Panasonic TX-26LXD8 stuck in standby.

[py3m4n]

This thread is a continuation of this thread - https://www.badcaps.net/forum/showthread.php?t=19620

Model - Panasonic TX-26LXD8.

Mainboard - TNP8EAL96.

Link to service manual - http://elektrotanya.com/panasonic_tx.../download.html

Fault - When turned on Standby light lights up then goes off immediately. PSU clicking.

Testing - The PSU has been tested and it is outputting the correct voltages to the mainboard and inverter as shown in the service manual. I also have another PSU that belonged to the previous owner that I have tried and the symptoms are the same.

On the mainboard I found that the voltage at SUB5V on connector A1 - pin 8 is unstable (page 19 service manual). Having bought this T.V to better my understanding of block diagrams, schematics etc. I started to investigate, here's what I have found so far.

I traced back from A1 - pin 8. Which led me to IC8404 and IC8405. According to the SCH I should be reading 5v at pin 20 on both of these. I have no reading at both. Same goes for IC8401, no 5v reading at pin 33.

Realising I might be going about this back to front I decided to trace back from connector A7 (PSU connector) pins 1 and 2. Which led me to IC5602, after checking I found that Vo on IC5602 is unstable (previous convo with budm I said no reading, I have found with all connectors connected it's unstable). Here is where the thread in the link at the top comes in.

Now.

@ budm.

IC5602.

Pin 1. Vc - Unstable
Pin 2. Vin - 5v
Pin 4. Vo - Unstable

Vo ohm to ground - no short (no beep, does read 2K ohm on resistance setting on meter? interested as to why this is if you can be bothered explaining)

So it looks like IC5602 is o.k like you mentioned, as it isn't getting a stable 2v at VC pin 1.

I have begun tracing back from here, I will update my findings, lol. I am determined to find the fault with this board. It is slowly peeling back the curtains to my being able to use SCH and I admit it, I am thoroughly enjoying it!

Relevant pictures below.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

@budm

It's looking like Q5610 could be the culprit.

Q5610 - Markings TPC8105.

Datasheet

http://pdf1.alldatasheet.com/datashe...TPC8105-H.html

From what I can work out I should have 5v at pin 1 and 5v at pin 8.

Testing it I have 5v at Pin1 and unstable at Pin 8.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

Is the VIN stable? If it is, you can force the IC5602 to turn on by forcing Q5605 to turn OFF by short out R5654, this will cause the Collector voltage of Q5605 to go high toward 5V.
The IC5602 is trun on when the following happens:
Q5608 is turn on when 3~5 is applied to the resistor R5640 (left leg, I cannot read the lable on the SCH), which will turn on Q5607 whick will allow 5V on the Emitter of Q5607 to appear at the Collector or Q5607, this voltage will then turn on Q5606, hwich will turn off Q5605, so the Collector of Q5605 to go high and turn on IC5602 5V REGULATOR.
at the same time the 5v from the Collector of Q5607 is also fed into Q5609 which will turn it on causes the Collector of Q5609 to go LO which in turn will tun on the P channel MOSFET Q5610 which will allow STB_5V6 to appear at the VIN of the IC5602.
So for you to get 5vdc at the output of IC5602,the above actions have to happen.

[randtek]

Re: Panasonic TX-26LXD8 stuck in standby.

I'm curious what you mean by "unstable". How much is it varying, and how? Is it cycling? It would not be possible to have stable 5v at pin 2 of IC5602, and unstable at pin 8 of Q5610, as they are the same point electronically - connected directly together. I'm guessing you may have a bad test lead on your meter, or a poor ground connection to you rmeter, causing erroneous readings, because some of the readings you are posting are not making sense electronically.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

If VC is not stable and it is swinging hi and lo then the output will be unstable, he needs to look at why VC is unstable.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

@randtek

Two different DMMs were used during testing (I considered this myself at the time) The readings fluctuate between 0v and 5v.

It's possible I have not allocated the correct numbers to the pins. My assumption was pin 1 is where the green STB_5V6 is (left top) and pin 8 is the yellow SWITCH_5V6 going to the Vin of IC5602.

@budm

The Vin reading at IC5602 is stable, I will look into what you have explained tomorrow.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

@budm

Just to double check, is the resistor circled in the attached picture the correct one I want to short.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

That is, great clear picture!

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

@budm

Shorted R5654, no change, still clicking going Hi-Lo.

Took voltage readings at the left and right pins of R5654.

Left pin - 0v
Right pin - 0.5 - 0.65 Hi-Lo.

