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

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Thursday 20 December 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,

Look for a common bypass capacitor between the vertical output stage and some 
point in the hscan stage that also has a resistor between it and the main 
supply, which if it opened would allow the vertical rise in current to 
modulate the hdrive amplitude.  Also for any medium power resistors that show 
signs of losing paint, 2 watt fireproof and up types.  The contact ferrel on 
the end connector might not be making good contact with the resistor film.

This is a case where the ESR meter like a Capacitor Wizard can be very handy, 
and so can an oscilloscope.  Borrow it.

Around the solder joints on the bottom of the scan/hv tranny is a good place 
to take a low power microscope, looking for solder cracks too.

I'd hesitate to condemn the scan tranny itself in an intermittent scenario, 
normally cuz any kind of a failure there is both permanent, and takes the 
scan transistor, a big switcher type, out with it in milliseconds.  Usually 
to protect the 50 cent fuse. :)

A couple of other things to ponder.  Hscan drive circuits are expected to 
fully saturate the hscan output transistor when its on.  If they do not, the 
heat generated by the transistor will rise very rapidly, often destroying the 
transistor is just a second or so.  Modulation of its drive level for the 
length of times you are describing would seem to indicate that the transistor 
may already be damaged.  Check the base-emitter forward breakdown with a 
digital meter, anything on the diode scale of over .8 volts would make me 
wary, I'd expect to see something below .7 volts but not as low as .6 volts.

From an old, almost retired C.E.T.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Where there's a will, there's a relative.


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