need help with odd CRT monitor image

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ive got a tovis mtg-1901cn cga color monitor. in use, or with a test patter
n, the image starts out perfectly at the top. as you move down toward the m
iddle , it gradually starts to compress the image, with more compression th
e closed you get to center.  and when it gets to mid screen, its a full ver
tical collapse. bright while line across the center,  with no image whatsoe
ver 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 fl
yback.  
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 t
he image is exactly the same. now im stuck.  

thanks

Re: need help with odd CRT monitor image
On Tuesday, July 4, 2017 at 8:44:59 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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ern, 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 v
ertical collapse. bright while line across the center,  with no image whats
oever in the lower 50% of the screen. also sold as a vision pro .  
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flyback.  
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n 2      ( output) is a nice 24 v square wave.  i recapped the monitor, and
 the image is exactly the same. now im stuck.  
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Sounds like a shorted turn in the deflection yoke, but not sure what you me
an by a square wave on the output.



Re: need help with odd CRT monitor image
On 2017/07/04 5:44 PM, snipped-for-privacy@gmail.com wrote:
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Your vertical circuit is missing the bottom deflection half of the  
vertical circuit. If the vertical driver is an IC, replace it, if it is  
a pair of transistors then I would suspect the transistor pulling to  
ground (typically the lower of the two on schematics) is the one at fault.

The vertical drive is just a very slow audio amp, apply audio amp  
service methods.

John :-#)#

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Re: need help with odd CRT monitor image
And if it has a large value (1000-4700uF) cap inside the feedback loop that
 is failing, that can cause severe linearity issues.



Re: need help with odd CRT monitor image
On 2017/07/05 12:35 AM, snipped-for-privacy@yahoo.com wrote:
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that is failing, that can cause severe linearity issues.
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Granted, however the OP did say he recapped the monitor and I didn't  
want to doubt his statement.

John ;-#)#

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Re: need help with odd CRT monitor image
On Wednesday, July 5, 2017 at 3:39:44 AM UTC-4, John Robertson wrote:
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that is failing, that can cause severe linearity issues.
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its an IC, an LA7833( nte1773) . i'll change it out and see what happens. i
 did change out every cap, and it was exactly the same. no better, no worse
.

thanks much

Re: need help with odd CRT monitor image
On Wednesday, July 5, 2017 at 9:34:41 AM UTC-4, snipped-for-privacy@gmail.com wrote:
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p that is failing, that can cause severe linearity issues.
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 i did change out every cap, and it was exactly the same. no better, no wor
se.
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just got done replacing the vertical IC, la7833. no change in the image. no
 better, no worse.

Re: need help with odd CRT monitor image
On 2017/07/05 10:00 PM, snipped-for-privacy@gmail.com wrote:
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:
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op that is failing, that can cause severe linearity issues.
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ns. i did change out every cap, and it was exactly the same. no better, n
o worse.
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. no better, no worse.
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Any schematic you can provide a link to? Someone suggested a bad yoke,  
which is possible if the bottom half of the vertical deflection shorted  
out. Maybe. However a schematic would help.

John :-#)#

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Re: need help with odd CRT monitor image
snipped-for-privacy@gmail.com wrote on 7/6/2017 1:00 AM:
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Looks like the 24 volt supply is generated by doubling the 12 volt supply.  
Measure the 24 volt supply at pin 6.  I bet something is wrong that this is  
only 12 volts, maybe the diode?

--  

Rick C

Re: need help with odd CRT monitor image
On Thursday, July 6, 2017 at 7:17:12 AM UTC-4, rickman wrote:
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:
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op that is failing, that can cause severe linearity issues.
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ns. i did change out every cap, and it was exactly the same. no better, no  
worse.
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. no better, no worse.
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.  
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is  
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https://na.suzohapp.com/pdf/service_pdfs/support/visionpro/49-1329-vp2_manu
al.pdf

https://na.suzohapp.com/pdf/service_pdfs/support/visionpro/49-1329-vp2_sche
matic.pdf

B+ is 12v3, 12V and 24V good. i was thinking yoke, but they ohm out good, a
nd look like brand new. no physical damage.  looks like a few transistors a
re after the vertical ic, i'm looking at them now.  not real good at follow
ing this schematic, so any help is appreciated.  

thanks

Re: need help with odd CRT monitor image
snipped-for-privacy@gmail.com wrote on 7/6/2017 11:18 AM:
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I'm not so good at following this schematic myself.  It looks like W402 is  
the connector to the yoke.  If so, I would say Q404 sets the vertical  
position.  Q360 and D307 link the vertical drive signal to I201, the color  
gun controller.  Perhaps this is creating too large a load from a bad part,  
so the drive is clipped?

Did you say I302 pin 2 has a square wave on it?  Shouldn't that be a sawtooth?

--  

Rick C

Re: need help with odd CRT monitor image
snipped-for-privacy@gmail.com wrote:
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nual.pdf
hematic.pdf
 and look like brand new. no physical damage.  looks like a few transistors
 are after the vertical ic, i'm looking at them now.  not real good at foll
owing this schematic, so any help is appreciated.  
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In the yoke there are two coils for vertical and two for horizontal connect
ed in series or parallel depending on the design, if in parallel one coil c
onnection could be broken. The coils are connected together in the solder p
osts on the yoke, you may check there if some wire looks loose.

