Panasonic TV, vertical osc. issue

[Elusive_Cure]

Hey guyz. i need some advice here

I have a panasonic TX25 MD3P tv (euro spec) that after 10 or so years of operation it stopped displaying picture but had sound.

I took it to the local repair shop where the technician changed the vertical oscilation IC (LA7845N) and gave it back to me.

After a couple of days (literally a couple) it gave up again.

I haven't tried to repair it myself, but I'm sure I'd do a better job than him (now that i have the schematics on hand).

Can someone experienced give me some advice on what to be taking care of so as not to burn the IC yet another time??

Here is the schematic from the Vertical oscillation circuit





Thanx in advance

[Elusive_Cure]

Re: Panasonic TV, vertical osc. issue

[kc8adu]

Re: Panasonic TV, vertical osc. issue

replace c456 and c455
should prevent a repeat failure.

[Krankshaft]

Re: Panasonic TV, vertical osc. issue

Yep dried up lytics can lead Deflection ICs to an early demise.

It looks like this "Tech" if they so can be called was just looking for a quick fix.

Also replace C462 the vertical ICs output capacitor and that will clean sweep any suspect lytics from the vertical section.

[Elusive_Cure]

Re: Panasonic TV, vertical osc. issue

thanx for the quick response fellas.....

will order the parts tomorrow and see what happens.

.

[Elusive_Cure]

Re: Panasonic TV, vertical osc. issue

It looks like this "Tech" if they so can be called was just looking for a quick fix.

.

the bloody bastard took more than a month to fix it....
i thought he'd have it done by the time i returned from a seminar i had to attend,
i didn't have the time to trace out the problem myself, that's why i gave it elsewhere to be fixed.

p.s.

although i have a bachelor's degree on elec. engineering, i'm not that confident
on working on tv's and such (lack of experience), i mostly focus on the IT sector as a freelance comp./network tech.

however, since i was a child what my eyes saw, my hands could fix, that's why i studied electronics.

anyway, thanx again 4 your advice....

keep up the good work

[gonzo0815]

Re: Panasonic TV, vertical osc. issue

Yes mate, TV`s are tricky. I have repaired a few for fun, and with any set my knowledge is rising.

But without tip database it could be expensive & time consuming.
I always hoped, that no one will come back, if i have not found any improvement tip. And i replace capacitors more frequent with high grade ones, if the set is old.

The usual TV tech can`t do this, if he want do make a living from this business.
And most people are not willing to spent a huge amount on high grade capacitors for an old TV set. But after 3 to 10years, the caps are usually bad.

[tvtech]

Re: Panasonic TV, vertical osc. issue

This was an easy,tv repair 101 fix,First you check the solder connections at the ic and the pumpup diode and the rest of the output circuit.Then replace the ic.Whenever you replace the vert output ic you look for whatever caused the chip to fail,overheating,bad pumpup cap,loose solder,bad yoke(rare)etc...........Because the la78XX series ic's are pretty reliable 90%of the time something in the vert output circuit casued the failure.Then after you replace the ic you probabaly will have to replace the resistor feeding the b+ to the ic because when the ic shorts it usually takes the resistor with it.The resistor is usually near the flyback and is usually less than 1 ohm.Make sure that you replace the caps in the vert. circuit mentioned.The best indicator of a bad pumpup cap is small lines in the top 1/4 of the screen that look like retrace lines,if you let the set continue running this way you will kill the output ic.Make sure the heatsink for the vert ic has enough thermal grease on it as the ic will overheat if you have too little or the old grease has dried out.If you have trouble sourcing the IC I can send you one I have plenty new ones.Be careful removing the old ic,paying attention to not damaging the traces as they will be very fragile due to the heat from the ic baking them,use good solder wick.If the tech took a month to repair the set how does the guy stay in business????When I had a shop I would do at least 10 of these vertical repairs a WEEK in addition to the normal business,these were usually the easiest repairs in the shop along with the old RCA ctc185's with the bad grounds in the tuner.

