Medion 42" plasma not turn on

[PlainBill]

Re: Medion 42" plasma not turn on

Okay. Then I don't understand how this means a bad Z board. If 32v is not fully up, even when everything is disconnected, then some under volt protection is probably shutting the unit down.

Toast

In simple steps:

The hybrid on the Z-sustain is prone to shorting.
The Z-Sustain draws a lot of power.
The same hybrid is used on the Y-Sustain
The Y-Sustain draws even more power
The power supply shuts down when the Z-sustain is connected, but comes up when the Z-sustain is disconnected.
Disconnecting the Y-Sustain has no effect on the performance of the power supply.
THEREFOR It is a logical conclusion that at least one of the problems is caused by the Z-sustain.

It is noted that this TV seems to have at least three problems. I am concerned that this may be a 'junk' chassis made up of a number of bad cards all placed in a TV with a bad panel. But even that can be educational.

(If I learn the part number of the Y-Sustain I MAY be able to verify a short. It is even possible that by checking resistance between the various power lines and ground on the Z-Sustain forgonati may be able to verify a short.)

PlainBill

[Toasty]

Re: Medion 42" plasma not turn on

Okay. You're looking at the shutdown problem, and I'm looking at the power supply not providing the correct voltage.

My difficulty is that, if the 32v is not correct, why go looking elsewhere for the shutdown problem until that was fixed? Wouldn't (Couldn't) the shutdown problem be the low supply voltage?

Evidently you know these Z board hybrids are a known (common?) failure. Perhaps I'm being thick headed, but I would not see the logical conclusion of the Z failure unless the PSU was working correctly.

Toast

[PlainBill]

Re: Medion 42" plasma not turn on

Okay. You're looking at the shutdown problem, and I'm looking at the power supply not providing the correct voltage.

My difficulty is that, if the 32v is not correct, why go looking elsewhere for the shutdown problem until that was fixed? Wouldn't (Couldn't) the shutdown problem be the low supply voltage?

Evidently you know these Z board hybrids are a known (common?) failure. Perhaps I'm being thick headed, but I would not see the logical conclusion of the Z failure unless the PSU was working correctly.

Toast

I wouldn't be quite that harsh. Perhaps 'deliberately obtuse' would be more appropriate.

First, I'm not ready to give the power supply a clean bill of health. However, there are only a few points to be checked. You have mentioned one of them; the PFC voltage. If that is incorrect, certainly that must be corrected.

Another question is 'Is that 32V label correct?' Look at first the Vs supply - it uses PFC+ as it's source, and in turn is the source for the Vsetup supply.

Even more telling is IC9 and IC10 - they monitor the output voltages. Would that approve a voltage that is 25% low?

Also of interest is what uses the 32V supply? Other than the 12V and 15V supplies, it appears nothing on the power supply does. Does the signal board use it?

More importantly, IMHO is the fact that the power supply will not work with this Z-Sustain connected. Fixing a possible power supply problem is rather difficult if it won't stay on.

Don't forget, the training manual for this chassis has a simple sequence of steps to troubleshoot 'protection' problems. One of the first is to disconnect the Z-sustain card. If that clears the protection fault, they suggest replacing the Z-sustain. I'm suggesting instead that we investigate the Z-sustain.

PlainBill

[Toasty]

Re: Medion 42" plasma not turn on

>>Don't forget, the training manual for this chassis has a simple sequence of steps to troubleshoot 'protection' problems. <<

Sorry, I must have missed that memo. What training manual?

>>Perhaps 'deliberately obtuse' would be more appropriate.<<

My name is not i4004. Go insult somebody else. I'm trying to understand this and I get sarcasm.

Toast

[forgonati]

Re: Medion 42" plasma not turn on

I checked the input voltage of the T2. I measured 380V, so i think, there is enough "drive", and probably those big electorlyts (220uF/450V) are OK. I have no idea, why the 32V is only 24V ...

But i checked, what uses the 32V supply. The 32V goes to the 2x20W audio amplifier IC, LM1876. This IC needs supply voltage 20V-40V (The absolutely minimum is 12V and the maximum is 64V). So, i think, when i have only 24V istead of 32V, this should not cause a problem...

