I have an NEC MS75 CRT monitor that's 3 or 4 years old. Lately there
have been times when the horizontal sweep shrinks slightly, for less
than a second. It can happen every couple of minutes or not happen for
several hours.

I seem to recall something similar with my last monitor before it
suddenly failed. There's a relay that clicks when I change resolutions
or put the computer to sleep. I suspect this relay because I do these
things much more than other computer users I know. I have found reports
online that with other monitors this relay has been associated with
narrowing of the display.

What is the function of this relay? NEC says it closes when you go to
800x600 or higher. That seems to be true; of my five available
resolutions, there are two transitions where the relay doesn't click.
However, it clicks twice when I go to or from 1024x768.

It clicks once when the monitor is put to sleep or waked. The screen
goes momentarily black any time I switch resolutions, whether or not the
relay clicks, so the relay doesn't cut power to the flyback.

Not understanding the function of the relay, I'm afraid it could cause a
sudden monitor failure.

This relay has an SA surrounded by a C.
It is labeled VBT12TBU-5.
1/6 HP 120 VAC
5A 24VDC.
0124L2.
How can I get a replacement?

Choreboy wrote:
I have an NEC MS75 CRT monitor that's 3 or 4 years old. Lately there
have been times when the horizontal sweep shrinks slightly, for less
than a second. It can happen every couple of minutes or not happen for
several hours.

I seem to recall something similar with my last monitor before it
suddenly failed. There's a relay that clicks when I change resolutions
or put the computer to sleep. I suspect this relay because I do these
things much more than other computer users I know. I have found reports
online that with other monitors this relay has been associated with
narrowing of the display.

What is the function of this relay? NEC says it closes when you go to
800x600 or higher. That seems to be true; of my five available
resolutions, there are two transitions where the relay doesn't click.
However, it clicks twice when I go to or from 1024x768.

It clicks once when the monitor is put to sleep or waked. The screen
goes momentarily black any time I switch resolutions, whether or not the
relay clicks, so the relay doesn't cut power to the flyback.

Not understanding the function of the relay, I'm afraid it could cause a
sudden monitor failure.

This relay has an SA surrounded by a C.
It is labeled VBT12TBU-5.
1/6 HP 120 VAC
5A 24VDC.
0124L2.
How can I get a replacement?

I expect different vendors use different circuits.
I remember an old monitor that used the relay to switch a capacitor
that changed the resonant frequency of the flyback circuit.
mike

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Hi!

What is the function of this relay? NEC says it closes when you go to
800x600 or higher. That seems to be true; of my five available
resolutions, there are two transitions where the relay doesn't click.
However, it clicks twice when I go to or from 1024x768.

This relay is most likely used to activate different sections of the
monitor's power supply as required by different resolutions and scan rates.

As you may have noticed, some monitors use different methods of adjusting
the voltages and outputs of their power supplies upon detecting a resolution
change. Some use totally electronic methods that are usually silent (unless
you can hear the deflection whine, which I can) and others use one or more
relays. Some use "combination" designs...for example, I have a Dell 15"
"UltraScan" (made by Sampo) monitor that will click in some resolutions or
just change the pitch of the slight deflection/flyback whine in others.

I doubt very much the problem is with the relay. If it were then the
monitor's picture should not ever come up or it will have great difficulty
in doing so. The monitor's electronics might even keep trying the relay to
make it work, resulting in a "chattering" noise.

If your monitor is anything like one of the NEC MultiSync 95 units I have,
then you have capacitors working on going bad in the monitor. This will
cause the display disturbance you are seeing. With my monitor it was fine at
times and bad at others. Sometimes it would be working fine and start
malfunctioning out of the blue, even after it had been on for a while. Some
resolutions would work and others were unusable or at least took several
minutes for the picture to stabilize. I replaced some of the capacitors in
my monitor that tested out of spec and this brought it back to perfect
working order. Ever since then it has been rock-solid and the application
that it is used with switches display resolutions quite frequently.

