Hello all Problem with above Samsung, the bottom third of the screen is not there. Replaced the vertical output IC TDA8356, still the same. The drive signals coming from the TDA8842,pin 46 and 47, only one is there, the other one is just a dc level. It is possible that someone was in the service menu and did change the vertical settings. Will try to get the original remote, which I do not have. Anyone know how to come in the service menu and which options to change. Thank you very much for any reply Bert.
Thanks for the reply Art. Pin 1 of the TDA8356 has got a nice sawtooth. Pin 2 of the TDA8356 has a dc level of about 4 volt. The dc lines are clean and the right value according to the datasheet. The 2 outputs are there, but the positive and negative outputs do have a flat part when showing on the scope Did download the datasheet of the TDA8842, the same I do measure on pins 46 and 47. I do not know if it is possible to adjust the linearity and shift so far that one of the outputs is only a dc level. Only problem is the 8842 is quite expensive but it seems I do have to get one. Greetings Bert.
Firstly, playing Devil's advocate here, you already replaced the chip, why would another new chip solve anything? How many chips would you buy until you either give up the job or conclude, correctly, that the fault is elsewhere? It's possible, though not highly likely, that the replacement was DOA, but if it is dead, it is more likely that it was killed by fitting it into a circuit with other faults.
How do you mean the bottom third is not there? Are you saying the picture has normal proportions, but the bottom is black, or do you mean the bottom of the picture is squashed and distorted? If it's the former, the fault is not necessarily in the vertical output circuit. If it's the latter, try changing electrolytic caps in the vertical output circuit. Given the low cost of electrolytics, it's not a bad idea to replace them as a matter of course when replcing the vertical output chip, as bad ones can destroy the new chip. After all, electrolytics are probably the least reliable components there are.
One thing is certain, making adjustments to attempt to cure the fault is a
*very* bad idea. You cannot adjust faults out, you have to repair them! Making adjustments now then finding you have to make a repair afterwards will make a satisfactory job a real hassle, or worse still, cause more damage.
Firstly, playing Devil's advocate here, you already replaced the chip, why would another new chip solve anything? How many chips would you buy until you either give up the job or conclude, correctly, that the fault is elsewhere? It's possible, though not highly likely, that the replacement was DOA, but if it is dead, it is more likely that it was killed by fitting it into a circuit with other faults.
How do you mean the bottom third is not there? Are you saying the picture has normal proportions, but the bottom is black, or do you mean the bottom of the picture is squashed and distorted? If it's the former, the fault is not necessarily in the vertical output circuit. If it's the latter, try changing electrolytic caps in the vertical output circuit. Given the low cost of electrolytics, it's not a bad idea to replace them as a matter of course when replcing the vertical output chip, as bad ones can destroy the new chip. After all, electrolytics are probably the least reliable components there are.
One thing is certain, making adjustments to attempt to cure the fault is a
*very* bad idea. You cannot adjust faults out, you have to repair them! Making adjustments now then finding you have to make a repair afterwards will make a satisfactory job a real hassle, or worse still, cause more damage.
Dave
Ok Dave please hold your horses, there is a little mixed up. The IC I did replace was the TDA8356, the vertical output chip. It is a dual input and output IC, have a look at the datasheet please, only caps are on the suply line. Boost cap has be replaced, very high ESR. There was fresh solder on the legs, that is why the replacement. The set was at another place before I got it to repair. All electrolytic caps around the vertical and jungle IC has been checked for the right ESR. The only thing which does not look right is pin 47 of the 8842, just a straight dc line on the scope, that is all I do get wrong. DC supply pins on both IC's are close enough and clean of any spikes. Deflection coils does ring OK, even checked it with a replacement for a few seconds, do not want screen burn. That is why my first question was there, is it possible to adjust Lin and Shift that far so it looks faulty???? Pin 46 has got the right waveform, a nice sawtooth with the flyback pulse on top of it. DC levels of both pins the are the same. On the bottom third of the screen is nothing, the top two thirds are ok, linearity not far out and shifted a bit upwards when viewing the screen with a test picture displayed. It was not my intention to start adjusting everything in the service menu, to much experience with sets who where like that. I do not have much experiance with sets who have a srvice menu, that is why I came over here. But thanks for the reply Dave D Bert.
The snag is a capacitor can have an excellent ESR reading, especially if it's a dead short! The only way to reliably rule out caps IMO is replacement. ESR testing is very useful, but not conclusive. With vertical linearity issues I'd suspect electrolytics before anything else, especially the small ones. Caps might show a good ESR reading, but fail when under working conditions.
The snag is a capacitor can have an excellent ESR reading, especially if it's a dead short! The only way to reliably rule out caps IMO is replacement. ESR testing is very useful, but not conclusive. With vertical linearity issues I'd suspect electrolytics before anything else, especially the small ones. Caps might show a good ESR reading, but fail when under working conditions.
Dave
Hello Dave Will have a look again tomorrow, the funny thing is the one of the dc coupled inputs from the 8365 is not there, also the output of the of the
8842. The only thing left is replacing the 8842 and holding thumbs, will report back. Also I will change the few caps. Customer his to decide it is his money. Bert.
The snag is a capacitor can have an excellent ESR reading, especially if it's a dead short! The only way to reliably rule out caps IMO is replacement. ESR testing is very useful, but not conclusive. With vertical linearity issues I'd suspect electrolytics before anything else, especially the small ones. Caps might show a good ESR reading, but fail when under working conditions.
Dave
Hello Dave Will have a look again tomorrow, the funny thing is the one of the dc coupled inputs from the 8365 is not there, also the output of the of the
8842. The only thing left is replacing the 8842 and holding thumbs, will report back. Also I will change the few caps. Customer his to decide it is his money. Bert.
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