Re: need help with odd CRT monitor image
On Thursday, July 6, 2017 at 2:47:59 PM UTC-4, Jeroni Paul wrote:
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manual.pdf
schematic.pdf
d, and look like brand new. no physical damage.  looks like a few transisto
rs are after the vertical ic, i'm looking at them now.  not real good at fo
llowing this schematic, so any help is appreciated.  
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cted in series or parallel depending on the design, if in parallel one coil
 connection could be broken. The coils are connected together in the solder
 posts on the yoke, you may check there if some wire looks loose.

thats a great idea, i didnt know the coils were doubled up. i will check th
at for sure tonite. i was thinking of inverting the image, there are solder
 pads for an inverted header on the chassis board, and see what the image d
oes.  that would tell me if its the yoke or a chassis issue. does that make
 sense?

thanks much

Re: need help with odd CRT monitor image
snipped-for-privacy@gmail.com wrote on 7/6/2017 3:32 PM:
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I don't get why your idea is not good.  If you can flip the drive signal  
polarity the image will invert.  If the problem is in the drive circuitry  
the half that is working will swap with the half that loses drive.  If the  
problem is in the yoke the image will invert, but the problem won't.  No  
need for fancy testers or widgets.  Or maybe there is something I don't  
understand about this?

--  

Rick C

Re: need help with odd CRT monitor image
On 2017/07/06 8:20 PM, rickman wrote:
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Great point Rick, that would indeed be a good test to see if the yoke is  
linear or not.

John :-#)#

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Re: need help with odd CRT monitor image
On Friday, July 7, 2017 at 12:48:26 AM UTC-4, John Robertson wrote:
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p2_manual.pdf  
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p2_schematic.pdf  
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l  
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s  
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i soldered a header into the reverse position on the chassis, and gave it a
 try. it did the same thing as before, just inverted. now the bottom half o
f the screen has an image that compresses toward the center, a total collap
se on the center, and no image in the upper half of the screen. does that r
ule out a yoke problem?  

thanks

Re: need help with odd CRT monitor image
On 2017/07/06 10:04 PM, snipped-for-privacy@gmail.com wrote:
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Based on Rick's points above I think we can rule out the yoke. Nice work  
Rick and yourself for that test!

If you have a 'scope then now is a good time to check the outputs of the  
LA7851's sawtooth wave generator. Also 'scope the output of the vertical  
drive IC to is if it too is still a saw tooth or not. Should swing  
pretty much the entire source voltage (24VDC?) if I am not mistaken...

John :-#)#
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Re: need help with odd CRT monitor image
On Thursday, July 6, 2017 at 11:18:38 AM UTC-4, snipped-for-privacy@gmail.com wrote:
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te:
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:
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loop that is failing, that can cause severe linearity issues.
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t
pens. i did change out every cap, and it was exactly the same. no better, n
o worse.
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ge. no better, no worse.
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ly.  
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s is  
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nual.pdf
hematic.pdf
 and look like brand new. no physical damage.  

Not saying the yoke is the problem, but a single turn shorted to an adjacen
t turn will not change the resistance reading on your dmm, but a single sho
rted turn will totally collapse the magnetic field of the coil.

A ringer is the best instrument to check for shorted turns.

Re: need help with odd CRT monitor image
On Thursday, July 6, 2017 at 3:58:22 PM UTC-4, John-Del wrote:
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rote:
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te:
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k loop that is failing, that can cause severe linearity issues.
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n't
appens. i did change out every cap, and it was exactly the same. no better,
 no worse.
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mage. no better, no worse.
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pply.  
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his is  
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manual.pdf
schematic.pdf
d, and look like brand new. no physical damage.  
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ent turn will not change the resistance reading on your dmm, but a single s
horted turn will totally collapse the magnetic field of the coil.
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not sure what a ringer is, but im pretty sure i dont have one. you mean lik
e a tone generator, and you move it around the yoke ?

Re: need help with odd CRT monitor image
On Thursday, July 6, 2017 at 5:18:47 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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:
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 wrote:
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rote:
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ack loop that is failing, that can cause severe linearity issues.
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idn't
p)
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 happens. i did change out every cap, and it was exactly the same. no bette
r, no worse.
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 image. no better, no worse.
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supply.  
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 this is  
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2_manual.pdf
2_schematic.pdf
ood, and look like brand new. no physical damage.  
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acent turn will not change the resistance reading on your dmm, but a single
 shorted turn will totally collapse the magnetic field of the coil.
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ike a tone generator, and you move it around the yoke ?

A dedicated instrument that checks inductors for shorted turns by injecting
 a waveform and counting the "rings" (like echos).  A shorted coil won't ri
ng or will ring very low.  In the old days, several companies made "flyback
" testers that would check flybacks and yokes for shorted turns.  I have a  
Sencore LC75 that has a ringing feature and it works well.

Another way of checking the yoke out of the circuit can be done with a scop
e. If you can identify and electrically separate the two halves of the vert
ical yoke, you can connect each section to the calibration output jack of y
our scope and monitor the resulting waveform of each.  Unless the top secti
on shorted to the bottom, you can be pretty confident you have one good sec
tion and one bad. If the waveform on one of the sections is severely distor
ted compared to the other, it's a yoke issue.  If both waveforms look the s
ame, the yoke is most likely good.

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