Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

[dskall]

Video on tl431
https://www.youtube.com/watch?v=k_sw...eebElectronics

[R_J]

The volt supplied by D505 that feeds ic501 input is not regulated, The 15 volt supply line is regulating the circuit.
The schematic numbers are hard to read, R445 & R446(?) form a voltage divider, the center of the divider connects to pin 2 of ic400, pin 1 is ground and pin 3 controls the optocoupler.
When the secondary 15 volts (or the 5v) gets loaded, and the voltage drops, this tells the optocoupler to change the feedback to ic200 and increase its output to compensate for the voltage drop in the secondary.

You say that when you load the +5v which is the output of ic501 the voltage drops, but what happens to the input voltage of ic501? does it also drop? It could be that ic501 is all that is bad.

[dskall]

So the voltage out of t200 after the rectifier diode or at the 2 test points I can't read, is the only voltage that is on. Which is 15vs. And the input of ic501.

[R_J]

Originally Posted by dskall

So the voltage out of t200 after the rectifier diode or at the 2 test points I can't read, is the only voltage that is on. Which is 15vs. And the input of ic501.

Ther are 2 voltages supplied by T200, one, D501 which is likely around 16 volts and is switched by Q502. and one that is supplied by D505 which is likely around 6~8 volts and supplies the input of ic501. Both voltages are there in standby.

[dskall]

Originally Posted by R_J

Ther are 2 voltages supplied by T200, one, D501 which is likely around 16 volts and is switched by Q502. and one that is supplied by D505 which is likely around 6~8 volts and supplies the input of ic501. Both voltages are there in standby.

Wow, block diagrams and schematics don't even look the same. I need to do lots more work on deciphering schematics.

[Ierwin]

Originally Posted by R_J

You say that when you load the +5v which is the output of ic501 the voltage drops, but what happens to the input voltage of ic501? does it also drop? It could be that ic501 is all that is bad.

The input voltage drops also under load. I think it was 7.x unloaded and 5.x under load.
So this seems to be where the regulation fails.
Of course the ic501 has been suspicious all the time. So far I refused to unsolder it because I fear I might break it. I disconnected it's input and output and bridged it with a 7805 regulator I had. The behavior was almost the same: voltage drop on the input.

I'll watch the video later and educate myself

[vinceroger69]

Originally Posted by dskall

Wow, block diagrams and schematics don't even look the same. I need to do lots more work on deciphering schematics.

I also have trouble understanding schematics/block diagrams, but what i do appreciate is how the likes of budm and r_j take time out to explain them too us i learn a lot more this way.

[Ierwin]

just minor update while Im still trying to understand the regulation:
The voltage divider for Reference should be R443(82k) and R443 (15.4K).

I guess that explains the 2.4Volts I measured on Ref.

*Edit*
I checked again and realized something:
The input voltage on the regulator drops (currently to 6.4V) but the voltage on the voltage divider does NOT. It stays at 15.x V.
So I guess the Feedback loop does not give any indication to IC400 to increase power.

[Ierwin]

moving forward I now tried to verify that IC501 is working which is what I thought.
I connected an external adjustable powersupply to the input side of IC501 and hooked up one voltmeter to the input and another to the output.
Once the input reaches around 5V the output goes to 0.9V which is also what it does in normal operation. Increasing the input to 7V does NOT change the output.
Is it time to take out IC501 and test it outside of the circuit?

[dskall]

So you are maintaining 15v, but not at 5v reg input?

[dskall]

Originally Posted by vinceroger69

I also have trouble understanding schematics/block diagrams, but what i do appreciate is how the likes of budm and r_j take time out to explain them too us i learn a lot more this way.

I see budm hasn't posted for around a year.

[R_J]

Originally Posted by Ierwin

moving forward I now tried to verify that IC501 is working which is what I thought.
I connected an external adjustable powersupply to the input side of IC501 and hooked up one voltmeter to the input and another to the output.
Once the input reaches around 5V the output goes to 0.9V which is also what it does in normal operation. Increasing the input to 7V does NOT change the output.
Is it time to take out IC501 and test it outside of the circuit?

If the 5v is dropping and the 15v is staying up, Check R501? 0.47Ω (between t200 pin9 and D505)
It could be that the 7 volt supply is not even working and you are getting the 7v from the 15v via R527/D505?

[vinceroger69]

Originally Posted by dskall

I see budm hasn't posted for around a year.

yes ive noticed that too i hope hes ok as was going through a health issue he made a thread on.

[Ierwin]

Originally Posted by R_J

If the 5v is dropping and the 15v is staying up, Check R501? 0.47Ω (between t200 pin9 and D505)
It could be that the 7 volt supply is not even working and you are getting the 7v from the 15v via R527/D505?

Right after pin 9 is R531. Should be 0.47 an measures at 0.6 so it seems okay.
I just now realized that ic501 is connected to d505 as well as D506. Hard to tell where the power is actually coming from. If it helps I previously opened the line between D506 and Ic501 when I tested with the other regulator. The gap is closed right now but should be easy enough to open again and see if power comes from the D505 side

*Edit*
So I did just that, I cut the connection between d506 and ic501. The d506 side of the gap reads 6.5V while the ic501 side that is still connected to D505 reads 6.3V.
I have absolutely no idea what that means.
But also my PC broke down and while I wait for crucial parts I only have my cellphone to read the schematics which makes it a little extra challenging

[R_J]

What happens if you place a load on the 16v line, does that voltage also drop?

[Ierwin]

Where exactly should I put that load?
I measured the 15.5v across the two resistors of the voltage devider. Is it okay to just put a resistor there?

[Ierwin]

I wanted to stay on the safe side so I connected 100Ohm on the 15.5V right on the points where I measured 15.5 before.
Voltage did NOT drop.
Then I went to 68Ohm and still no voltage drop on the 15.5V.

I d9nt want to go smaller until I know its safe, so Im hoping for your advice

[dskall]

It should say on board voltages and current for each supply.

[R_J]

Originally Posted by Ierwin

I wanted to stay on the safe side so I connected 100Ohm on the 15.5V right on the points where I measured 15.5 before.
Voltage did NOT drop.
Then I went to 68Ohm and still no voltage drop on the 15.5V.

I d9nt want to go smaller until I know its safe, so Im hoping for your advice

You just need to place the load on the output. You see the transformer T200, it has two windings that supply two different diodes, these are the two secondary supplies. To load the 16v line place a load across one of the three filter capacitors, 470µf/25v. 68Ω will only load it to 235ma so if you have two of them put them in parallel to double the current
If loading the 16v line does not change it, but loading the 6v line causes it to drop, and if the 0.47Ω resistor is ok, it could be that D205 is going open under load.

[Ierwin]

I did as you suggested and put two 68Ohm Resistors in parallel across one of the filter Caps. Only for a couple of seconds as they got a little warm (I think those are 1W resistors at best, maybe even just 1/2W).
Voltage remained at 15.45 or something so it did not drop significantly.

How can I test D205?

I have performed so many tests around IC501, cut the tracks and reconnected them and so on when I tried to measure the behavior without load. I thought it would be a good idea to verify the voltage drop on the 7V line changes with the load.

But I must admit I still have WAY to little knowledge to perform this by myself.

Either that maybe some of the information I gave earlier was not correct.
Because my latest observation is when I tried to increase the load on the 7V line by putting a Resistor in parallel to IC501 (connected from Vin to GND) the Voltage on the input did not change. It stayed at 6.2V with and without the resistor (I used the 34Ohms from the 15V test before).

I think at this point I am completely confused and everything I thought I had learned so far doesn't seem to make sense anymore