Toshiba 26AV600A

[budm]

Re: Toshiba 26AV600A

BTW, did you use the GND pin of the IC for the meter probe?

[budm]

Re: Toshiba 26AV600A

I should add that because the IC was located on the underside of the PS board I had to perform the voltage measurements with the board on the bench. All other connectors were off and I did not use the force-on resistor.

OK, now I see why you are not getting the Voltage at the VCC pin, you need to have forced-on applied.

[budm]

Re: Toshiba 26AV600A

Here is how the VCC gets to the ICs

[Markus1]

Re: Toshiba 26AV600A

Bud, thank you very much for your assistance here. I am most impressed by your knowledge and the efforts you put in to get to the bottom of the problem.
Your tracing of the Vcc path is very helpful. Thank you again.
Based on the above I did more testing. This time I forced the PS board on with the 1k resistor between 5V and PS-ON pins.
Voltage on the main filter cap jumps to 397V. The switched transistor Q953 (delivering Vcc to the IC970) appears working OK, with the voltage readings as follows: B 9V, C 9.15V, E 8.45V.
That voltage signal goes all the way to the IC970, pin 12 (Vcc) where it reads 9.6V (not sure why it is higher there).
These are the voltages on the IC 970 under above conditions:
pin 1 (Css) 0V
pin 2 (Del) 0V
pin 3 (CF) 0.3V
pin 4 (RFmin) 0V
pin 5 (STBY) 0V
pin 6 (Isen) 0V
pin 7 (Line) 6.2V
pin 8 (DIS) 0V
pin 9 (PFC-Stop) 0V
pin 10 (Gnd) 0V
pin 11 (LVG) 9.2V
pin 12 (Vcc) 9.6V
pin 13 n.c
pin 14 (Out) 0V
pin 15 (HVG) 0V
pin 16 (Vboot) 0V

For GND I used the neg pin of the main filter cap, and it is the same as GND on pin 10.

Voltage on LVG is present but not on HVG?

Still, behavior of the front indicator light remains the same, green on AC-On them red after about 2 sec.

One observation here. The IC970 supposed to trigger, via pin 11 (LVG) and pin 15 (HVG), two Mosfets Q970 and Q971, which operate as a pair in a complementary mode to each other. One of them was earlier replaced due to a failure. The replacement is not as per the original p/n but it seems to be a close match.
For your info: Mosfet Q970 is K4100, Q971 was replaced and it is now K2141. So, they are not exactly the same, but close. I am not sure if this is of any significance here. I thought I should mention that too.

[budm]

Re: Toshiba 26AV600A

OK, according to the spec sheet of the IC, the startup voltage at pin 12 is between 10~11.4VDC, once it starts it will keep on running as long as the VCC does not drop down below 8.15V (the range is between 7.45~8.85V), if it does then it will go into shut down.
So right now it can be that the Voltage at pin 12 is too low for it to start up or, it is in shutdown due to false some where, or the IC is bad so it is not sending any Gate drive to the MOSFETs.
I need you to check the rectangular grey cap just below the model number tag 715G3370-1.
Can you read the number as printed on the body? we need to check its capacitance.

[Markus1]

Re: Toshiba 26AV600A

I checked that cap earlier. If I remember correctly, the number printed on it was 333J. The capacitance measurement indicated 33.3nF. Looks to be OK, but I could't check its ESR value, have no meter.
To be sure I will verify it again.

[budm]

Re: Toshiba 26AV600A

OK, it is OK, when that cap failed the capacitance will drop to less than half the value.
At this point you may want to try a new IC.

[Markus1]

Re: Toshiba 26AV600A

Oh, replacement of that IC may present a challenge for me. I am not sure if I can handle taking out and then soldering back the 16-pin chip using just a soldering iron. It is in a tight place and with many other SMD's around. May find it to be too difficult. Alternatively I can perhaps look for a new PS board.

Now, please allow me to ask you a few questions here. It is purely for my learning, knowledge seeking I would say. I am just trying to find some explanation for the system operation.
1. As the switched transistor that supplies the Vcc to the IC delivers 8.45V on its Emitter, and that voltage signal proceeds further to pin 12 (Vcc) of the IC, so why pin 12 reads 9.6V which is above the supplied voltage?
https://www.badcaps.net/forum/attach...6&d=1415852958

2. What are these SMD's marked as "D" along the trace path to pin 12?

3. What's the purpose/reason for ACD (AC Detect)? Why does the Main board need to sense AC, it already receives 5VSTBY DC? Where is the AC signal originating from on the PS board?

4. How critical is it that a pair of Mosfets working in complementary mode to each other be very closely matched? Can they be just "close enough" or need to be "very exact"?

That would be much appreciated.

[Agent24]

Re: Toshiba 26AV600A

2. What are these SMD's marked as "D" along the trace path to pin 12?

0-Ohm SMD resistors. It's a 0, not D. They are basically wire links. But sometimes it's easier to design them in for ease of manufacture or to change values during prototyping.

4. How critical is it that a pair of Mosfets working in complementary mode to each other be very closely matched? Can they be just "close enough" or need to be "very exact"?

I would say they need to be pretty well matched.

