Toshiba 26AV600A

[Agent24]

Re: Toshiba 26AV600A (back again)

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.

I had a similar problem in an LCD monitor when one of the dual rectifiers went half-shorted. Have you checked for shorts on the secondary rectifiers?

In your schematic that is D956, D973, D972.

[Markus1]

Re: Toshiba 26AV600A

Agent24 - All 3 dual rectifiers were checked out of the circuit. All appear to be good.

[mmartell]

Re: Toshiba 26AV600A

"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.

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.

I thought I read somewhere that pin is for the detection of a/c removal so the uP can initiate proper shutdown so as not to corrupt memory ic's and such before working voltages disappear ?

[Agent24]

Re: Toshiba 26AV600A

I thought I read somewhere that pin is for the detection of a/c removal so the uP can initiate proper shutdown so as not to corrupt memory ic's and such before working voltages disappear ?

Perhaps some designs do that, it sounds like a plausible concept I suppose, but I have no idea really.

[Agent24]

Re: Toshiba 26AV600A

Agent24 - All 3 dual rectifiers were checked out of the circuit. All appear to be good.

OK, that's good.

So what you're saying is that 5vSB is OK in standby, but as soon as you try to start the PSU the 5vSB line starts going crazy?

Seems to suggest something shorted\bad on the VCC_1 net, which is derived from IC950's own VCC supply via the PS_ON transistor Q953.

What I also find interesting is that you said earlier IC950's VCC (Pin 5) was around 14v, yet the VCC_1 rail is only 9.6v. There should not be that much voltage dropped across Q953 as far as I can see.

Also, as budm said earlier, the IC970 needs at least 10v to start, so this 9.6v is borderline. It could be why the 12v/24v rails it controls never come up.


From your previous posting, it seems the PFC circuit is OK, but now it is not. Seems likely that's due to no VCC_1 now.

Try removing VCC_1 from IC970 but not IC901. Desolder one end of J917 and try starting the PSU again. Check the voltage on the main capacitor. You should get the boosted voltage now.

If that all works, see what the voltage is on that VCC_1 rail. It should be closer to 14v than 9.6v I expect.


If that is all correct, then it seems something is still wrong with IC970 or its circuitry.

[Markus1]

Re: Toshiba 26AV600A

Thank you Agent24.
As mentioned in earlier posts the set goes into the Red front indicator stage straight after plugging it into a wall socket, without pressing Power On button.
Actually, in the first 1-2 seconds the front indicator shows Green then goes Red and stays there, again without any from the Power On button. It is a sort of shut down. So, yes 5VSTBY is obviously present.

However, when forced-on is applied 5VSTBY drops a bit and varies a bit, Vcc (Vcc-1 as per SCH) fluctuates strongly and the front indicator blinks Green.

you said earlier IC950's VCC (Pin 5) was around 14v, yet the VCC_1 rail is only 9.6v.

Yes, voltage reading on IC950 Vcc pin is 14.7V (during steady Red indicator stage) but no Vcc-1 voltage is present on either IC901 or IC970. It is because at this stage Q953 transistor in not On as there is no PSON signal.

Try removing VCC_1 from IC970 but not IC901. Desolder one end of J917 and try starting the PSU again. Check the voltage on the main capacitor. You should get the boosted voltage now.

I will do all that and report back tomorrow.

[Agent24]

Re: Toshiba 26AV600A

What is the VCC_1 and IC950 Vcc Pin5 voltage when you force the PSU on?

[Markus1]

Re: Toshiba 26AV600A

Here are the latest checks:
Test 1
-all connectors in place
-PSON wire cut between PSU and Main
-PSU forced on via 1k resistor
-front LED blinks Green

Main filter cap voltage 350V.
Vcc on IC970 and IC901 (it's Vcc-1 on SCH) fluctuates widely, can not read.
Vcc on IC950 fluctuates around 11V value by about +/-0.5V.

Test 2
-all connectors in place
-PSON wire cut between PSU and Main
-PSU forced on via 1k resistor
-front LED blinks Green
-jumper J917 lifted to remove Vcc supply to IC970

Main filter cap voltage 398V. (boost occurred, good))
Vcc-1 steady at 13.56V. (good)

Further investigation indicated a short on IC970 between Vcc pin and GND. Adjacent capacitors C977 and C978 checked, appear good. Short on the IC970 confirmed by un-soldering its Vcc pin and measuring resistance to ground.

So it is the IC970, the one that has been recently replaced. Thank you Agent24. Your logic has been spot on.
Now, replacement of that chip may be difficult for me. Will see how to do it. I hope no other components are also contributing to the problem.

[Agent24]

Re: Toshiba 26AV600A

Strange that IC970 is shorted if you just replaced it. I guess either two things:

1) It was a fake part (Where did you get it?)
2) Something else caused the original and the replacement to fail.

Before you go replacing IC970 again, I would check all the semiconductors in the associated circuitry for shorts.

You say you also replaced the MOSFETs it drives earlier? What was wrong with the originals?

Is the original IC970 also shorted?

[Agent24]

Re: Toshiba 26AV600A

Just to add... I checked the datasheets of the 2SK4100 and the 2SK2141 and while it seems like a pretty good alternative with regard to most specifications, I see the replacement has an input capacitance (Ciss) of almost double that of the original. (1150pF vs 600pF typical)

I do not know if this difference is enough to cause a problem.
Also I notice these are measured with different Vds voltages (10v vs 30v). I don't know if that makes a difference. Maybe someone else can comment on this - datasheets attached.