William

"William R. Walsh" wrote:

Hi!

What is the function of this relay? NEC says it closes when you go to
800x600 or higher. That seems to be true; of my five available
resolutions, there are two transitions where the relay doesn't click.
However, it clicks twice when I go to or from 1024x768.

This relay is most likely used to activate different sections of the
monitor's power supply as required by different resolutions and scan rates.

As you may have noticed, some monitors use different methods of adjusting
the voltages and outputs of their power supplies upon detecting a resolution
change. Some use totally electronic methods that are usually silent (unless
you can hear the deflection whine, which I can) and others use one or more
relays. Some use "combination" designs...for example, I have a Dell 15"
"UltraScan" (made by Sampo) monitor that will click in some resolutions or
just change the pitch of the slight deflection/flyback whine in others.

I doubt very much the problem is with the relay. If it were then the
monitor's picture should not ever come up or it will have great difficulty
in doing so. The monitor's electronics might even keep trying the relay to
make it work, resulting in a "chattering" noise.

If your monitor is anything like one of the NEC MultiSync 95 units I have,
then you have capacitors working on going bad in the monitor. This will
cause the display disturbance you are seeing. With my monitor it was fine at
times and bad at others. Sometimes it would be working fine and start
malfunctioning out of the blue, even after it had been on for a while. Some
resolutions would work and others were unusable or at least took several
minutes for the picture to stabilize. I replaced some of the capacitors in
my monitor that tested out of spec and this brought it back to perfect
working order. Ever since then it has been rock-solid and the application
that it is used with switches display resolutions quite frequently.

William

I used to hear flyback whine. When I was in sixth grade I was home from
school with a severe cold. When I turned the TV off I still heard the
whine. The infection must have given me tinnitis. I haven't heard a
flyback since.

Your suggestion sounds good. In the early 1950s there was a popular TV
that was notorious with repairmen for the failure of a capacitor. When
a repairman had lunch with the manufacturer's head engineer, he asked
why they didn't use a better capacitor. The engineer said a better one
would have cost eight cents more. From the manufacturer's perspective,
that was prohibitive.

How can I get a schematic? It would show me what capacitors are
involved. If I discover that the problem affects only some resolutions,
I might really narrow down the possibilities. It might also enable me
to test most of them without unsoldering.

As the problem happens only briefly, I wonder if a bad capacitor could
appear to be in specs.

Hi!

How can I get a schematic? It would show me what capacitors are
involved. If I discover that the problem affects only some resolutions,
I might really narrow down the possibilities. It might also enable me
to test most of them without unsoldering.

I was not able to get a schematic from NEC on this monitor. I had to make
some educated guesses about where the problem was.

As the problem happens only briefly, I wonder if a bad capacitor could
appear to be in specs.

For a while my monitor was able to "drift" into working order if I would
leave it alone for a few minutes after switching to a problematic
resolution. As the capacitors got worse though, the monitor became unusable.
It was still quite new at the time (and just out of warranty) so I didn't
want to throw it out. I decided to do the fix and the monitor has been in
excellent working order ever since. That's been about two years ago.

William

William R. Walsh wrote:
Hi!


How can I get a schematic? It would show me what capacitors are
involved. If I discover that the problem affects only some resolutions,
I might really narrow down the possibilities. It might also enable me
to test most of them without unsoldering.


I was not able to get a schematic from NEC on this monitor. I had to make
some educated guesses about where the problem was.


As the problem happens only briefly, I wonder if a bad capacitor could
appear to be in specs.


For a while my monitor was able to "drift" into working order if I would
leave it alone for a few minutes after switching to a problematic
resolution. As the capacitors got worse though, the monitor became unusable.
It was still quite new at the time (and just out of warranty) so I didn't
want to throw it out. I decided to do the fix and the monitor has been in
excellent working order ever since. That's been about two years ago.

William


ESR meters are nice for testing in circuit.