[budm]

Re: Toshiba 26AV600A

"1. As the switched transistor that supplies the Vcc to the IC delivers 8.45V on its Emitter, and that voltage signal proceeds further to pin 12 (Vcc) of the IC, so why pin 12 reads 9.6V which is above the supplied voltage? " That has to be error in reading as you can see that the Connection from the Emitter of the switch transistor is connected directly to the VCC pin of both ICs(PFC, SMPS), you can verify the resistance between the Emitter and the VCC pin of both ICs, it should be <1 Ohm (depend on the probe wires resistance).

"3. What’s the purpose/reason for ACD (AC Detect)? Why does the Main board need to sense AC, it already receives 5VSTBY DC? Where is the AC signal originating from on the PS board? " I never find any good explanation in the any training manuals as to the purpose of ACD when the standby power supply need to have AC present to operate to run the main board, if the AC is not present, then the TV will be dead.

[Agent24]

Re: Toshiba 26AV600A

Going by this: http://www.google.com/patents/US8441270, the AC Detect may be to check not that AC is present but that the voltage is high enough for the PSU to safely turn on. This makes sense to me as I have read on RepairFAQ that SMPS testing with a Variac on low line voltages can be a bad idea and cause damage. (Seems it was an old trick used on early CRT TVs instead of the series lightbulb idea)

The 5v standby section is relatively low power and can probably run OK from reduced line voltages so is likely not an accurate way to determine if the line voltage is OK.

[budm]

Re: Toshiba 26AV600A

I saw some of the circuit section in the TV SMPS for detecting Over voltage or under voltage, so I would think the correct term they should have used is AC OV/UV detect.
AC detect make it sounds like it is looking for the AC is present or not.

[Markus1]

Re: Toshiba 26AV600A

Thank you Bud and Agent24 for clarifications. It is great to to have this forum with people like you offering support to anyone, advanced or beginner.

[saularciga]

Re: Toshiba 26AV600A

Here is the schematics

CHANGE THE IC L6599 , ITS ALL THE PROBLEM

[saularciga]

Re: Toshiba 26AV600A

Here is the schematics

CHANGE THE IC L6599 , ITS ALL THE PROBLEM

https://www.badcaps.net/forum/newrep...ote=1&p=503110

[Markus1]

Re: Toshiba 26AV600A

Here is the schematics

CHANGE THE IC L6599 , ITS ALL THE PROBLEM

Thanks saularciga. I will try with the IC replacement.

[Markus1]

Re: Toshiba 26AV600A

Sill waiting for that IC. Will post an update when replacement done.

[Tech 57]

Re: Toshiba 26AV600A

Hi everyone,
I would like to ask you for some help in identifying the problem with this LCD TV.
It is Toshiba, Model 26AV600A. Its Power Supply board is marked 715G3370-1.
I did try to locate the circuit diagram for the PSU but no luck, and without the schematics I am a bit blind in diagnostic.
The symptoms are:
-at powering the set the front LED shows Green
-one or two seconds later it turns to Red
-no response from the remote control or the side buttons
-voltage at the main filter cap 340V
-STBY 5V OK
-no 24V, no 12V

Power IC (designated on the board as IC950) is an 8-pin chip A6069H. I took some voltage measurements there, and they are:
-pin 1 0V
-pin2 about 6V
-pin 3 GND
-pin4 0.8V
-pin 5 around 14V but varies
-pin 6 not connected
-pin 7 and 8 340V

Attached below: picture of the Power Supply board and datasheet of the Power IC.

I would be very grateful for your guidance here.

Regards
Markus1

HI EVERYONE I found this to be a interesting thread
as a owner and tech of a tv repair shop i repair these type of power supplys
all the time .thought id give my two cents on this kind of a problem it might help someone in the future.yes ic L6599D goes bad kills the the 12 and 24v
supply.this ic shorts that causes the problem the best way to test this is with a volt ohm meter.there are smd caps that surround the ic check if their are any shorts across the smd caps if you read 0-10ohms that ic is shorted.
you can also take the ic out and check for shorts also.you will find the shorts across the smd caps goes away.
hope this helps
phil

[Markus1]

Re: Toshiba 26AV600A

Thanks Phil.I will check those SMD caps before replacing the IC.

[Markus1]

Re: Toshiba 26AV600A (back again)

Back to the old problem. It is stage 2 now.
The IC970 (L6599D Resonant Controller) has been changed, as suggested in the previous postings but still no joy.
The external signs are the same. As soon as the set is plugged into the wall socket the front LED shows Green and in 1-2 sec turns Red. No response to any commands from the remote or buttons on the side of the TV.
Voltage readings on CN92 with everything connected as normal are:
pin 1 On/0ff 0V
pin 2 Dim 0V
pin 3,4 +12V 0V
pin 5,6,7 GND 0V
pin 8,9 +24V 0V
pin 10 PSON 0v
pin11,12 +5V 5.17V
pin13 ACD 0V

Filter cap 349V.
Vcc on both IC901 pin 8 (PFC) and IC970 pin 12 (Resonant Controller) show 0V.
----------------------------------
With the force on (pin +5V via 1k resistor to pin PSON and the PSON wire to the Main bd disconnected) the front LED blinks constantly Green, filter cap still at 349V, Vcc on both IC's fluctuate widely, 5V STBY varies visibly between 3.98 and 4.15V.
----------------------------------
Well, we have no voltage boost above 349V and no 12 and 24 V rail present.
Where do we go next.
At least this time the PSU circuit diagram is available, thanks to saularciga.
https://www.badcaps.net/forum/attach...2&d=1416429735
That Sch is not exactly for the same board number but appears to be very close.

Any further advice would be greatly appreciated.