Panasonic TX28PB50 Euro 7

[Turnip]

Hi folks,
Have one of these where at switch on, PS pulses for perhaps 10 seconds then cuts. Thought to check LOP collector which shows little initial voltage, but resistance wise (Avo 8 of course), it's 12 Ohms one way and 100 Ohms the other. Suspect 100 Ohms is too high for a PS secondary diode short - any thoughts ?  - Chris.

[Contrast John]

Hi Chris
Assume you mean 100 ohms to deck. Probably a leaky line output transistor. Usually caused by dry joints in line stage or high HT from the psu.
Regards
John.

[sparky]

hi
in that model line-out
 transister as internal diode
best to measure out of circuit
check line drive transformer pins become
dry can see with magnifier
sparky

[Turnip]

Hi, and thanks,

Checked LOP, found it leaky. Replaced checking for dries esp around L/drive tx. Plugged in and sound came up with picture rather narrow E/W with some foldover. LOP then died. Suspect I have a duff LOPT to replace - Chris.

Original LOP is 2SC5591 (non integral diode) I replaced with BU4508AX, nearest I had.
Checked ChasHyde, tranny some £75 ish - ah well.

[brainstorm]

Hi,

If line op transistor is leaky then always check for drys on line driver transformer on panasonics,
and check yout HT hasnt gone high. but you are probably right that its the Line op transformer

Brian

[Turnip]

Hello, and thanks Brian,

Had the thought that with a Panasonic, dries in the line drive area might be the problem, but foldover after resoldering and demise of replacement LOP suggested an excessive load.

One more for China, lets hope it comes back as an LCD - Chris.

[tv john]

Do you not have to use the origional o/p tr for that chassis ?

John.

[Turnip]

Hi and thanks John,

You may well be right. Have in the past had trouble with Sony FE2 in particular where similar'ish LOPs got hot after a short while, but generally checking specs first gets me out of trouble with minimum stock.

Like most repair folks I have a quantity of semiconductors salvaged from scrap and am reluctant to spend possible profit for even more that might just sit on the shelf - especially these days.

Thanks again - Chris.