[Elusive_Cure]

Re: Panasonic TV, vertical osc. issue

tvtech thanx for the useful post...

i live in the countryside of Greece, so I'm waiting till next week when i'll travel to Athens for business, so i'll buy myself the components from an electronics store i know. Thank you for the offer though....

the tech stays in business because he's the only one around here, so he never runs out of customers....

i opened the tv two days ago and i saw that he changed C456 and C455 with old caps, not fresh ones, and when i looked at the bottom of the board to see the solder joints he had refreshed most of the solder (just to make sure???) around the IC. I'm going to change all the electrolytic caps around the IC just to be on the safe side.

Now, just a few quick notes because as i mentioned i have little experience on tvs. Bare with me, i appreciate all help you can give.

Then after you replace the ic you probably will have to replace the resistor feeding the b+ to the ic because when the ic shorts it usually takes the resistor with it.The resistor is usually near the flyback and is usually less than 1 ohm

i can't see a resistor lower than 1 ohm in the schematic, but as you say near the flyback i suppose its R470 (5.1k). am i right??

First you check the solder connections at the ic and the pumpup diode

By pump up diode you mean D470 near the flyback?? Or is it D456 on the VCC1 pin?

I would appreciate if you could point what components (besides the IC) i should change by naming them or better by editing the schematic i have attached and reposting it.

Thank you again for your advice, its always nice to see people offering their 2 cents when someone needs it...

[Krankshaft]

Re: Panasonic TV, vertical osc. issue

This resistor is supposed to be located in the VCC line going into the deflection IC.

I know for a fact that these resistors are always 1/2 watt or more and always are in the low ohms range.

Just trace back from the deflection ICs VCC line until you come across the resistor it shouldn't be miles away from the IC.

Then after you replace the ic you probabaly will have to replace the resistor feeding the b+ to the ic because when the ic shorts it usually takes the resistor with it.

Wow B+ its funny we still use terms from the old tube radio days the past is never completely dead .

[Elusive_Cure]

Re: Panasonic TV, vertical osc. issue

......i see ... that B+ got me confused ...... that must be R472 (4,7 1/2 W)

[gonzo0815]

Re: Panasonic TV, vertical osc. issue

B+ is very common language regarding TV circuits. Most service manuals are referring to it and it is probably one of the most important voltages in a TV.

[tvtech]

Re: Panasonic TV, vertical osc. issue

tvtech thanx for the useful post...

i live in the countryside of Greece, so I'm waiting till next week when i'll travel to Athens for business, so i'll buy myself the components from an electronics store i know. Thank you for the offer though....

the tech stays in business because he's the only one around here, so he never runs out of customers....

i opened the tv two days ago and i saw that he changed C456 and C455 with old caps, not fresh ones, and when i looked at the bottom of the board to see the solder joints he had refreshed most of the solder (just to make sure???) around the IC. I'm going to change all the electrolytic caps around the IC just to be on the safe side.

Now, just a few quick notes because as i mentioned i have little experience on tvs. Bare with me, i appreciate all help you can give.



i can't see a resistor lower than 1 ohm in the schematic, but as you say near the flyback i suppose its R470 (5.1k). am i right??



By pump up diode you mean D470 near the flyback?? Or is it D456 on the VCC1 pin?

I would appreciate if you could point what components (besides the IC) i should change by naming them or better by editing the schematic i have attached and reposting it.

Thank you again for your advice, its always nice to see people offering their 2 cents when someone needs it...

No problem,besides the caps you changed and checking the solder you should be ok,Sorry I cant see your schematic you posted because my IE is screwed up on my computer but I will gladly give you a run down of the major componets fuctions as soon as I refromat.When you go to buy your replacement IC make sure it is an oem chip not a nte or ecg replacement,as they are crap and most likely you will have a repeat failure of the ic very quickly.Grab a tube of heatsink grease while you are at the electronics store if you dont have any laying around because you will want to replace the grease during the installation of the new ic.