Oscillating Sony Focus

[Me]

Hello,

I picked up a Sony KV36FS100 for cheap... it had been dropped and the
boards had been fractured.

I was able to fix all the fractures and glue the plastic parts back
together, etc. After fixing everything, it powered up the first time I
tried it, so I was happy.

After watching it for about an hour or so, I wondered about the focus. I
had drilled a hole through the back cover before I put it on so I could
adjust the focus with the back on. I attempted to adjust the focus and
figured it was ok.

I switched to the satellite menu (high contrasting white and blue colors)
and while I was reading the menu I noticed the TV was going out of focus.
It got progressively worse for about 10 seconds and then snapped back into
focus. It does this over and over and over. It doesn't appear that the HV
is changing, since the picture size doesn't change, it doesn't bloom and the
brightness doesn't seem to change either. The focus just goes from crystal
clear to slightly fuzzy and then snaps back to crystal clear... all in 10
seconds or so.

It only appears to do this when I'm viewing the satellite menu, but it's
hard to tell. It could be doing it constantly.

Unfortunately, I gave the TV to my parents (100 miles away) before I noticed
the problem, so I can't look at it right now, nor do I watch it often. It
also explains why I haven't torn it down to make any measurements yet.

I was just curious if this sounds odd to anyone? I always thought the focus
and HV were tied together. What could be making the focus oscillate like
this?

Thanks for you input.

Barry

 

[David]

Seen this before, on a dropped Sony tube. There is internal damage in the
tube on the focus control grids inside the tube. To fix it you have to
replace the tube.

This is of course assuming there is no damage to the focus controls ground
return line or the dynamic focus circuits (if this model has it) for the
corner focus fine tuning of the tube.

David

 

[Me]

Ewwwww... a bad tube would be a bummer. I'll have to check on the
presence of the dynamic focusing circuit. I'll also check the focus ground
return line.

On a good note, my parents don't seem to be too concerned about the problem,
it's almost unnoticeable.

Are manuals hard to come by for the Sony TVs?

BTW, thanks for the info!

Barry

 

[Sofie]

Me / Barry::
Check the simple and cheap stuff like the screen filtering ...... look for
low uf/high voltage electrolylics near and around the flyback ...... i.e.
10uf @ 250V or similar values...... or for additional cracked circuit board
traces or connections near and around that area.

 

[Andy Cuffe]

I've seen several sony computer monitors with bad CRTs causing this
problem. The white ceramic resistor on the side of the electron gun
sometimes gets cracked. Try looking at the neck of the tube in a dark
room for any signs of internal arcing.