Re: gEDA-user: Any TV repair gurus lurking?

[Stuart Brorson <sdb@xxxxxxxxxx>]

Back when dinosaurs roamed the earth, I used to fix TVs in a TV repair
shop.  Maybe I remember some of what I knew then, but the jury is out
on that one.

I'd take a look at the driver transistor for the vertical oscillator,
or any of it supporting components.  It sounds like the vertical sweep
osc is having a hard time producing a nice, solid saw-tooth wave.
Putting a scope probe on it would be a good idea.

FWIW, the horizontal oscillator is often the culpret in these bad
sweep problems, so take a look at it too, using the scope.

HTH,

Stuart


On Fri, 21 Dec 2007, Peter Clifton wrote:

> Hi,
>
> I'm fixing (or trying to) a TV for a friend, and wondered if anyone had
> any wisdom relating to the following symptom.
>
> This is a Sony BE-3D chassis, and exhibits an intermittent fault. When
> its doing it, the picture is still visible, but collapsed (jitteringly)
> upwards towards the top of the screen. It doesn't stay collapsed for
> particularly long. The sections of image which collapsed upwards also
> narrowed horizontally, leading to a tapered looking screen
> ---------------
> |\           /|
> | \         / |
> |  \_______/  |
> |             |
> ---------------
>
> (Although this diagram probably exaggerates it somewhat).
>
> None of the obvious supply caps tested for bad ESR on my meter (main PSU
> cap, the various secondary side PSU caps including B+, the flyback
> derived 200V supply etc.) Even when raster / scan has been lost
> completely, sound works fine. At one extreme I saw in person today, the
> vertical collapse left three horizontal lines (R,G,B) separated by an
> inch or so on the screen towards the top. No picture was visible (and I
> turned it off).
>
> I know this set has a common fault with flyback transformers failing due
> to internal arcing. I almost convinced myself this was probably the
> case, but now I'm not so sure. Could it cause the symptom? (It usually
> supposed to trip the sets overcurrent protect on the B+ line and shut
> down H drive, throwing the set into standby though - which it didn't
> seem to do for me today).
>
> Unfortunately its been several years since I last fixed a TV, and I
> don't have my handy box of bits for insulated prodding / light-bulb
> inserting / freezing / HV testing. I don't even have an oscilloscope
> here (although I could borrow one from the lab). I'm working with
> multimeter and ESR meter. I do have the schematics and service manual
> though, which is a big plus.
>
> Best wishes,
>
> -- 
> Peter Clifton
>
> Electrical Engineering Division,
> Engineering Department,
> University of Cambridge,
> 9, JJ Thomson Avenue,
> Cambridge
> CB3 0FA
>
> Tel: +44 (0)7729 980173 - (No signal in the lab!)
>
>
>
> _______________________________________________
> geda-user mailing list
> geda-user@xxxxxxxxxxxxxx
> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
>


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