need help with odd CRT monitor image

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?

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Rick C

Great point Rick, that would indeed be a good test to see if the yoke is linear or not.

John :-#)#

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p2_manual.pdf

p2_schematic.pdf

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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

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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MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 
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        "Old pinballers never die, they just flip out."

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 .

flyback.

n 2 ( output) is a nice 24 v square wave. i recapped the monitor, and the image is exactly the same. now im stuck.

I didn't read back through all the posts, but if you haven't, check the pum p 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 havo c (don't check it with a dmm - they often sag under load). Also, I know yo u said you recapped the monitor, but make sure the yoke coupling capacitor was included. This is typically a high value capacitor (around 1000uf or mo re).

ttern, the image starts out perfectly at the top. as you move down toward t he middle , it gradually starts to compress the image, with more compressio n 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 wha tsoever in the lower 50% of the screen. also sold as a vision pro .

e flyback.

pin 2 ( output) is a nice 24 v square wave. i recapped the monitor, a nd the image is exactly the same. now im stuck.

ump up diode. The anode will be connected to the Vcc and the cathode to th e pump up pin of the vert IC. A weak diode here will cause all sorts of ha voc (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 capacito r 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 wav eform. pin 17 has nothing. on the la7833 pin 4, which i believe is the inp ut, 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 defi ned 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?

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Rick C

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 compress ion the closed you get to center. and when it gets to mid screen, its a fu ll vertical collapse. bright while line across the center, with no image w hatsoever in the lower 50% of the screen. also sold as a vision pro .

the flyback.

d pin 2 ( output) is a nice 24 v square wave. i recapped the monitor, and the image is exactly the same. now im stuck.

e 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 kn ow you said you recapped the monitor, but make sure the yoke coupling capac itor was included. This is typically a high value capacitor (around 1000uf or more).

th. 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 t hink 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 t hink the pump up is ok, as the output is 24v, but maybe im wrong. im think ing of just replacing the 7851 as a hail mary, and then bailing if that doe snt work.

.

s e p

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3

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pin 3 of 7833 is 25.65 v. pins 2 and 15 are opposite polarity. pump driv e 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.

Ramp generator, probably at whatever that unit's equivalent of a jungle IC is. An IC is only as good as what is fed into it. Yoke problems are rare an d thus unlikely.

You need a proper sawtooth wave driving that IC or it will not put out what you want. It is that simple. So go backwards and find that ramp generator. If you cannot find it, put up a link to the print and I will. If you do, p lease use something that an older PC can use.

If pin 7 pump drive signal is only half a volt, I would say your problem is there. Pin 3 should clearly be more than 25 volts and it is this signal that drives that voltage. Is pin 3 at all steady or is it also a waveform? You should measure the voltage with the scope, not the meter. They don't use a smoothing cap so I guess the voltage will pulse with the drive. During retrace you don't need a high voltage on pin 3. Retrace is when the voltage goes below 24 volts and the diode is forward biased to charge up the cap again.

I think you said you already replaced the LA7833, so there are only two other parts. When you say pin 7 is "upside down", do you mean it is normally high and drops low or it is going negative? I would expect it to be near 24 volts during the picture time and drops low during retrace as the cap charges. I guess the question is how does the waveform on pin 3 look? I would expect it mostly near 48 volts dropping slightly below 24 volts during retrace. Pin 7 should definitely have a significant pulse voltage on it.

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Rick C

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...

John :-#(#

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I tried the DSE ringer on pretty much anything laying around - it doesn't like very low inductances.

Scan yokes should be fine.

The one in the vertical output section can and does.

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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(Please post followups or tech inquiries to the USENET newsgroup) 
                      John's Jukes Ltd. 
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 
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                      www.flippers.com 
        "Old pinballers never die, they just flip out."

C is. An IC is only as good as what is fed into it. Yoke problems are rare and thus unlikely.

at you want. It is that simple. So go backwards and find that ramp generato r. If you cannot find it, put up a link to the print and I will. If you do, please use something that an older PC can use.

matic.pdf

Did you check D360 (the "bootstrap" diode from 24V in the pump circuit)?

It may be shorted and pulling the "pumped" supply "down" to 24V. The IC will most likely need the pumped supply for one half of the screen only.

Look up "impulse winding tester", that's the high voltage version. Ring testers work the same way, with V rather than kV scale drive.

t's

m a d e

i did pull up one leg of D360 and checked it, it checked good. pin 7 never goes anywhere near 24 volt. there is a .5 volts poorly defined waveform, a lmost looks like ripple. im thinking it should be a 24v sawtooth? i am thin king something on the board may be pulling down pin 7?

thanks