I've started looking into a problem with a Viewsonic PF790 monitor
that I've had sitting in the closet for the last year or so. The
problem is that about 1 second after turning the switch on, it stops.
The green led on the front panel remains on, but the degauss relay
abruptly turns off, as does the high voltage. Every once in a while
it will turn on without a problem, and work perfectly for hours at a
time. There was a time when it would reliably turn on if there was no
video signal connected, or if a low resolution signal was connected,
but that is no longer the case.

I have read the group's FAQ, and I have access to schematics. My
first thought after reading the faq was to check the x-ray protection
circuit. According the service manual, it is activated when the
voltage at the relevant pin on the controller IC exceeds 8V. With a
DMM it was difficult to get an accurate measurement within the second
of activity, but it was almost certainly below 8V.
I tried halving the voltage divider ratio to see if that would make a
difference, but it didn't. I presume that something else is shutting
things down.

If anyone has any suggestions as to where I might look next, I'd very
much appreciate it.

Dave

David Dawes wrote:
I've started looking into a problem with a Viewsonic PF790 monitor
that I've had sitting in the closet for the last year or so. The
problem is that about 1 second after turning the switch on, it stops.
The green led on the front panel remains on, but the degauss relay
abruptly turns off, as does the high voltage. Every once in a while
it will turn on without a problem, and work perfectly for hours at a
time. There was a time when it would reliably turn on if there was no
video signal connected, or if a low resolution signal was connected,
but that is no longer the case.

I have read the group's FAQ, and I have access to schematics. My
first thought after reading the faq was to check the x-ray protection
circuit. According the service manual, it is activated when the
voltage at the relevant pin on the controller IC exceeds 8V. With a
DMM it was difficult to get an accurate measurement within the second
of activity, but it was almost certainly below 8V.
I tried halving the voltage divider ratio to see if that would make a
difference, but it didn't. I presume that something else is shutting
things down.

If anyone has any suggestions as to where I might look next, I'd very
much appreciate it.

Dave

PS caps....

jak

"jakdedert" <jakdedert at (no spam) bellsouth.net> wrote in message
news:JSH1k.10945$vP5.137 at (no spam) bignews6.bellsouth.net...
David Dawes wrote:
I've started looking into a problem with a Viewsonic PF790 monitor
that I've had sitting in the closet for the last year or so. The
problem is that about 1 second after turning the switch on, it stops.
The green led on the front panel remains on, but the degauss relay
abruptly turns off, as does the high voltage. Every once in a while
it will turn on without a problem, and work perfectly for hours at a
time. There was a time when it would reliably turn on if there was no
video signal connected, or if a low resolution signal was connected,
but that is no longer the case.

I have read the group's FAQ, and I have access to schematics. My
first thought after reading the faq was to check the x-ray protection
circuit. According the service manual, it is activated when the
voltage at the relevant pin on the controller IC exceeds 8V. With a
DMM it was difficult to get an accurate measurement within the second
of activity, but it was almost certainly below 8V.
I tried halving the voltage divider ratio to see if that would make a
difference, but it didn't. I presume that something else is shutting
things down.

If anyone has any suggestions as to where I might look next, I'd very
much appreciate it.

Dave
PS caps....

jak

I agree. Mine needs some new PS caps. It has been sitting waiting to be
repaired when I have time.

Mike

On Jun 4, 10:54 pm, James Sweet <jamesswe... at (no spam) trashmail.net> wrote:



Michael Kennedy wrote:
"jakdedert" <jakded... at (no spam) bellsouth.net> wrote in message
news:JSH1k.10945$vP5.137 at (no spam) bignews6.bellsouth.net...
David Dawes wrote:
I've started looking into a problem with a Viewsonic PF790 monitor
that I've had sitting in the closet for the last year or so. The
problem is that about 1 second after turning the switch on, it stops.
The green led on the front panel remains on, but the degauss relay
abruptly turns off, as does the high voltage. Every once in a while
it will turn on without a problem, and work perfectly for hours at a
time. There was a time when it would reliably turn on if there was no
video signal connected, or if a low resolution signal was connected,
but that is no longer the case.

I have read the group's FAQ, and I have access to schematics. My
first thought after reading the faq was to check the x-ray protection
circuit. According the service manual, it is activated when the
voltage at the relevant pin on the controller IC exceeds 8V. With a
DMM it was difficult to get an accurate measurement within the second
of activity, but it was almost certainly below 8V.
I tried halving the voltage divider ratio to see if that would make a
difference, but it didn't. I presume that something else is shutting
things down.

If anyone has any suggestions as to where I might look next, I'd very
much appreciate it.

Dave
PS caps....

jak

I agree. Mine needs some new PS caps. It has been sitting waiting to be
repaired when I have time.

Mike

The 4N35 optoisolator in the power supply fails on these, causes the
voltage to slowly drift up, takes out capacitors and eventually pops the
vertical output IC. I've fixed probably a dozen of these and the similar
17" models with the same problem.

Thanks for the pointers. I took a look at the feedback circuit, which
senses off the 180V line. I adjusted the trimmer to lower the voltage
a
little, and was able to get it to start. The voltage at that point
was 175V.
After adjusting it back up, it will now start with no or a low freq
hsync,
but not with a high hsync. Once started, it will remain on when a
high
hsync is connected. Judging from the trimmer position, the voltage
was probably a little high originally, but not more than 185V. Is
this the opto-isolator drift you've seen?

Following a hint from the diagnostics in the service manual, I
temporarily disconnected the over-voltage circuit (an SCR fed
from a zener, from a primary coil on the transformer). The monitor
would power up correctly in all situations. The 180V line appears
stable. Next chance I get, I'm planning to remove and test these
parts. In-circuit testing didn't show an obvious problem.

I don't have a lot of experience with switching supplies, so in the
meantime I'll do some more reading on the subject...

Dave