Panasonic 27" Picture Problem

[Krankshaft]

Re: Panasonic 27" Picture Problem

Well I got the IC today I originally had another part on backorder that was holding the IC up it was in stock. I removed the hold on Wednesday because I got impatient.

Anyhow I replaced it and success!!!

The final body .... erm part count was the Chroma / Deflection / Microcontroller IC and the vertical IC and associated capacitors.

Included in the cap replacement was the 100 uf pump up capacitor the 1000 uf yoke coupling capacitor and two more in the vertical section.

The caps checked out fine but I had the vertical IC on hand and both it and the caps were cheap.

So most likely the vertical cracked joint probably killed something in the old IC. That's the only anomaly that makes sense at this point.

Thanks for all of the help on this. It was a real strange ordeal.

[Krankshaft]

Re: Panasonic 27" Picture Problem

Oh yeah and I made sure to apply gray RTV to the side of the IC that had it.

I still don't know why it had it though?

I remember hearing insulation but insulation from what arcing between pins?

In any case I used Permatex Water Pump RTV since that is the gray stuff.

I let it sit for an hour before firing the TV up it fully cures in 24 but it is no tack in 1 hour.

That's how long I wait when I use RTV for gasketing anyways.

[Wizard]

Re: Panasonic 27" Picture Problem

On scope waveforms will be very apprent if vertical IC is iffy. And vertical IC is cheap and I simply replace it regardless if TV still acts funky after this resolder and *always* replace 100uF cap with good one too! If it is still funky, I replace the pump up diode as well. Uncovered few surprises on that diode. :p

Geeh, I advised that earlier on.

Cheers, Wizard

[tmwalsh]

Re: Panasonic 27" Picture Problem

I will assert my grave ignorance and ask: Couldn't the intermittent vertical collapse have induced a current that back-emf'd the chip? I don't have the schematic in front of me, but the thought occurred...
tom

[Wizard]

Re: Panasonic 27" Picture Problem

Usually I see fractured solder joints on vertical since IC is linear transistor, basically a linear regulator! There is second transistor to switch in a charged cap a pulse of 22-25V on top of ramp around 20-22V at beginning of ramp for tall thin ~50V kick to slam vertical winding on the yoke back to start of vertical progress due to strong inductuce built into this vertical winding of this yoke (this is ferrite core wound).

All this especially the ramp transistor develops themendous thermal stresses and current stresses get worse if that pump cap ESR rises and stresses the ramp stage of the tranistor driver part considerably trying to yank voltage back up to the beginning because of no assisting kick (can be bad cap, fractured solder).

Caught early, all good, but left alone too long the vertical IC fail and sometimes take out diode as well.

Cheers, Wizard

[Wizard]

Re: Panasonic 27" Picture Problem

Usually I see fractured solder joints on vertical since IC is linear transistor, basically a linear regulator! There is second transistor to switch in a charged cap a pulse of 22-25V on top of ramp around 20-22V at beginning of ramp for tall thin ~50V kick to slam vertical winding on the yoke back to start of vertical progress due to strong inductuce built into this vertical winding of this yoke (this is ferrite core wound).

All this especially the ramp transistor develops themendous thermal stresses and current stresses get worse if that pump cap ESR rises and stresses the ramp stage of the tranistor driver part considerably trying to yank voltage back up to the beginning because of no assisting kick (can be bad cap, fractured solder).

Caught early, all good, but left alone too long the vertical IC fail and sometimes take out diode as well.

Cheers, Wizard

[Wizard]

Re: Panasonic 27" Picture Problem

Updated:

This is block diagram of a vertical IC but they are 2 power transistors associated with the driver and forming waveforms if input is pulses (LA7838). There are other vertical IC that take sawtooth waveform as well (LA7830 is one of these).

http://www.pixelsplasher.com/_knowle...Yoke.is.Dirty/

The waveform of a good vertical output (ingore the FET drivers, commonly used transistors in majority of vertical circuits) in other circuit following this diagram on OUTA:

http://www.nxp.com/pip/TDA8357J_3.html

PS: if using FET design, the vertical circuit becomes more efficient and need less heatsink. In fact, CTC169 used 15.7KHz isolated supply from flyback transformer through a SCR with very cheap tiny, really tiny heatsink. Used a LM511 to develop ramp and develop a pump pulse in one circuit to drive the SCR transistor. Efficient!

Cheers, Wizard

[i4004]

Re: Panasonic 27" Picture Problem

>Anyhow I replaced it and success!!!

excellent!

>So most likely the vertical cracked joint probably killed something in the old IC. That's the only anomaly that makes sense at this point.

yes.

>Thanks for all of the help on this. It was a real strange ordeal.

but imagine how strange could problems get if this tv was all discrete devices, like many years ago...
mhm...

[Wizard]

Re: Panasonic 27" Picture Problem

>Anyhow I replaced it and success!!!

excellent!

>So most likely the vertical cracked joint probably killed something in the old IC. That's the only anomaly that makes sense at this point.

yes.

>Thanks for all of the help on this. It was a real strange ordeal.

but imagine how strange could problems get if this tv was all discrete devices, like many years ago...
mhm...

Depends. discrete component TV rarely have the cut out circuit built in for direct view TVs, I can count on one hand for certain brands/models combination with this feature: JVC, Pansonic and rarely found in other brands. The projection TVs always have the cut out circuits to prevent burning out phorphors if vertical or horizontal circuit breaks down.

Cheers, Wizard

[Krankshaft]

Re: Panasonic 27" Picture Problem

>
but imagine how strange could problems get if this tv was all discrete devices, like many years ago...
mhm...

I would have welcomed that I'd probe every point of the chroma circuits until I found the culprit. But you are right ICs for the most part simplify troubleshooting. If this IC wasn't SMT I would have shotgunned it from the start.

The price of the IC was a bit excessive at 28 bucks but I got the TV for free from Craigslist so I figure it's worth it for a Panasonic.

A special thanks about the Chip Quik it made the process alot less painful buzzing off all the pins with a cutting wheel on a Dremel was my old way.

I think the whole thing that set this off was the vertical blanking failure circuits only cut out the RGB signals and not the OSD.

When the vertical IC had it's VCC pin cut off the only way I knew I had no vertical sweep was when I changed channels or raised the volume which displayed the thin horizontal line. If it wasn't for that a vertical cutoff circuit wouldn't have crossed my mind.

I thought those circuits were mostly on RPTVs since their phosphors burn much more easily.

[i4004]

Re: Panasonic 27" Picture Problem

they're probably just using (slightly modded?) chassis from RPTVs(or at least mpus), because it doesn't make much sense for direct view crt to have this: when only one line remains on the screen, who's gonna use it like that for prolonged periods of time?

yes, i was thinking how ics simplify troubleshooting, not necesarilly just thinking about this case and the weird protection circuit...


now i'm wondering if my panasonic has it...but..naah...too lazy to check it out now...

[Wizard]

Re: Panasonic 27" Picture Problem

CRT Proj drive the CRTs real hard and need coolant to keep heat even across face. In extreme case, very occasionally a CRT projection stuff cracked across face when there was bright image displayed too long if the CRT had a flaw in it. Opposed to direct view CRTs BUT the line can burn the phorphors if left too long, plenty of time to react and hit power button. Projection CRTs burned once you say "Oh no!" and go for the power instant, no matter what if deflection collaposes to a line.

Oh yeah, if someone forgot to keep CRT board off the tube when testing the dismounted yoke. Dot in exact center of tube's face, bye!!

Cheers, Wizard