panasonic tx-p50s10b

[ronhayes]

Re: panasonic tx-p50s10b

Hi Paulstef
The power led is always off.
Have checked the resisters R555 this reads 1.004 in 2K range and R556 reads 180.8 in 200 range.
I do have a scope but it is very old have not used for a while would need to dig it out from where ever i have stored it would need to have a think ?

[paulstef]

Re: panasonic tx-p50s10b

Sorry but the resistor would need to be measured out of circuit. This combination doesn't make sense.

Isn't there anything printed on them?

[ronhayes]

Re: panasonic tx-p50s10b

Hi Paulstef
They are really tiny I will sort out my eye magnifier glasses tomorrow and see if I can read anything failing that I will have to remove them to re-check them.
My eyes are not what they used to be.

[tom66]

Re: panasonic tx-p50s10b

Doh, I just noticed @Paul said check 4.73V and I say the same two posts later. I can't read... Anyhow...

The voltage across IC551s ref to cathode pins should be about 2.495V, if this is different IC551 can be bad or optocoupler can be bad, if not it can be the resistors or something else. You can check this across R556.

(IC551 will always act to keep the voltage between ref and cathode to 2.495V as it will sink current through the opto if the voltage exceeds the threshold, telling the controller IC to back off on the duty cycle. Unless it's faulty, or the power supply can't give any more, e.g. excessive load, bad capacitor...)

[ronhayes]

Re: panasonic tx-p50s10b

Hi Paul/Tom
Thank you both for all your help with this set i feel i am getting close now to finding the fault.
Just a couple of questions that i hope you can help me with ?
Paul i am attaching a picture of ic551 on track side so you can see R555 and R556 and as you will see these are smaller than all the other smr resisters and you should be able to see from the picture there is no markings on them?
Also Tom you say to measure the voltages at these resisters to see if their is a voltage of 2.945v this would be quite difficult while board is in situ unless i were to solder fly leads to them or take measurements from legs of ic551 on component side ?
Also would taking measurements while set is powered up could this cause any damage elsewhere or would i have to connect set through an isolating transformer?
And do i take measurements in stby and while pressing power button?
Any more help would be much appreciated.

[vinceroger69]

Re: panasonic tx-p50s10b

Maybe just the picture but jc15/ jc17 /jc18 solder joints looks iffy? maybe worth re soldering.

[paulstef]

Re: panasonic tx-p50s10b

Ok, ideally you would need to remove and measure them. You may damage them though by doing this... :-/
These two resistors form a voltage divider and in order to obtain 5V on STB_IN they would need to be almost equal in value (2.5V at reference pin of IC551).

I actually do not think they are damaged , I only wanted to find out if the STB voltage corresponds to the voltage determined by these two resistors. You could measure the 2.5V directly at the reference pin of IC551 from the top.

There is no danger to the set if measured with a common handheld multimeter.
Also with an oscilloscope there is no problem on the secondary side.

The problem arises if using a non-isolated, common oscilloscope on the primary side of such a power supply. Neutral and Earth are connected somewhere in your home installation. The scopes probe ground is also connected to earth via the power cord. Therefore you need either an isolation transformer or isolated probes or a handheld scope.

For this specific measurement you can use your old scope without problems.

[paulstef]

Re: panasonic tx-p50s10b

This power supply is always running. You should measure the voltage before Q557 (STB_IN and also VCC for the uC) . No need to press the standby button for now.

EDIT: sorry I meant, the voltage before Q557 (that's the 4.73V) AND the voltage on the reference pin of the TL431 (supposed to be around 2.5V)

[ronhayes]

Re: panasonic tx-p50s10b

Hi Paul
I reinstated the P board after checking all iffy joints as kindly pointed out by vinceroger69 (thank you).
Then checked volts on ref pin ic551 it was 2.8v in stby and same when power button pressed?
I then removed P board again to locate Q557 which i located but cannot locate TL431? to measure this voltage.

[tom66]

Re: panasonic tx-p50s10b

IC551 is TL431. That Vref of 2.8V indicates an issue with IC551.
(The voltage should stay the same regardless of run or standby, as this part of the power supply is always running when mains power is connected.)

[paulstef]

Re: panasonic tx-p50s10b

Yes, that's weird. If the reference pin is at 2.8V than the STB voltage should be way higher than 5V.

I would proceed to removing and measuring the two resistors and/or replacing the IC551.

[ronhayes]

Re: panasonic tx-p50s10b

Thank you tom
The IC551 has the markings KIA 431A 907 I have located a similar IC in another power board its marking are TL431C yo4o
Would this be compatible?

[tom66]

Re: panasonic tx-p50s10b

Yes, that's weird. If the reference pin is at 2.8V than the STB voltage should be way higher than 5V.

I have not yet seen it with TL431, but I have seen other devices with feedback pins that became leaky wrt to the supply as well as ground, i.e. current getting injected into the REF node, this could cause REF to rise, which would cause the power supply to reduce duty cycle too early. (Opto-coupler current would be injected into the REF node in this instance.)

[paulstef]

Re: panasonic tx-p50s10b

Thank you tom
The IC551 has the markings KIA 431A 907 I have located a similar IC in another power board its marking are TL431C yo4o
Would this be compatible?

Yes, take anything with a 431 on it (not 2431)

They are very common in feedback loops.

[tom66]

Re: panasonic tx-p50s10b

Thank you tom
The IC551 has the markings KIA 431A 907 I have located a similar IC in another power board its marking are TL431C yo4o
Would this be compatible?

I think TL431C has a reversed pin out. (See datasheet) This will work but you will have to install it in reverse. In any instance, when testing it, disconnect all loads from the power supply, such as the main board. This will ensure in case the TL431 is installed backwards, higher output voltage cannot damage anything.

[paulstef]

Re: panasonic tx-p50s10b

I think TL431C has a reversed pin out. (See datasheet) This will work but you will have to install it in reverse. In any instance, when testing it, disconnect all loads from the power supply, such as the main board. This will ensure in case the TL431 is installed backwards, higher output voltage cannot damage anything.

Yes, needs verifying but seems to be the same pinout to me. TI shows top view, KEC bottom view... (I think)

[paulstef]

Re: panasonic tx-p50s10b

I marked the pinout on the PCB.

Question: Somebody scratched around on this board. Where the blue circle is are these two traces connected?

You may also measure if C553 is shorted. (I don't think so though)

[ronhayes]

Re: panasonic tx-p50s10b

Hi Tom/Paul
I have checked the data on both these shunt regulators and included a picture showing them side by side and from what i can see the pin orientation are the same ? would you mind looking to confirm this please.
Also Paul i have checked the two traces not connected ! I also checked the schematics for c553 and it does not give value only slate plate capacitor ?
Would hand held capacitor meter measure this and in what range ?

[paulstef]

Re: panasonic tx-p50s10b

As for the pinout I included a pic in my previous post. Yes they look the same, only the KEC seems to show it from the bottom (thanks for not indicating it, KEC)

The traces should not be connected, that's ok then.

Only do a Ohmmeter test on the cap. I don't believe it's shorted but I would test it nevertheless.

[ronhayes]

Re: panasonic tx-p50s10b

Hi Paul/Tom
I checked C553 did not seem to be short i then carried on with replacing IC551 then reinstated the P board reconnected plugs but not p6.p7,p25 to main A board still nothing not even stby light then disconnected power lead and reconnected p6,p7,p25 reconnected power lead still no joy no stby light no relays clicking nothing at all? .