Just repaired a CRT. Now have some odd image issues.

[fastline]

Repaired an industrial color CRT. Monitor would not show any image. Replaced a pile of capacitors and resoldered the big transformer (flyback?), and the incoming 120VAC power supply terminals.

CRT powers up normally, sounds good, image quality otherwise is much better than it was. Brighter anyway. However there are some shadows and vertical lines that are more visible on the left side of the screen and fade towards the right. Also, any vertical lines on the outsides seem to oscillate from wavy to straight sort of at random.

I tend to think this is interference induced from having certain wiring touching each other but figured it was worth asking first. That high voltage wire is what I am looking mostly at.

 

[cowboybob]

It does look like some cross talk is occurring.

It also looks very dusty (dirty) inside. Try cleaning up all the static dust. Then try to arrange the various (especially the high voltage) wires and ribbon cables such that they are as far as possible from each other and running as close to perpendicular to each other as is practical.

 

[dr pepper]

Nige is the man to tell you whats up with that.
I have seen similar when its been a power supply fault.
If its worse on one side maybe its a linearity issue, some monitors have an adjustment for it, see if you can find out if it does.

Last time I saw a display like that I think it was a mazak, what machines it off?

 

[fastline]

Well, to update, The monitor seemed to work so I reinstalled it only to have it not power up yesterday so I pulled to covers off again and could not hear the system whining like it should (obviously not powered up) so I unplugged and replugged and it turned on! Previously I did find a solder joint for the main power plug cracked so I reflowed them all. If the PSU is indeed at fault here, that might make some sense.

The vertical lines seem to be present most of the time I think but the wavy vertical edges seem to be wavy for a while, then straight for a while, at random. The ghosting to the right of characters seems to be consistent.

This device is going to be converted to LCD soon as it plugs right into any modern monitor and works but I think I am more or less trying to learn from the experience. I don't know much about CRTs so most of this is just combing for obvious issues found visually or with meters.

 

[dr pepper]

I'm not really a crt man, however I'm kinda thinking its a horizontal o/p transistor or lopty thing, if the latter put up with it till you get the new monitor, sounds like you've sussed it anyway, however there are firms that specialize in converting vdu's for old kit like this.

 

[Nigel Goodwin]

I would suggest scoping the supply lines, particularly those fed off the LOPTX - there's probably a high voltage electrolytic on the CRT base which often gives problems.

 

[dr pepper]

Excuse me for being nosey Nige, but would that be to do with the screen voltage, or one of the other grids?

 

[Nigel Goodwin]

Excuse me for being nosey Nige, but would that be to do with the screen voltage, or one of the other grids?

The HT rail for the video output transistors (around 200V or so), the reservoir capacitor for it is often on the CRT base and thus easily overlooked - so it's actually for the cathodes.

 

[ronsimpson]

Nigel's idea of looking noise on the power supplies on the video board is good.

When I first looked at this I thought the grounds were not connected on the video cable. Look at "X Y and Z". See the ringing. It looks like the video is repeated 5 times. Left side third line. "T0H0D0" Seems to have a ring or oscillation. On the right side the "7" repeats and repeats. Could be the video board is not grounded to the main board well. This long a ring and delay looks like a long ungrounded video cable.

I think this monitor also has yoke ring or linearity coil ring or maybe damper diode ringing. It probably has all ways had that.
There is text on the bottom right buttons. Must a burned CRT.
------------------
I noticed "System Status" has no ring but "T 0 H 0 D 0" has great ring. That said; the problem, as seen on a scope, will have noise at 20 to 50 times the horizontal rate. It should be easy to find. The noise is related to white text on black and not white on brown. The black/white is where all three video amplifier are working hard. So this points to noise on the video amplifier's power supply. (Nigel)
----------------
So video coax grounding AND bad capacitors.

 

[fastline]

To add to this. I have connected 2 different LCDs, both have a clean image without distortion. As mentioned in the other post, I replaced all electrolytic caps on the main board. I did not, however, even think to look at the base board on the end of the CRT tube.

One thing I think is probably related is the CRT failed to power up the other day, after the repairs! As well, before the CRT initially failed with no image, the image produced was relatively clean and no ringing was present. This brings me to wonder if I did something wrong in my repairs or selected inappropriate caps? I compared ripple ratings on all caps and all were general use, not low ESR. There is only one low ESR cap on the board and I did not replace it as it was testing ok, we in circuit anyway.

Could any of this be caused by the way the wiring is laying in the box? Or did I somehow mess something up moving things around? This thing is built tight and a real pain to work on. There is only 1 screw attaching the board to the frame and I know it is present. There is also screws on the VGA and AC input ports that go to ground.

Yes, the CRT is old and has burn in. Also, you cannot really see it in the pic but the edges with the little red pin stripe on both outsides are wavy. They will go straight for a bit, then wavy again.

 

[dr pepper]

Thanks Nige,
Yes I get that, if you recall a while back I built the xy monitor from a 14" telly, originally it had a driver ic for each cathode, and I seem to remember that each of the 3 ic's had its own decoupling electrolytic, didnt later telly's have one ic with 3 amps inside?, they look like a wideband high voltage op amp with low z o/p stages to drive the capacitance of the cathode.

 

[Nigel Goodwin]

To add to this. I have connected 2 different LCDs, both have a clean image without distortion. As mentioned in the other post, I replaced all electrolytic caps on the main board. I did not, however, even think to look at the base board on the end of the CRT tube.

Most people don't think to look there

One thing I think is probably related is the CRT failed to power up the other day, after the repairs! As well, before the CRT initially failed with no image, the image produced was relatively clean and no ringing was present. This brings me to wonder if I did something wrong in my repairs or selected inappropriate caps? I compared ripple ratings on all caps and all were general use, not low ESR. There is only one low ESR cap on the board and I did not replace it as it was testing ok, we in circuit anyway.

You don't need low ESR capacitors, pretty well nothing requires them, and certainly nothing domestic

Could any of this be caused by the way the wiring is laying in the box? Or did I somehow mess something up moving things around?

It could be, it's hard to know.

 

[cowboybob]

Could any of this be caused by the way the wiring is laying in the box? Or did I somehow mess something up moving things around?

It also looks very dusty (dirty) inside. Try cleaning up all the static dust. Then try to arrange the various (especially the high voltage) wires and ribbon cables such that they are as far as possible from each other and running as close to perpendicular to each other as is practical.

 

[dr pepper]

The only domestic circuits I've seen low esr caps is in switching power supplies.

 

[Nigel Goodwin]

The only domestic circuits I've seen low esr caps is in switching power supplies.

They would be EXTREMELY rare in those as well, the vast majority even use the cheapest crappiest caps they can buy, hence their huge failure rates.

There's no need for low ESR, a decent quality cap is very low ESR anyway, and will last for many, many years (I've never seen one fail again, as long as you use good quality ones).

I've changed absolutely 1000's of duff caps in switch-mode PSU's

 

[dr pepper]

I'm not really a domestic appliance man so i'm not an expert on domestic stuff.
I had made an assumption that the tall narrow caps found in domestic stuff psu's were in fact low esr, as in servers, motor drives and industrial controls low esr caps are usually tall & thin, 'cept for some really odd ones I cant remember the name of that are resistant to drying out if left discharged.