PlainBill:

I checked the part numbers of the sustain boards, see attached pics. How can i test the Z sustain, or these hybrid ICs???

Thanks

[PlainBill]

Re: Medion 42" plasma not turn on

>>Don't forget, the training manual for this chassis has a simple sequence of steps to troubleshoot 'protection' problems. <<

Sorry, I must have missed that memo. What training manual?

>>Perhaps 'deliberately obtuse' would be more appropriate.<<

My name is not i4004. Go insult somebody else. I'm trying to understand this and I get sarcasm.

Toast

Would you prefer 'unobservant'?

Notice the manuals (documents) I suggested in post 17. These are two early LG Electronics training manuals, plus a more recent set-up guide. (They are also marvels of inept translation).

PlainBill

[PlainBill]

Re: Medion 42" plasma not turn on

I checked the input voltage of the T2. I measured 380V, so i think, there is enough "drive", and probably those big electorlyts (220uF/450V) are OK. I have no idea, why the 32V is only 24V ...

But i checked, what uses the 32V supply. The 32V goes to the 2x20W audio amplifier IC, LM1876. This IC needs supply voltage 20V-40V (The absolutely minimum is 12V and the maximum is 64V). So, i think, when i have only 24V istead of 32V, this should not cause a problem...

PlainBill:

I checked the part numbers of the sustain boards, see attached pics. How can i test the Z sustain, or these hybrid ICs???

Thanks

Score a possible hit for Toasty. I don't know if the LG audio amps had the same weakness, but many Philips audio amps had a fault in which a failure would overload the 12V output of the power supply, generating a protection fault. I suggest you see what happens to +32V when you disconnect the audio amp.

As far as the Z-Sustain, the part number you photographed appear to be for the pc board, not the assembly. It cross references to 6871QZH020B.

Check the resistance from the power input connectors to ground, and between power inputs. A picture of the connectors would help.

In post 17 I suggested you download three documents. Two of them are training manuals for LG Plasma TVs. I suggest you look through them.

PlainBill

[forgonati]

Re: Medion 42" plasma not turn on

Score a possible hit for Toasty. I don't know if the LG audio amps had the same weakness, but many Philips audio amps had a fault in which a failure would overload the 12V output of the power supply, generating a protection fault. I suggest you see what happens to +32V when you disconnect the audio amp.

As far as the Z-Sustain, the part number you photographed appear to be for the pc board, not the assembly. It cross references to 6871QZH020B.

Check the resistance from the power input connectors to ground, and between power inputs. A picture of the connectors would help.

In post 17 I suggested you download three documents. Two of them are training manuals for LG Plasma TVs. I suggest you look through them.

PlainBill

So, i checked all 4 hybrids on each panel. On Y sustain i measured between Vs and SUS_OUT 5 Mohms. But on the Z sustain i measured only 4,5 ohms on the IC5, so that hybrid is shorted. The other hybrids are OK. So i have 1 shorted hybrid IC. So i need to be rebuild that.

In te next days i order the needed components. I will inform you guys here, when i'm done with the hybrid.

PlainBill, Toasty, thank's for the help

[Toasty]

Re: Medion 42" plasma not turn on

It still does not explain the low 32 volt line.

Did you disconnect the audio board and retest as PlainBill indicated the audio problem?

PlainBill,

>>Notice the manuals (documents) I suggested in post 17. <<

Yes, but the links didn't work Now I realize they are not links, but rather something you meant to be looked up and searched for. My apologies.

The term "deliberately obtuse" is inflammatory. It means I was consciously/deliberately ignoring something. I was not. Unobservant is accurate.

Is there a schematic for this Z board? In another sets SM?

Forgonati,

Between what points did you measure 4.5 ohms?
Are you are sure the 2 circuits are identical? 4.5 ohms almost sounds like a diode....
Did you reverse leads and get the same reading?
Was the board disconnected from everything else when you did this?

Toast

[Toasty]

Re: Medion 42" plasma not turn on

Okay, tried the scribd bit. No go. Gets first 2-3 pages and then they want $9 to download.