Took some other V readings not sure how helpful they will be.

Q5608 -

0v - 5.55v Hi-Lo @ collector.

C5671 -

5.55v @ Top pin/Pos
0v @ Bot pin/Neg

Q5610 -

Pin 1/Source - 5v stable
Pin 4/Gate - Hi-Lo.
Pin 5/Drain - Hi-Lo.
Pin 8/Drain - Hi-Lo.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

If that resistor is shorted out (both ends of the resistor are tied to ground when we short out this resistor) then there cannot be any voltage on it.
If Q5608 Collector is going Hi and LO then the Base must be driven HI-LO also, this Collector voltage sholud be 0.6vdc or less when Q5608 is on.
And If Q5608 Collector voltage is going up and down, then Colloector voltage of Q5607 must be going up and down also.
Are these voltages steady at the following points: RELAY_5V6, STB_5V6

You should use the ground points (such as the Emitter of Q5608, Q5609, or Q5605) as shown in the diagram as the ground ref for troubleshooting this section of the circuits.
It must be the way you are attaching the meter probes to the circuits.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

I have been using the screw in the top right of the mainboard as my ground point.

The readings I took on R5654 were after removing the short, same for the rest. The only reading I took whilst the short was in place was at the Vin of IC5602.

Q5605 is an NPN type transistors, is that right.

Having to differentiate between which transistors are NPN and PNP from the schematic and using the SCH in conjunction is hurting my head, but the only way I will learn is to persist!

Edit - I have just realised the Base, emitter, collector are in the same location whether it's NPN or PNP from the datasheets. I have been allowing myself to get confused by the SCH.

These are the two datasheets I am using for the transistors.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

To make it easy, see PDF.
These voltage MUST be steady: RELAY_5V6, STB_5V6

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

Did as you said. I used the emitter of Q5609 as ground.

Q5608 Base and collector are both going Hi-Lo.

B - 0vdc - 0.4vdc
C - 0vdc - 5vdc

Collector of Q5607 is 0vdc - 2.7vdc.

Just as you described!

Now I see you have made it easy. I appreciate it, although I was kind of enjoying the brain pain, lol.

Cheers bud.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

How about the DC Voltage on that Collector of Q605 when you shorted out the resistor? How about the two important DC voltages I ask to check: RELAY_5V6, STB_5V6 They have to be stable, I need that information. See the last PDF.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

Checked RELAY_5V6 using the test point from emitter of Q5607 (see pic)

RELAY_5V6 - 5.48vdc - 5.55vdc
STB_5V6 - 5.4vdc steady

With jumper in place. 3.5vdc @ collector of Q5605.

Here's the frustrating part, after I did this I retested the Vo of IC5602 and now it's going Hi-Lo ?!?! (jumper was removed)

Now has me wondering if I have done something to cause this.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

What do you get at the Vo when the jumper is in place since Vc is now steady, the on way that Vo is goinh HI/LO has be due to Vin is going up and down.
If Vin is going up and dopwn then that is because the first transistor (Q5608) which controls the circuits is switching on and off due the to signal feeding it is going HI and LO, we need to find out where that signal is coming from, if you put the meter on the point as shown in the diagram it will more likely to be going up and down.
By the way, is there any indicator lights on the front of the TV blinking?
I think your TV is in sfaety shutdown and it is try to start and restart due to fault some where.
What do you get at the Vo when the jumper is in place since Vc is now steady, the on way that Vo is goinh HI/LO has be due to Vin is going up and down.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

Let me explain further, that regulator IC has the Vc pin which will turn the output Vo pin on and off, if Vc is being switched on and off, then the out put will be going on and off, then you also have Vin pin, let says that we force Vc to be high by jumping the resistor but the Vin is also going up and down , then the result will be that Vo will begoing up and down even if the Vc is high.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

Vc is steady - 3.69vdc.

At Vo with jumper in place - Hi-Lo.

Meter at point shown in diagram is indeed going Hi-Lo.

Indicator lights - I had hoped for this to be the case when I bought it because the service manual has the section covering it, no luck. The red standby light comes on and goes off immediately, no error codes to be had.

I am currently working on tracing where the 10k resistor you mentioned leads us, I will update soon.

[budm]

Re: Panasonic TX-26LXD8 stuck in standby.

How about the Vin? Is it jumping up and down? Pictures of the power supply board? The service manual does not give any info about the power supply.

[py3m4n]

Re: Panasonic TX-26LXD8 stuck in standby.

Path of 10k resistor circled in red on rear of board.

Vin is Hi-Lo.