OT: Nokia 21-inch 445 Series CRT / Monitor Schematics

[bob p]

I know this is a little OT for the site, but I thought I'd ask for help anyway.

I'm looking for schematics and related repair info for the following Nokia 21" CRT computer monitors:

445Xav, 445Xi, 445Xpro

These are all 21-inch CRT displays that use the same chassis and differ primarily in their support circuitry.

I've already contacted Nokia, and I got the reply that the monitor business was sold to ViewSonic, go ask them.

I've already contacted ViewSonic, and they won't help. They bought Nokia's monitor business to shut down a competitor. Instead of providing help, they recommend buying a new ViewSonic monitor.

I've got 4 of these monitors, 2 that are dead and 2 that are nearly dead. I'd like to do my part to keep them out of a landfill.

When working, these monitors are wonderful devices. They're worth the effort to repair if I can only find the repair documents.

Please let me know if you've got a lead on any tech info. I've been passively searching for years without any luck.

Thanks.

PS - Seems that my inbox here is always full. I've subscribed to the thread, so if you post a reply I'll know to come back. Thanks again.

[Enzo]

See if Sams Photofact covers them, I don;t know if they got into computer monitors, but they got TVs covered wall to wall.

And instead of this guitar amp forum, try posting in a computer hardware geek forum.

And having spent many years in the service industry supporting the arcade business repairing video monitors for arcade games, I'd say the majority of monitor problems are cured by replacing dried out e-caps. If these are more modern, and have switching powr supplies, then try to determine if the SMPS is faulty or just shutting down, assuming they won;t power up.

[bob p]

Thanks for the help, Enzo. You're someone I can always count on.

I've been through Sams Publishing -- nothing Nokia in their database.

I don't doubt that I've got some dried out or drying out e-caps. But I'm trying to avoid taking the shotgun approach and just doing a blind recap of the working monitor and hoping for the best. Although that method might help me to get one problem fixed, I'm going to need technical manuals for the other monitor, whose problem is more complex:

One monitor got killed by being driven with an overscanning HF input. It'd dead -- no picture at all. A little tick when I try to turn it on. I've never repaired something like that, and I'm not sure how the multisync-type input circuit works -- I imagine that the problem should be a blown semi of some kind. Docs would sure be helpful. Without docs, my other recourse is just testing semis for shorts.

One monitor that is working is showing some interesting display artifacts. My hypothesis is that they are cap related.

If you look at the image below, there is white to gray streaking moving across the screen.

<insert pic here>

The horizontal dark bands happen after dark text. I think what I'm seeing is ultra white followed by the dark banding. Digital data loss shows terrible random hash. I don't have that. So it's an analog problem in the video path.

The RC time constant of the bands is several inches of screen.
1600 @75Hz has a 202.5MHz pixel clock.
Horizontal frequency is 93.75KHz.

Video timing calculator
I entered VESA 1600x1200 75Hz

The period of 93.75KHz is 10.66uS, of which 0.95uS is the sync pulse.
So the screen width is about 9uS.

The DC coupling of the video signal should have a time constant much larger
than 9uS to avoid sagging and banding.

The video problem looks like one of DC restoration.

Since this is RGB, there are 3 video signals, and we don't see colored bands,
so the problem is common to all of them.

I thought it might be a grounding problem in the video connection. Perhaps ground is capacitively coupled somewhere and the cap has lost value.

Perhaps I have no ground, and residual capacitance is returning the signal path.

Checking the cable connections, swapping a video card didn't help. Seems to be in the display. Now I get to hunt down dry caps and bad solder connections.


FWIW I have tried posting on the computer geek sites. Everyone with these monitors is having the exact same support problem that I'm having -- no docs. I have encountered a couple of people who have described the same streaking artifact, though none of them have solved the problem. Interestingly, one guy was an EE but had no clue about the cause. I'm hoping that I'm on track with the DC restoration idea.

[Steve Conner]

To me, it looks like either bad capacitors in the video amp, or a bad screen supply or worn-out cathodes in the CRT. However, the red cathode tends to go first, but your problem affects all three colours, so it's quite unlikely.

The video amp is usually a board mounted directly on the neck of the tube, so you can limit your recapping efforts to that area.

How many hours of runtime do the monitors have on them? CRT cathodes are rated for about 10,000, ironically about the same as the backlight tubes in the new TFT monitors.

The one that just ticks when you turn it on: Blown horizontal output transistor, shorting the power supply, which goes into hiccup mode. Hopefully just replacing the HOT will fix it. If not, maybe the flyback is arcing internally. (That's what happened to the last CRT monitor I messed with, when I tried to push it to a scan rate it wasn't designed for.)

[Steve L]

Schematics for Nokia Monitor

I have the service manuals for the 445P and 445 Pro versions.

They might not be exactly what you want, but they are likely very close.

They are too big to upload here, however if you want them you can send me a PM with an email address and I will be happy to send them to you.

Steve

[bob p]

I just realized that the scan rate on the monitor in the photos is 70 Hz, not the 75 Hz that I referenced in my calculations. I doubt that anyone else is playing the numbers game and holding a ruler up to the screen like I was, but I just thought I'd correct the mistake.

[bob p]

To me, it looks like either bad capacitors in the video amp, or a bad screen supply or worn-out cathodes in the CRT. However, the red cathode tends to go first, but your problem affects all three colours, so it's quite unlikely.

The video amp is usually a board mounted directly on the neck of the tube, so you can limit your recapping efforts to that area.

Good idea.

How many hours of runtime do the monitors have on them? CRT cathodes are rated for about 10,000, ironically about the same as the backlight tubes in the new TFT monitors.

I don't know how many hours are on the monitor in the photo. I acquired it used at the end of it's lease. I imagine that it got a fair amount of use in a work environment, probably 40 hours per week. If it came off of a military lease it could have been lots more. I've had it for maybe 5+ years and I leave it on all day (with power saving set to blank the display after 10 minutes and turn it off after 20. The blank works but the power down doesn't). The monitor doesn't have an hour counting circuit, so the best that I can do is guess at a figure. Realistically speaking, it's possible that I've reached the service life of the capacitors and I maybe the cathodes too.

The dead one that ticks has far fewer hours. I bought that one at retail, and killed it with a faulty linux driver. It's probably got 1/3 to 1/2 as many hours on it.

The one that just ticks when you turn it on: Blown horizontal output transistor, shorting the power supply, which goes into hiccup mode. Hopefully just replacing the HOT will fix it. If not, maybe the flyback is arcing internally. (That's what happened to the last CRT monitor I messed with, when I tried to push it to a scan rate it wasn't designed for.)

Regarding the flyback transformer -- were you able to find a replacement for yours and get the monitor repaired? Or are those the kind of things that render the monitor unrepairable? Just wondering about the suitability and availability of replacement parts. With this monitor, OEM parts aren't available.

Thanks for your help!

[bob p]

Steve L: ygpm.

Thanks.