High pitched sound from monitor

I am looking for any idea regarding a strange problem with my monitor. After
about 10 minutes of operation it starts to produce high pitched sound. This
sound is generated only in certain scan rates - within the range of 66 to 70
KHz of horizontal scan. It covers, however the most usable modes for this
box. More peculiar to this is that the frequency of the pitch is fixed and
independent of the scan rate, i.e. using either 67 KHz scan or 69 KHz the
sound pitch is exactly 11.7 KHz. I recorded it and analyzed - its FFT looks
like f.ex. stereo broadcast pilot tone, a spike. The construction of the
monitor doesn't allow me to isolate the problem - without the cover it can't
warm up as much and its weight is balanced in a bit different way thus
preventing from generating any accoustic effects. With the cover the sound
is practically ominidirectional. The deflection yoke is fixed firmly so I
suppose that's not it. I also fixed the screen width coil (within flyback
shielding). So I can't find any mechanical solution, perhaps there is a
control on the pcb to set to get rid of the sound. It's also remarkable that
the HOT transistors are getting really hot. The sound wasn't present from
the begining, nor it does have any impact on the picture. Any idea?

cheap design, hmm.




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"Jonie" <jonie@capella.ae.poznan.pl> wrote in message
news:bd2i33$one$1@atlantis.news.tpi.pl...

I am looking for any idea regarding a strange problem with my monitor.
After
about 10 minutes of operation it starts to produce high pitched sound.
This
sound is generated only in certain scan rates - within the range of 66 to
70
KHz of horizontal scan. It covers, however the most usable modes for this
box. More peculiar to this is that the frequency of the pitch is fixed and
independent of the scan rate, i.e. using either 67 KHz scan or 69 KHz
the
sound pitch is exactly 11.7 KHz. I recorded it and analyzed - its FFT
looks
like f.ex. stereo broadcast pilot tone, a spike. The construction of the
monitor doesn't allow me to isolate the problem - without the cover it
can't
warm up as much and its weight is balanced in a bit different way thus
preventing from generating any accoustic effects. With the cover the sound
is practically ominidirectional. The deflection yoke is fixed firmly so I
suppose that's not it. I also fixed the screen width coil (within flyback
shielding). So I can't find any mechanical solution, perhaps there is a
control on the pcb to set to get rid of the sound. It's also remarkable
that
the HOT transistors are getting really hot. The sound wasn't present from
the begining, nor it does have any impact on the picture. Any idea?

Almost any coil or ferrite material in the horiz deflection may be causing
the resonating noise. this is inclusive of transformer cores, ferrite beads,
or coil assemblies. You may try reseating the coils and transformers by
reflowing the solder to the pcb. Or you may try flowing a bit of shellac
into the core pieces of the offending coils and transformers. A bit of
material applied to the ferrite beads also may stop them from vibrating and
radiating the noise. May take a bit of time to actually determine what
device actually is causing the offence and to eliminate the noise.
"Jonie" <jonie@capella.ae.poznan.pl> wrote in message
news:bd2i33$one$1@atlantis.news.tpi.pl...

I am looking for any idea regarding a strange problem with my monitor.
After
about 10 minutes of operation it starts to produce high pitched sound.
This
sound is generated only in certain scan rates - within the range of 66 to
70
KHz of horizontal scan. It covers, however the most usable modes for this
box. More peculiar to this is that the frequency of the pitch is fixed and
independent of the scan rate, i.e. using either 67 KHz scan or 69 KHz
the
sound pitch is exactly 11.7 KHz. I recorded it and analyzed - its FFT
looks
like f.ex. stereo broadcast pilot tone, a spike. The construction of the
monitor doesn't allow me to isolate the problem - without the cover it
can't
warm up as much and its weight is balanced in a bit different way thus
preventing from generating any accoustic effects. With the cover the sound
is practically ominidirectional. The deflection yoke is fixed firmly so I
suppose that's not it. I also fixed the screen width coil (within flyback
shielding). So I can't find any mechanical solution, perhaps there is a
control on the pcb to set to get rid of the sound. It's also remarkable
that
the HOT transistors are getting really hot. The sound wasn't present from
the begining, nor it does have any impact on the picture. Any idea?

"Jonie" <jonie@capella.ae.poznan.pl> wrote in message
news:bd2i33$one$1@atlantis.news.tpi.pl...

I am looking for any idea regarding a strange problem with my monitor.
After
about 10 minutes of operation it starts to produce high pitched sound.
This
sound is generated only in certain scan rates - within the range of 66 to
70
KHz of horizontal scan.

<snip>

Classic flyback transformer squeal; a ubiquitous and, as you well know, a
highly irritating problem.
You can try reseating the FBT -- desolder all the pins then push it down
squarely against the PCB, making certain all the plastic legs are resting on
the PCB, then solder in place. This "readjustment" sometimes helps. There
are also times when the interior FBT construction (insulation, epoxy dip)
develops flaws which can lead to squeal.

So I can't find any mechanical solution, perhaps there is a
control on the pcb to set to get rid of the sound.

Fiddling with the technician adjustments w/o having the appropriate test
gear and consulting the service literature is more likely to create new
problems than to solve your existing one.

It's also remarkable that the HOT transistors are getting really hot.

That's why they're mounted on large heatsinks. If the only prob with the
unit is the squeal, the HOT(s) is/are functioning normally.