Visio VF551XUT 55’’no sound no picture

[bender302]

I have a Visio VF551XUT 55'' flat screen tv. The TV no longer turns on. The “visio” power indicator is amber when off. When the TV is turned on, the indicator changes to white and several “feature” lights sequence on as well. However, there is no picture or sound.
I have read several posts and attempted to troubleshoot the issue. This is where I am:

The issue appears to be related to the “ps on” output from the control bd.
When the power is turned on, I measure approx. 0.18v on the “ps on” signal.
If I isolate the “ps on” signal by disconnecting the black wire that goes to J1-1 on the control bd and pull that signal up on the pwr supply bd by using an external resistor (~1.6K) tied to 5v, the TV powers up and I get normal picture and sound.
If I leave the black wire connected between the control bd and power supply and try to pull up using the external resistor, the signal goes to approx. 1.45v – not high enough to turn on the main supplies.
I'm trying to track down what the issue is on the control bd. It is a bit difficult to trace the signals without having a schematic given that the control bd is multi-layer.
I tried to trace back from J1-1 but am having difficulty. I can trace thru R11, R1, R23, C328 to Q7; some interconnect between Q7 & Q5 – but then I loose it.
I have made the following measurements using a dmm:
See attachment


I feel like I am close, but just can't seem to hone in on it.
Does anyone have either a schematic or some suggestions?
Thank you very much for taking the time to read this post.

Steve

[CapLeaker]

Re: Visio VF551XUT 55''no sound no picture

Congrats on getting this far by yourself!!!
What happens to your TV is this: The main board simply can't turn the power supply on because the PSon switching transistor is bad (not enough voltage on the PSon pin coming from the main board). You have to trace and find that transistor and replace it. Either you trace it back from the PSon pin, or you find a datasheet of the main chip and see what pin is used and trace it from there.
Also... double check the model number: Is it really A VF551XUT not a VF551XVT?
As usual, high res pics would be nice and some board numbers.

Is that your main board?
http://assets.shopjimmy.com/media/ca...0150-top_1.jpg

[bender302]

Re: Visio VF551XUT 55''no sound no picture

Thank you so much for the rapid reply.
Yes, that is my main bd.

I had tried to trace from "ps on" backwards and got a bit lost as this is a multi-layer bd. I did provide an attachment with several voltage measurements for many of the regulators and transistors.
The main processor is under a heat sink and I was trying not to disturb it. Don't know if I'd be able to find a datasheet for it as I'm not sure if its a custom or not.

Most of the measurements I took for the regulators and transistors "looked ok". I'll take another look and see if perhaps I've missed a transistor that's in proximity of processeor and area of connector J1.

Steve

[bender302]

Re: Visio VF551XUT 55''no sound no picture

Yes, sorry about the typo (or my bad eyes), it is a VF551XVT

[budm]

Re: Visio VF551XUT 55''no sound no picture

The typical setup is the ps-on pin is connected to a resistor, the other end of the resistor will be connected to the Collector of the switched transistor, there will be another resistor (pull-up resistor) with one end connected to the VCC (+5V), another end of the resistor will be connected to the Collector.
The switched transistor is normally turned on by the output (feeds the BASE of the switched transistor through a resistor) of the processor to keep the PS-ON low, when TV power switch is activated, the output of the processor goes low which will turn off the switched transistor so the Collector Voltage will swing high.
You may be able to use other circuit that swing high when the power switch is activated to use as the PS-ON command.
I would check and see if one end of R11 is connected to Collector transistor Q1 or Q2.

[bender302]

Re: Visio VF551XUT 55''no sound no picture

Hi, thanks for taking the time to reply, I really appreciate it.

Yes, the scenario you describe makes sense to me and that's what I was looking for (and haven't quite got there yet.)

I was able to trace J1-1 to series R - R11 (550ohms). Other side connects to R1 (3.5K); other side of that R connects to another R - R23 (10K) which connects to collector Q7 and base Q5.

I was expecting to find a connection to a collector on the R11/R1 net but haven't yet. I was trying to find it. Perhaps I should be seeing either 5v or 3.3v on the R1/R23 node? The regulators I checked, "lookd decent" (see attatchment) and so did the transistor behavior of Q1, Q2, Q5, Q7 where Q1 looks to have 5v on its collector when TV is off and 0v when TV is on (opposite of what I would expect if it was the transistor responsible for driving the ps on ckt)
Q5 has 3.3v on its collector when tv is on and ov when off so that might be a a candidate.
Voltage on Q2 and Q7 collectors looked a bit odd to me at first glance (see attachment).

Another detail is that I measure approx 23 ohms dc resistance to gnd at R1/R23 node (probes swithced results in same reading).
Per your description perhaps I should have 3.3 or 5v on this node.
Maybe I missed a regulator along the way ??

I will keep poking around.

Thanks
Steve

[budm]

Re: Visio VF551XUT 55''no sound no picture

Sounds like you are getting close, so if you look at the E-B DCV of the transistor, if it has 0.6~0.7V then the E-C Voltage should be low, and goes high if the E-B Voltage goes <0.2V. If you can see that the E-B V changes state but E-C V does not change state, then you will know that there is problem with that transistor.
The setup can also be two transistors for active high, processor output goes high (TV ON) to turn on the first transistor which then turn off the second transistor, that will cause the second transistor E-C to go high which will give PS-ON = HIGH.
The first example I give is active LOW.

[bender302]

Re: Visio VF551XUT 55''no sound no picture

Well after a bit more poking around, I'm still pretty much stuck.
I do see 3.3v on one side of R1 (3.5K) and the other side is connected to R11 (550) that is the series resistor for the PS-ON signal to J1-1.

I tried to trace the R1/R11 node (which is what I see approx 0.19v on when attempting to turn the tv on) to a collector of a transistor. This is what I can't find. It looks like there is a trace that would appear to go directly to the processor. Is it possible that a processor gpio directly drives this node? Perhaps its an open drain type I/O or tristate buffer that is driving this. Maybe that I/O has been damaged?

Appreciate any additional insights.
Thanks
Steve