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

[dskall]

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

By holding oscilloscope probe on top of transformer you should see pulses. I think you must have a feedback issue.

[Ierwin]

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

Puliing this up again, hoping ot get another great advice on how to proceed.
I now have the oscilloscope here and Im learning how to use it. I have nothing to compare it to besides a very very old analog device that my dad used to have when I was a kid. This one is battery driven, digital, with auto range and auto hold. Its a Tektronics THS-720 if that brand/type is anything common.

So to sum up the current state:
loading the 15V line (measured and loaded over the 3 filter caps C503-505) the voltage stays stable.
loading the 7 V line (measured and loaded between the Vin and GND Pin of IC501) the vltage drops. At the moment IC501 is completely removed.

R_J suspected R501 (the 0.47Ohm Resistor on Pin 9 of T200) to be open => not the case
or D205 to open under load. Testing the Diode unloded with my meter it seems okay.
I dont know how to test the diode under load, but I think with the osci I should be able to.

So from T200 comes some kind of AC current that is generated by IC200, right? Its not just a sine wave because IC200 does all kind of fancy stuff, like skipping cycles and so on but it is AC.
The diode would be "cutting" one halve of the wave, because it blocks the current in one direction, but until a lot of filtering happens this still not a "flat line". Its waves cut in halve and it should be visible on the Oscilloscope.

I will try to make the function of D205 visible, but to be hones I do not have high hopes that it is the faulty part.

If anyone has another idea what I could test, I would verymuch appreciate it.

[Ierwin]

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

Let me say that Im sorry for all the confusion.
I realize I have had some practical issues here that screwed up my results and its hard to perfrom analyzis when measurements are not reliable. One simple reason was the way I proped on the board.
I now soldered some wires to critical testpoints so I can clamp on the probes and get reliable readings.

So with the IC501 removed completely I can confirm that 15V indeed does NOT drop (loaded with 34Ohm, Resistors heating up) while the 7V line does DROP under load (same 34Ohm Resistor, drops to 4.2V, no heat)

[Ierwin]

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

All the latests tests I had done with the IC501 fully connected so since you mentioned reconnecting it, I took it out in the first place.
And this time I really took it our completely. I think I should have done that earlier because all my attempts to measure around it by cutting lines on the board just messed it up. Still to a point where all the lines are okay, it just doesnt look as good.

So without the IC501 I get 9.8V where Vin would be.

I realized I made a mistake yesterday by putting the Load resistor in Series with my Voltmeter. Of course it has to be in parallel

So now when I put the 15Ω load between Vin and Gnd of IC501 the voltage drops significantly to 3.5V. With the 35Ω it drops to 4.2V.
So I thought there was a load dependant voltage drop on the second supply.

Knowing my stupid mistake with the measurement I wanted to retest the 15V line with a load in parallel, but found it wasnt there anymore. And now the 9.8V on Vin are gone too.
To my understanding of the Block diagram that is because without IC501 RL100 wont switch.
I guess when I measured earlier it was still closed on the charge of some cap and I still dont know if the voltage really drops.

I need to do more testing

[R_J]

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

If the 16v is staying up and the 68Ω got warm, I would say the standby circuit is working. and the primary side and feedback/regulation is all ok.
Adding a load to the 6v line is the only way to check if it is working and able to supply current. About a 15Ω resistor should draw around 500ma, and the circuit should be able to supply that without a problem.
Reconnect ic501, does the vin stay up?

These power supplies were very reliable and the original problem was likely not on the power supply. I would check all the work you have done on it so far.

[Ierwin]

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

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

[R_J]

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

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.

[dskall]

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

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

[Ierwin]

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

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

[Ierwin]

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

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?

[R_J]

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

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

[Ierwin]

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

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

[vinceroger69]

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

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.

[R_J]

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

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?

[dskall]

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

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.

[dskall]

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

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

[Ierwin]

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

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?

[Ierwin]

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

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.

[vinceroger69]

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

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]

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

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