[PlainBill]

Re: Medion 42" plasma not turn on

Okay, tried the scribd bit. No go. Gets first 2-3 pages and then they want $9 to download.

Let me check that. They may have changed it and I'll see if I can attach them, or find another source.

Well, you can bypass the charge for downloading from Scribd if you upload something. Let's see if I can do better.

Here's another source for one of the files. For some reason, the PDF upload failed; the board is supposed to be able to handle a 9 Meg PDF, this was only 5.5.

Well, this may be an improvement. Go to http://search.4shared.com and search for Plasma Training Manual. That gives 16 hits, some for other manufacturers.

PlainBill

[forgonati]

Re: Medion 42" plasma not turn on

It still does not explain the low 32 volt line.

Did you disconnect the audio board and retest as PlainBill indicated the audio problem?

PlainBill,

>>Notice the manuals (documents) I suggested in post 17. <<

Yes, but the links didn't work Now I realize they are not links, but rather something you meant to be looked up and searched for. My apologies.

The term "deliberately obtuse" is inflammatory. It means I was consciously/deliberately ignoring something. I was not. Unobservant is accurate.

Is there a schematic for this Z board? In another sets SM?

Forgonati,

Between what points did you measure 4.5 ohms?
Are you are sure the 2 circuits are identical? 4.5 ohms almost sounds like a diode....
Did you reverse leads and get the same reading?
Was the board disconnected from everything else when you did this?

Toast

You're right, it not explains the 32V line, but i have no idea, where is the problem. I have already mentioned, on primary side of the T2 i measured 380V. And on the secondary only 24V. Those big caps (C33, 35, 36) are ok, and those small caps on the other side (C66, 67, 68) are ok. What now? Please give me ideas, on where to look for problems.

I'll explain it again, i have tested the PSU WITHOUT ANY PANEL CONNECTED, and i still have only 24V on the 32V line, so i think, not the audio amp cause the problem.

These 2 circuits looks like identical. Anyway, i have already removed from the board that hybrid, and i still measure only 4,5 ohms. On the other 3 hybrids i measure megaohms. I have a schematic of that hybrid, look the attached pic. Between SUS_OUT and Vs (D and S of the IRFP250 FET) should be couple of megaohms, and not only 4,5 ohm...

[PlainBill]

Re: Medion 42" plasma not turn on

You're right, it not explains the 32V line, but i have no idea, where is the problem. I have already mentioned, on primary side of the T2 i measured 380V. And on the secondary only 24V. Those big caps (C33, 35, 36) are ok, and those small caps on the other side (C66, 67, 68) are ok. What now? Please give me ideas, on where to look for problems.

I'll explain it again, i have tested the PSU WITHOUT ANY PANEL CONNECTED, and i still have only 24V on the 32V line, so i think, not the audio amp cause the problem.

These 2 circuits looks like identical. Anyway, i have already removed from the board that hybrid, and i still measure only 4,5 ohms. On the other 3 hybrids i measure megaohms. I have a schematic of that hybrid, look the attached pic. Between SUS_OUT and Vs (D and S of the IRFP250 FET) should be couple of megaohms, and not only 4,5 ohm...

Very good work, I'm impressed. You've gone beyond anything I would have been willing to try.

As far as the 24V vs 32V issue, a few ideas come to mind. The output of the 32V supply will vary, depending on the load on the 5V supply. The less load there is on the 5V supply, the lower the voltage on the 32V output. Normally, I would expect a change of less than 10%, but if the load on the 5V supply is very light, it could be greater.

Another possibility is Medion requested that output be 24V rather than 32V. This could be accomplished quite easily by reducing the number of turns on the appropriate secondary of T2. And the power supply manufacturer (Powerwel) simply used the same board, but assigned a different dash number to the revised supply.

PlainBill

[Toasty]

Re: Medion 42" plasma not turn on

Wacky thought... Could the MOSFET be latched "on". Have you tried to turn it off?

