Monitor Vert Deflection Problem

[Tony Benham]

I have a mitsubishi 17" monitor, TFS6705SETKL which has an irritating
problem.
The size of the vertical raster keeps wandering up and down randomly,
sometimes it's stays fine for a period, but mostly keeps moving about.

I looked at the vertical deflection schematic, and the vertical deflection
circuit is pretty much an "IC", with some external caps/r's, and the size,
position is controlled by processor controlled dacs on a separate card (for
which I don't have the schematic). The size is controlled by a current into
the IC so I can't look at this with a scope to see if this is at fault.

The ramp generator looks fine, but the ramp on the output from the vertical
deflection IC that drives the coil is wandering up and down, and running
into the supply rail where it limits. The centre voltage is stable on the
other side of the coil.

It does look like the vertical delfection IC could be at fault, but it could
be the DAC ?Does anyone have any ideas what type of faults might be causing
this problem in either scenario ?

Regards
Tony

 

[u1061771156]

In article <3f3d0651$0$46009$65c69314@mercury.nildram.net>, Tony Benham wrote:

I have a mitsubishi 17" monitor, TFS6705SETKL which has an irritating
problem.
The size of the vertical raster keeps wandering up and down randomly,
sometimes it's stays fine for a period, but mostly keeps moving about.

I looked at the vertical deflection schematic, and the vertical deflection
circuit is pretty much an "IC", with some external caps/r's, and the size,
position is controlled by processor controlled dacs on a separate card (for
which I don't have the schematic). The size is controlled by a current into
the IC so I can't look at this with a scope to see if this is at fault.

The ramp generator looks fine, but the ramp on the output from the vertical
deflection IC that drives the coil is wandering up and down, and running
into the supply rail where it limits. The centre voltage is stable on the
other side of the coil.

It does look like the vertical delfection IC could be at fault, but it could
be the DAC ?Does anyone have any ideas what type of faults might be causing
this problem in either scenario ?

Regards
Tony

You could check the current drive by putting a resistor in series and
measuring across it, really needs a proper differential probe to get an
accurate signal though.

I fixed a TV with identical symptoms last year. One of those annoying
cases where I spent some time convincing myself it must be the IC, and
replaced it with no change. :-( Turned out to be a small (100pF?) SMT
capacitor connected between input and output of the amplifier IC, which
was intermittently leaky. Taking it off fixed the problem, I couldn't find
anything wrong with the cap afterwards, either the heat fixed it or the
cause was something like conductive flux trapped under it.

Mike.

 

[Asimov]

"Tony Benham" wrote to "All" (15 Aug 03 17:11:26)
--- on the topic of "Monitor Vert Deflection Problem"

I've seen a somewhat similar vertical problem lately in a monitor. I
thought it was something with the supply but the B+ never wandered.
Turned out to be an intermittant 3.6 volt zener diode in the vertical
yoke return. You might try sniffing around the yoke socket for a little
glass zener if your monitor uses a similar design.

On the other hand I had Toshiba tv which would sometimes shrink
momentarily about 2 or 3 lines at the top after it warmed up. Replacing
most of the vertical related electros seems to have fixed it but no cap
tested bad. Maybe it was a solder joint, I'll never know for sure, but I
could have stared at the scope waveforms a long time instead of getting
it done by shot gun. A scope isn't always the best diagnostic tool, the
best tool is the human brain.


TB> From: "Tony Benham" <tonyb@removekerrisway.freeserve.co.uk>

TB> I have a mitsubishi 17" monitor, TFS6705SETKL which has an irritating
TB> problem.
TB> The size of the vertical raster keeps wandering up and down randomly,
TB> sometimes it's stays fine for a period, but mostly keeps moving about.

TB> I looked at the vertical deflection schematic, and the vertical
TB> deflection circuit is pretty much an "IC", with some external caps/r's,
TB> and the size, position is controlled by processor controlled dacs on a
TB> separate card (for which I don't have the schematic). The size is
TB> controlled by a current into the IC so I can't look at this with a
TB> scope to see if this is at fault.
TB> The ramp generator looks fine, but the ramp on the output from the
TB> vertical deflection IC that drives the coil is wandering up and down,
TB> and running into the supply rail where it limits. The centre voltage is
TB> stable on the other side of the coil.

TB> It does look like the vertical delfection IC could be at fault, but it
TB> could be the DAC ?Does anyone have any ideas what type of faults might
TB> be causing this problem in either scenario ?

TB> Regards
TB> Tony

.... A couple of volts below threshold.