Vizio VW37L TV - no 5v standby voltage

[budm]

Re: Vizio VW37L TV - no 5v standby voltage

Pin 8 is connected to resistor R907, pin 5 is connected to R914.
Caps C915A, 915B, 917 are the filter caps for the STBY 5V.

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

Ahh! Ok, thanks budm! I think I have it now. That would make pin #1 the lower right corner of the shopjimmy photo. (next to the descriptor for C904).

[budm]

Re: Vizio VW37L TV - no 5v standby voltage

Ahh! Ok, thanks budm! I think I have it now. That would make pin #1 the lower right corner of the shopjimmy photo. (next to the descriptor for C904).

No, pin 1 is the lower left most pin.
Look at the spec sheet of the IC.

[budm]

Re: Vizio VW37L TV - no 5v standby voltage

8-pin IC, get it?

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

Thanks for the drawing budm, and your patience. The spec sheet lists "VH" as pin #8.. which is the upper right. However, and for whatever reason, I was thinking the sequence of the pins had pin #5 across from pin #1... not pin #8 across from pin #1.

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

Vdc Measurements of the IC pins with respect to pin 4 yielded the following:

Pin #1 - 0v
Pin #2 - 4.94v
Pin #3 - 8.04v
Pin #5 - 0v
Pin #6 - fluctuated between 10.3v - 11.2v
Pin #7 - 0.02v
Pin #8 - fluctuated between 54v - 159v

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

I replaced start up cap C903. However, there was no change in the output and still no 5v standby power.

[budm]

Re: Vizio VW37L TV - no 5v standby voltage

Pin 8 should not be fluctuationg like that at all since the source Voltage is from the 160VDC of the main filter cap.
There should be high resistance resistor R907 connected between pin 8 and the + leg of the main filter cap, check that resistor out.
It can be due to internal failure of the charge pump circuit on pin 8 of that IC.

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

Budm,

I will remove the resistor from the board and check it out.

If it checks good, is it safe to assume the IC is faulty and should be replaced next?

[budm]

Re: Vizio VW37L TV - no 5v standby voltage

Budm,

I will remove the resistor from the board and check it out.

If it checks good, is it safe to assume the IC is faulty and should be replaced next?

I do suspect the IC due to the DCV reading on that pin8.

[CapLeaker]

Re: Vizio VW37L TV - no 5v standby voltage

Budm,

I will remove the resistor from the board and check it out.

If it checks good, is it safe to assume the IC is faulty and should be replaced next?

That resistor could show good on the DMM but yet still be bad under load. That is what happened to me before repairing a PSU.

The IC could be faulty too. Check the gate drive output with a oscilloscope. Just be sure to use an isolation transformer. Other than that there will be a big bang!

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

I checked the resistor and it measured 99.4 K ohms. According to the color bands, it should be a 100K resistor +/- 5%.

I found a replacement IC online and should have it within a week. At capleaker's suggestion, I will go ahead and replace the 100k resistor.. just to be sure.

[CapLeaker]

Re: Vizio VW37L TV - no 5v standby voltage

Just verify the input voltage and the output voltage of the resistor.

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

A voltage check of the R907 resistor, using the negative leg of the large capacitor as ground, showed the following:

DC voltage at + leg of large capacitor = 162 v
DC voltage at input side of resistor = 161.7 v
DC voltage at output side of resister = 60v-159v, fluctuating

Looks like I will be replacing that resistor first, before trying to do anything with the IC. As capleaker suggested, even though the resistor measured correctly when removed from the board, it appears to be malfunctioning when placed under a load.

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

I replaced the R907 resistor. However, the voltage on one leg of the resistor still fluctuated. Coming from the big capacitor, the voltage was steady and correct. On the other side of the resistor (the side that connects to pin 8 on the IC) had fluctuating voltage. If I removed that leg of R907 from the circuit, the voltage into and out of the resistor was stable. When putting it back in circuit with pin 8, the voltage bounces around.

I found a direct replacement IC online. I replaced the IC, but the voltage on pin 8 still bounces between roughly 60 - 160 volts.

Anyone have an idea of what could be causing the voltage variances?

[dick_barton]

Re: Vizio VW37L TV - no 5v standby voltage

Can you check with your meter on ohms that one leg of C903 is connected to pin 6 of the IC FA5531?

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

Dick,

Yes, one leg of C903 shows continuity with pin 6 of the FA5531.

[dick_barton]

Re: Vizio VW37L TV - no 5v standby voltage

Been doing a bit of thinking and I hope I'm correct in my conclusion.

The supply voltage at pin 8 (HV) is bouncing at approximately 100V. If the series resistor is 100K from the 165V supply, then it indicates a current flow of 1mA into the chip

100K * 1mA = 100V drop across resistor (leaving 65V measured at Pin 8)

(I cannot find a reference to what the initial startup current is for the FA5531)

With this current flowing (provided by HV input), VCC is reaching the required voltage needed for the startup current circuit to switch off. So the voltage at Pin 8 would now be 165V since there's no current flowing. The output of the chip should now be driving the external mosfet to provide the secondary voltages as well as the VCC voltage needed to maintain the FA5531 running.

My thoughts are that the FA5531 is working but either the mosfet it drives is not being switched and therefore not driving the primary of the transformer and therefore there is no feedback voltage being fedback to VCC of the FA5531 to maintain it working.

The voltage at VCC falls to zero and the FA5311 tries to restart the chip which is why the supply voltage at pin 8 appears to be bouncing.

There is another fault around that circuit stopping the FA5531 working by not providing the feedback voltage from the transformer to VCC to maintain it operating..

[Drail24]

Re: Vizio VW37L TV - no 5v standby voltage

I'm not all that knowledgeable on checking the mosfet's in circuit, but with the board disconnected from power, Q905 shows a direct short or connection between two of the legs. A quick check of the other similar mosfets do not yield this result. I am not sure if Q905 is a direct piece of the IC FA5531 circuit, but is this normal for 2 of the legs to show continuity.. or could this be a function of other components in the circuit affecting this reading?

[dick_barton]

Re: Vizio VW37L TV - no 5v standby voltage

Best check is to remove it from the board and test it. There should be no short circuit between any pins.

Edit
What is the part number on the device?