I ran into this in a power supply. Thinking the FET was shorted, I removed it, after which it was not shorted, nor was it ever. I had turned it "on" when testing with the multimeter. Analog meters do this easily to FETs. Digital meters can too when using "diode test". Usually they have to be out of circuit, but in some cases they will not, especially when using an analog meter.

http://www.4qdtec.com/mostest.html#simple

Toast

[forgonati]

Re: Medion 42" plasma not turn on

PlainBill:

Maybe you're right, and the 24V is absolutely allright. I try to PM to Maxxarcade, maybe he knows the truth about the 32V line.

I was nothing to loose, so i opened up the hybrid. I have 2 FETs blowned, the 2nd and the 4th from left.

I don't understand, WHY these hybrids blows out I find many many people on the net, who have the same problem. Those crap hybrids...

Toasty:

The first thing i did, checking all MOSFETs, diodes, rectifiers, zeners. I have a test circuit for checking the MOSFET and JFET transistors. So the FETs are OK.

[PlainBill]

Re: Medion 42" plasma not turn on

PlainBill:

Maybe you're right, and the 24V is absolutely allright. I try to PM to Maxxarcade, maybe he knows the truth about the 32V line.

I was nothing to loose, so i opened up the hybrid. I have 2 FETs blowned, the 2nd and the 4th from left.

I don't understand, WHY these hybrids blows out I find many many people on the net, who have the same problem. Those crap hybrids...

Toasty:

The first thing i did, checking all MOSFETs, diodes, rectifiers, zeners. I have a test circuit for checking the MOSFET and JFET transistors. So the FETs are OK.

That would be a good idea. I'm uncomfortable with the idea of turning the set on with a possible problem in the power supply.

I would say there are several contributing factors to the demise of the hybrids.

1. Inadequate heat dissipation, especially after the thermal compound between the hybrid and the heat sink dries out.

2. Components that are used very close to their voltage limits.

3. Use of lead free solder when manufacturing the hybrids.

Perhaps if you attached a picture of the blown hybrid it will convince Toasty. Or perhaps not.

PlainBill

[Toasty]

Re: Medion 42" plasma not turn on

You are under the impression I am not convinced of the Z problem. That is incorrect. I am merely making sure a power supply problem is not the cause of the problem.**

Once you said that these have a "history" of failure, I was fine. But, to merely go along with replacing a board without evidence, is a "board hanger" routine. I despised it 30 years ago, and I still do today.

From the looks of the hybrid, (never saw one torn open before) these are similar to STK genre devices with BMF heat sinks glued on. Yes?

My schooling taught me to look at the power supply as a prime suspect and make sure it was working correctly. Once that is done, then you can move onto tracing the problem down. Aside from "obvious failures" such as chips missing chunks or burned components, it's the first thing I check (off) the diagnostic list.

>>I have 2 FETs blowned, the 2nd and the 4th from left.<<

Actually, I think you have 4. #5 & 6 don't look good either.
Is something wrong with the component near ZD1, also? May be just the picture.

Toast

[forgonati]

Re: Medion 42" plasma not turn on

"From the looks of the hybrid, (never saw one torn open before) these are similar to STK genre devices with BMF heat sinks glued on. Yes?"

Absolutely. The newer LG plasmas uses on Z and Y boards STK795-811A, for example.

"My schooling taught me to look at the power supply as a prime suspect and make sure it was working correctly. Once that is done, then you can move onto tracing the problem down"

Agree. I send PM message to Maxxarcade, he has repaired that PSU, maybe he knows the truth about 32V line

With that hybrid you're probably right, the 5. and 6. don't look good either.
That big black component near ZD1 is a schottky diode (30V 2A).

Sadly, i must build new hybrids, this will cost me cca. 70€.
The 2. hybrid on the Z sustain is OK, but i must change the 2. hybrid also, to maintain a symmetry between two hybrids.

[forgonati]

Re: Medion 42" plasma not turn on

So, i have good news, the TV is working, even without the Z sustain panel See attached pic.

Of course, the screen is not perfect, wihout Z sustain.

I let it run cca. 10min, no problems.

[Toasty]

Re: Medion 42" plasma not turn on

>>Sadly, i must build new hybrids, this will cost me cca. 70€. <<

Build?!?!

How the heck do you rebuild all that?

???