. when the internal mosfet in the ic shorted it may have damaged the source resistors, if it did the ic will likely not start. The ZD909 zener is more for spike suppression than regulation.

When power is applied ic901 will have 165vdc on its drain and it will internally supply a small current to the vcc pin, this will charge C904B, once charged to about 12~16v the ic starts to switch, then the transformer will suppliy the vcc voltage via R908A~C and D902B to about 25vdc.

Once the standby circuit is working, it will also supply the VCC voltage (via D902 and C907) to the pfc and main driver ic's, only when the power supply is switched ON will the pfc start to work and then you will have about 385vdc on the main capacitor.

[CapLeaker]

Yes, ZD909 tested shorted even when removed from the circuit.
I put an order in for the C904B and C907 and should have them in a day or so.
I missed that the IC generates its own VCC voltage, is this only during startup then the VCC power comes from the secondary side of the transformer ones it starts switching? Will it power up even if the optocoupler on the COMP pin is bad?
I will plan to replace the caps then retest the VCC, I also ordered another LD7913J as the first one came from e-bay, and I am not sure of an easy way to test that is fully operational.


I am on 120V 60Hz, shouldn't this be about the correct voltage?
Vpeak ≈ 120V × 1.414 ≈ 169.7V

Cuz there is a an active Power Factor Correction circuit that boosts the voltage to around 390 VDC. If that isn’t there, the TV is never going to work.

Thats why I said, either the PSU is off, or there is something wrong with the PFC circuit. VCC at 1.1Vdc it is in undervoltage lock out.

[R_J]

The power supply is OFF because it has no standby voltage

[CapLeaker]

The power supply is OFF because it has no standby voltage

Op didn’t specify…