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I'm getting an odd CRT problem. When cold, the tube has an area about 1/3 way down that isn't scanned. The area above is compressed and the area below a little stretched.

I've attached 2 pictures - one of a screen that should be completely white, and the other of a CP/M screen:

White screen

CP/M screen

Once the system has warmed up (usually 1-2mins at most), if I power cycle, the screen is completely normal.

I've attached the schematics of the drive board.

I've reflowed the neck board connector, flyback and yoke wires.

Can anyone point me in the direction of the problem?

Schematic

UPDATE:

Thank you all for the suggestions.

I've checked the test points on the schematic that looked relevant - 9, 12 and 13 and they are as follows: Test point 9 Test point 12 Test point 13

Here is the an image of the screen once it has warmed up running CP/M Working screen

I reflowed the ICs joints just in case they were to blame (the joints looked ok).

I don't have any proper cold spray, but, once the screen was running, I tried spraying a can of compressed air at the inductors, the two ICs and then just at random, but it didnt do anything to the picture.

Here is a picture of the area around the ICs Photo of board

(Ignore the red dots on the tops of the capacitors - that was me marking ones I'd replaced)

Archive.org have a nice high resolution schematic - link here as file size is too big to append directly: https://archive.org/details/service-manual-pcw-9256-engacme/page/8/mode/2up.

Thanks again

UPDATE 2: Finally had some time to work on it again. I tried probing the pins of IC 401 and seem to have now lost nearly all vertical deflection - not aware of having shorted anything.

I now get a single line with a bit of fluctation - I obviously dont want to damage to CRT so am not leaving it to see if it suddenly works.

I checked some of the other test points in it's current state: Test points 3 and 9 with a 1x probe setting, the test point 12 with a 10x setting Test Point3 Test Point 9 Test Point 12 with 10x probe

Pin 8 of IC401 is just held at 5v accoring to my scope, though my multimeter reads 12v on this pin (I'd be inclined to believe the multimeter) Pin8-401-1x

I was thinking IC401 may be the problem, but then as test point 9 seems upstream of this, am I wrong?

UPDATE 3:

Output at test point 13. R421 measures in spec at 31.6k (measured in circuit)

Now looks totally different from when I originally posted and way out of spec. All on a 1x probe - note the voltage! WELL below what it should be! It flickers between this: TP13-1x-very-bad and TP13-1x-verybadtoo which corresponds to the white line on the screen breifly deflecting down. This is it with longer timebase TP13-1x-longer and shorter TP13-1x-Shorter

Probing at IC401 pins, pin 1 shows this: pin1-1x pin2 is just held high at 6.7v (scoped, but voltage according to multimeter set to dc) Pin 3 is just high at 3.8v (again scoped but voltage according to multimeter set to dc)

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  • \$\begingroup\$ Have you checked electrolytic capacitors yet? \$\endgroup\$ Commented May 24 at 20:45
  • \$\begingroup\$ IC401 contains the vertical output driver. Make sure there are no cracked solder joints associated with that chip. \$\endgroup\$
    – Dave Tweed
    Commented May 24 at 21:08
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    \$\begingroup\$ Your schematic shows some helpful test point waveforms (I really wish they still did that...); can you probe those test points and check that the waveforms look as expected? \$\endgroup\$
    – Hearth
    Commented May 24 at 21:10
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    \$\begingroup\$ @squelch41 Since you mention about a difference after warming up and restarting, I'm curious about what you might observe from using directed "cold spray." Do the warm up and restart, make sure things look good, then apply cold air or spray to various areas and see if you can make it fail back. \$\endgroup\$ Commented May 25 at 4:39
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    \$\begingroup\$ You have vertical deflection over the entire screen. It looks to me like you have a problem with the vertical synchronization. \$\endgroup\$
    – JRE
    Commented May 25 at 7:05

3 Answers 3

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If this problem has remained after doing all the usual checks (for dried out electro caps, aged variable resistors, bad solder joints) and after the power supply rails have all been confirmed as being stable and in spec, etc), then I suggest taking a closer look at all the components associated with the vertical deflection sub-system, of which the key component is IC401.

Suggest starting with the adjustable devices, eg: VR485 (might be marked "VR405", it's a little hard to read your schematic). Then look at the capacitor on pin 2. (Maybe post a clearer image of the vertical deflection section of the schematic?)

enter image description here

If that fails, then the vertical deflection system associated with IC401 (UPC1379C) may need more investigation. The figure below is the application circuit from the datasheet. I'd start with the components connected to pin 8 (the output to vertical deflection coils) and the associated feedback signals on pin 6 & 3, followed by those components on pin 11 (vertical blanking adjust).

Good luck!

enter image description here


Comment after Update by OP
The scope image you posted for TP13 looks very different to the expected image as shown on the schematic: the edges of the scope image are far more rounded than expected. The problem is I cannot make out from the schematic exactly where TP13 connects to, even after looking at the link you provided.

What is the vertical scale on those scope images? According to the schematic, TP12 should be 124Vp-p. TP13 should be 24Vp-p. The scope images seem to have much lower amplitude than these. Did you use x10 or x100 probes? Only the image for TP9 matches the amplitude as indicated on the schematic.

Question 1: Is TP13 connected to the join of R421, R431, C418, & VR405?

If "yes" then:
The schematic says R431 is 3.9Ω; if this is true then according to the schematic waveform this means the current in R431 is 6.1Ap-p (24V / 3.9Ω), which sounds a little high to me, but is in rough alignment with the application circuit diagram of the datasheet for the chip UPC1379C.

I suggest checking all components in the current path for the vertical deflection coil, marked as "DY" connected to pins 1 & 2 of connector CP401. The most likely culprits are the capacitors (C430, C418, C416, C417).

It would be worth probing pins 1, 2 & 3 of IC401, since these set the inputs to the vertical amplifier.

Take care as many of the nodes associated with this chip are high voltage.

Good luck!

Images below: relevant section of schematic as discussed above, taken from the link provided by OP.

Image 1:
enter image description here

Image 2:
enter image description here

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  • \$\begingroup\$ thanks, I'll give those a check \$\endgroup\$
    – squelch41
    Commented May 25 at 14:54
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    \$\begingroup\$ AFTER UPDATE: Thank you very much for that -- I should have been clearer about the probe type - was using x10 for the 12 and pretty sure for 13 as well as my scope can only take up to 30v I can't work on it for a few days but will follow your advice once I get back. Thanks again \$\endgroup\$
    – squelch41
    Commented May 26 at 10:00
  • \$\begingroup\$ Be interesting to see what the waveform on TP13 looks like after the unit has warmed up & the problem has gone away. Does it look different to what you've posted? \$\endgroup\$ Commented May 26 at 13:25
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The first thing I would check is the power supply to the monitor. I had issue like with an old CRT. Changing the bridge rectifier and a the filter capacitor corrected the issue. The logic being that PSU powers the video card and everything that follows. The circuit board itself is probably fine

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  • \$\begingroup\$ power seems ok - solid 12v and 5v \$\endgroup\$
    – squelch41
    Commented May 25 at 19:09
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You said:

"Once the system has warmed up (usually 1-2mins at most), if I power cycle, the screen is completely normal."

Get a can of freeze spray (here is an example of one) and spray small amount on each component of the vertical deflection circuit. If you're lucky, you'll hit the right component and the problem will show up again.

Good luck.

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