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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Panasonic CT2702UB Hor. lines at top -sometimes
This set has what sort of looks like retrace for the first two or
three inches down from the top. I found a 100uf 16V high ESR electrolytic in the vertical circuit but replacing it didn't seem to help. The thing is when watching a program with full screen the problem is usually there constantly. However when the commercials come on sometimes they appear to be shown in wide format with the top and bottom black for the first inch or so. It is during these times that there doesn't seem to be a problem. It seems to depend on what commercial is showing and sometimes the luminence level that brings it on. Does this seem like anyhing common on this set? Thanks. Lenny. |
#2
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Panasonic CT2702UB Hor. lines at top -sometimes
You still have problems in the vertical deflection circuits, probably
another flakey electrolytic, may even be in the supply circuit to the vertical. FWIW, check the connections at the vertical iutput ic also, seen way too many of these with cracked solder connections at the pcb. wrote in message ... This set has what sort of looks like retrace for the first two or three inches down from the top. I found a 100uf 16V high ESR electrolytic in the vertical circuit but replacing it didn't seem to help. The thing is when watching a program with full screen the problem is usually there constantly. However when the commercials come on sometimes they appear to be shown in wide format with the top and bottom black for the first inch or so. It is during these times that there doesn't seem to be a problem. It seems to depend on what commercial is showing and sometimes the luminence level that brings it on. Does this seem like anyhing common on this set? Thanks. Lenny. |
#3
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Panasonic CT2702UB Hor. lines at top -sometimes
(PST) wrote:
This set has what sort of looks like retrace for the first two or three inches down from the top. I found a 100uf 16V high ESR electrolytic in the vertical circuit but replacing it didn't seem to help. I wonder how often in this NG it is simply advised to replace caps. Wouldn't I have a general confidence in people, I would have to suspect that some of the respective authors are cousins, nephews, husbands, sons, fathers or other relatives of cap manufacturers. Whereas it may be true, that aged caps do have their share in old device trouble, this does imho not justify to do trial and error exchanges without reason. Please correct me, if I'm wrong in the assumption, that retrace is blanked by a signal to the CRT, and if that signal is misshaped, does not arrive in time with necessary amplitude, or meets other unfavorable conditions at the CRT, then the retrace can become visible. Thus it would be nice in those instances to sit over the schematic, follow the path of the blanking signal and to ponder, what could interfere there. Regards, H. |
#4
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Panasonic CT2702UB Hor. lines at top -sometimes
Retrace: Persay, would be visable normally throughout the whole screen, in
all areas of the raster, especially if the g2 bias is adjusted too high or there is a problem is the supply circuit for the g2. Faults in teh video processing and biasing can also cause retrace lines beng visable. Easily visable on an unused video input when adjusting the G2 control on the LOPT. Advancing the control will cause the screen to brighten a bit before the retrace lines become visable, reversing the control will deminsh the intensity of the retrace lines with correct adjustment to a point where the retrace lines are not visable. Having "Foldover" at the top of the raster, indicated initially by the top portion of the raster being folder over onto the top of the rater. This symptom is normally caused by a fault in the vertical frame deflection circuitry, commonly either failing electrolytic capacitors and/or a defective active vertical deflection devices, be it an ic or descrete transistors. Systomatic description, in the original post, indicate the lines are visable only in the top of the raster, therefore the probable diagnostics of vertical frame deflection fault. BTW: Wish I had a nickel for every defective or leaky electrolytic capacitor that actually cures this specific symptom, and no, I do not own stock in any manufacturer using 'Fish Oil' as part of the dielectric within their components. "Heinz Schmitz" wrote in message news (PST) wrote: This set has what sort of looks like retrace for the first two or three inches down from the top. I found a 100uf 16V high ESR electrolytic in the vertical circuit but replacing it didn't seem to help. I wonder how often in this NG it is simply advised to replace caps. Wouldn't I have a general confidence in people, I would have to suspect that some of the respective authors are cousins, nephews, husbands, sons, fathers or other relatives of cap manufacturers. Whereas it may be true, that aged caps do have their share in old device trouble, this does imho not justify to do trial and error exchanges without reason. Please correct me, if I'm wrong in the assumption, that retrace is blanked by a signal to the CRT, and if that signal is misshaped, does not arrive in time with necessary amplitude, or meets other unfavorable conditions at the CRT, then the retrace can become visible. Thus it would be nice in those instances to sit over the schematic, follow the path of the blanking signal and to ponder, what could interfere there. Regards, H. |
#5
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Panasonic CT2702UB Hor. lines at top -sometimes
AJ wrote:
Retrace: Persay, would be visable normally throughout the whole screen, I have an old TV set which regularly shows a few retrace lines at the top of the screen if it receives a program with videotext data, but is fine with "normal" programs. I suppose the blank pulse timing is somewhat disturbed by the videotext signals, which they didn't know of at the devices creation day. Having "Foldover" at the top of the raster, indicated initially by the top portion of the raster being folder over onto the top of the rater. This symptom is normally caused by a fault in the vertical frame deflection circuitry, commonly either failing electrolytic capacitors and/or a defective active vertical deflection devices, be it an ic or descrete transistors. Yes. I had that once, the cause being a defective TDA-something. These get rather hot - more so, if the manufacturer tries to squeeze out the last nickel by saving on the heat sink. BTW: Wish I had a nickel for every defective or leaky electrolytic capacitor that actually cures this specific symptom, I can't say that to replace electrolytic caps would have saved me much work. Regards, H. |
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