Then again, maybe the difference is not so much of an issue by itself, but if you are running one of the 2SK4100 along with a 2SK2141 it may be causing some mismatching problem. A very wild guess, I may well be talking out my ass, but maybe you should look into it. At the very least, make sure the MOSFETs you are using are both the same, even if they are not the same as the originals.

[Markus1]

Re: Toshiba 26AV600A

Agent24, thanks for the advice.

Before you go replacing IC970 again, I would check all the semiconductors in the associated circuitry for shorts.

I will do that again. So far SMD caps (as suggested by Tech57, post 38) were checked and appear good, switching transistor Q953 seems OK, Vcc caps C976 and C977 also good.
The IC970 is definitely showing a short between Vcc pin and the GND. I've got that chip from a local supplier in Sydney "WES Componets".

You say you also replaced the MOSFETs it drives earlier? What was wrong with the originals?

One of the two Mosfets was short so it was replaced with a reasonably close matching substitute (could not find the exact part number).

Is the original IC970 also shorted?

Not sure whether it was clearly shorted but its Vcc seemed low at about 9.6V, and the replacement looked like a reasonable move (see post 27, 34, 38).
Still not sure about making the two Mosfets to a matching pair. As you have pointed out they are very close on the specification, except for the input capacitance, if it maters. Can't tell if it really affects operation of the IC970.
Anyway, I will report back later in the week.

BTW, on the side issue, and as a curiosity:
What is the reason for two caps in parallel on the Vcc pin of that IC970? There is one electrolytic C977 10uF 10V and one much smaller SMD C976 0.1uF. Please refer to the schematic https://www.badcaps.net/forum/attachm...2&d=1416429735
That arrangement is also at the Vcc pins of other chips IC901, IC950.
Why do we need the small SMD cap for?

[Agent24]

Re: Toshiba 26AV600A

As far as I know, the higher input capacitance of the replacement MOSFET means the driver IC has to use more power to turn it on. But if the current is fixed to both MOSFETs, that means the newer one will turn on slower.

This may or may not be a problem. If they are not switching on and off at the right time. I don't know if it would be enough to cause the IC to blow or not.



Those capacitors are decoupling capacitors for the ICs supply.

The 10uF capacitor will smooth out slow changes in voltage, the 0.1uF will react faster and smooth out quick changes in voltage.

[Markus1]

Re: Toshiba 26AV600A

I have something positive to report now. Well, at least in part.
The chip IC970 was replaced, Mosfets stayed as they were.
The PSU, when forced on comes alive. Main cap voltage goes to about 390V, 12v and 24V rails are present. So this is the positive part.

However, when all connected in a normal way (without forced on) the set is not turning on as there is no PSON signal coming from the Main board.
A few SMD fuses on the Main checked OK, no obviously swollen caps either.
Is there anything else I could possible check there?

[Agent24]

Re: Toshiba 26AV600A

Where does the PSON signal come from? Trace it back and find out if something along the way is faulty.

It might even be that the mainboard is not getting signal from the power button etc so it never sends PSON.

Check your power button and other buttons are OK, not shorted or open, check the IR receiver is not bad.


Do you have a schematic for the mainboard?

[budm]

Re: Toshiba 26AV600A

Boy, it is tough troubleshooting when the replacement IC is bad, I am glad Agent24 took over and hang in there and get this far.
BTW, what is ACD signal, HIGH or LOW since the PFC is working now.

[Agent24]

Re: Toshiba 26AV600A

Another thing I would check, the 5vSB probably feeds at least one secondary regulator and likely more on the motherboard (like 3.3v, 1.8v or such) check any such regulator(s) are functioning correctly.

Can you upload a photo of the mainboard?

[Markus1]

Re: Toshiba 26AV600A

Good news again, I am in luck.
After rechecking and respraying the side buttons with a contact cleaner/lubricant the TV turns on, picture and sound good. That was as per your suggestion Agent24.
It has been running for about 2 hrs now. I probably need to burn it for a bit longer but so far the signs are good. The two Mosfets, frequently mentioned in earlier posts, stay nicely warm and not hot, other components seem fine.

budm you asked for ACD signal check. It reads steady 0.7V (not sure whether it is High or Low).
I have noticed on the PSU circuit diagram that the ACD signal is just taken from a voltage divider on the +5V rail. See resistors R989 and R988 at the bottom right hand corner of the schematic https://www.badcaps.net/forum/attach...2&d=1416429735 . I know there were some discussions on this ACD in previous posts here but this concept is still a bit strange to me, particularly when it is called "AC Detection" and yet it derives from 5V DC. Can't say that I understand its purpose.

Anyway, a big thank you to all who responded to this tread, and special thanks here to budm and Agent24. Your support was fantastic, it got me to a stage where the TV is back on line, and what is more important I am learning a lot from it. Thanks again.

[Agent24]

Re: Toshiba 26AV600A

Good to hear

[Amraks]

Re: Toshiba 26AV600A

standby light green and side buttons function turns off the set.

same tv here but I have PFC boosted voltage on the filter cap at 396vdc

while on the 24v and 12v look like this
24v is at 2.191v
12v is 1.477v

[nomoresonys]

Re: Toshiba 26AV600A

Check all the diodes on the powerboard, good place to start and easy enough to do.