On 2017/07/07 1:52 PM, Ian Field wrote:


"John Robertson" wrote in message
...
On 2017/07/07 10:01 AM, wrote:
On Friday, July 7, 2017 at 12:09:33 PM UTC-4, rickman wrote:
wrote on 7/7/2017 10:55 AM:
On Friday, July 7, 2017 at 9:52:26 AM UTC-4, John-Del wrote:
On Tuesday, July 4, 2017 at 8:44:59 PM UTC-4,
wrote:
ive got a tovis mtg-1901cn cga color monitor. in use, or with a
test pattern, the image starts out perfectly at the top. as you
move down toward the middle , it gradually starts to compress the
image, with more compression the closed you get to center. and
when it gets to mid screen, its a full vertical collapse. bright
while line across the center, with no image whatsoever in the
lower 50% of the screen. also sold as a vision pro .

battery voltage is good at 123, 12v and 24 v are good. 123 volts
at the flyback.
the vertical output ic is a LA7833. pin 6 has good 24 volt
supply, and pin 2 ( output) is a nice 24 v square wave. i
recapped the monitor, and the image is exactly the same. now im
stuck.

thanks

I didn't read back through all the posts, but if you haven't,
check the pump up diode. The anode will be connected to the Vcc
and the cathode to the pump up pin of the vert IC. A weak diode
here will cause all sorts of havoc (don't check it with a dmm -
they often sag under load). Also, I know you said you recapped
the monitor, but make sure the yoke coupling capacitor was
included. This is typically a high value capacitor (around 1000uf
or more).

i may have hit the wall here. pin 14 of the LA7851 is a nice 5v
sawtooth. pin 15 is a 12v volts square wave. pin 16 is kind of an
ugly bowl shape waveform. pin 17 has nothing. on the la7833 pin 4,
which i believe is the input, has the same ugly bowl shape wave,
where i think it ought to have a saw tooth? pin two, the output
has a very nice 24V square, which i dont think it should have.
should be a 24v sawtooth? odd that it puts out a very defined
square, instead of just some ugly shape, if something is wrong. i
think the pump up is ok, as the output is 24v, but maybe im wrong.
im thinking of just replacing the 7851 as a hail mary, and then
bailing if that doesnt work.

This would be a lot easier if you had a working unit to compare
waveforms..
It doesn't make sense to me that pin 15 if the LA7851 would be a square
wave. I think I'd try to find something wrong with the parts
between pins
15 and 16. Or maybe the components from the LA7833 pin 2 to pin 17
on the
LA7851. I'm guessing this latter circuit is for linearization of
the ramp
and might cause the problem you see if a part is bad. It's a bit
hard to
figure out what all this does. Looks like the whole thing is pretty
highly
optimized.

Are the signals on pin 2 and pin 15 of the two parts the same
polarity or
opposite? I can't see how you would get a bowl shape signal on pin
16 if
they are the same. Have you played with the vertical size and
linearity
controls to see how they affect the problem?

The charge pump circuit was bothering me as the diode seemed to be
backwards. Then I realized it isn't making 24 volts from 12 volts,
it's
pumping up the 24 volts to something higher! What is the voltage on
pin 3
of the LA7833? The cap is only rated for 35 volts, so it isn't
making 48
volts. But clearly pin 3 should have something on it higher than 24
volts.
What does the pump drive signal on pin 7 look like?

--

Rick C

pin 3 of 7833 is 25.65 v. pins 2 and 15 are opposite polarity.
pump drive signal is a .5 volts upside down sawtooth.

i appreciate the time taken here. i could just send this out, but i
think i am close, just not there.


Afraid we don't have one on the service bench otherwise I would be
happy to send you waveform images. One of those monitors is in my
storage area I'm pretty sure, but...

Pretty sure you can wire up an adaptor to run VGA off a CGA output - for
some strange reason, many VGA monitors aren't backward compatible with EGA.

There are CGA - VGA adapters on eBay all the time. Cheap. Most work,
some don't so do some research first.

John :-#)#

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