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Old 17th Jul 2019, 2:48 pm   #1
dom10161
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Default Commodore 1084-D2 Monitor Repair

Hi

First repair post, Hope it's in the right section.

I have on the bench a Commodore 1084 Monitor which I am in the process of repairing for a friend. He said it was fine when put into storage a few years ago, But now dead. He replaced the main smoothing cap himself.

Fortunately the service manual is available online and working my way through the schematic, I have managed to bring the monitor back to life.
The 12v power rail was dead due to a short circuit Cap C605 220uf and a faulty 7812 Voltage Regulator I402.
However another fault has presented itself.

To test the monitor I am feeding it with a composite signal from a DVD player. The picture is OK for about 30 secs then black horizontal lines start appearing about every inch with odd bursts of colour.

I have attached an image showing the fault

Its been about 15 years since I worked on anything CRT related, any assistance would be much appreciated.

Thanks in advance
Dom
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Old 17th Jul 2019, 3:40 pm   #2
Argus25
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Default Re: Commodore 1084-D2 Monitor Repair

It appears as though a fast rise high amplitude pulse has found its way into the video circuits/signal. It is driving the CRT beam current to zero followed by an unlocking of the color decoder and some color phase disturbance that then recovers.

Also, the disturbance is about 5 times the vertical scan frequency, an unusual rate as such a signal at those intervals is not normally found in monitor circuitry. While it is possible this fault is in the monitor, most likely the video source (your DVD player)you are feeding the monitor from is defective.

First thing to do is to try a second video source, or second monitor.

If this is a monitor fault, it would be easy to find the cause of this with a schematic for the monitor and following the video signal from the video input with the scope to find where the disturbance is introduced.
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Old 17th Jul 2019, 8:41 pm   #3
JohnBHanson
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Default Re: Commodore 1084-D2 Monitor Repair

Just worth mentioning - is this monitor not responding well to macrovision?
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Old 17th Jul 2019, 11:41 pm   #4
SiriusHardware
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Default Re: Commodore 1084-D2 Monitor Repair

A good thought, but I would have thought that if that were the case the problem would be apparent as soon as playback started.

That monitor almost certainly has RGB-in capability, if dom is already using a fully wired SCART lead between the DVD player and monitor then it may be useful to try RGB-in instead of composite-in.

On my similar Philips monitor there is a physical switch on the front panel to select between RGB / CVBS input mode. You would need to enable RGB-out in the DVD player's menus as well (the default will be for composite out).
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Old 18th Jul 2019, 5:14 am   #5
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Default Re: Commodore 1084-D2 Monitor Repair

Do the lines stay fixed in position or slowly move up or down?

Looks like hum bars to me, you probably have more faulty caps in there.

Check the supplies coming off the EHT/Flyback transformer.
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Old 18th Jul 2019, 9:32 am   #6
cmjones01
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Default Re: Commodore 1084-D2 Monitor Repair

Without seeing the fault close-up, that looks to me like there's something amiss in the black level clamping/colour burst department, so I'd start looking for oddities around the line timebase.

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Old 19th Jul 2019, 7:33 am   #7
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Default Re: Commodore 1084-D2 Monitor Repair

Quote:
Originally Posted by cmjones01 View Post
Without seeing the fault close-up, that looks to me like there's something amiss in the black level clamping/colour burst department, so I'd start looking for oddities around the line timebase.

Chris
I don't think so.

Black level clamping and anything to do with the color Burst gating act at horizontal or line rate and you would see vertical disturbances/lines, not horizontal ones.

This fault acts at a frequency not too far above the vertical rate, much slower, and it's about 250Hz, nowhere near the line rate, which is why the lines are horizontal. As I mentioned, this rate would be an unusual frequency to find as a signal inside a monitor, but could easily appear out the output of a DVD or VCR etc, like the signals you can see when you fast forward or shuttle a DVDs or vcr tape.
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Old 19th Jul 2019, 11:11 am   #8
dom10161
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Default Re: Commodore 1084-D2 Monitor Repair

Hi

Thanks everyone for your input. Not made much progress. I am now feeding the monitor from the original Commodore Amiga via RGB. The colours have now stabilized, But the black lines are still on the screen, they seem to vary in width and frequency

I have checked the voltages around the fly back transformer, all seem to match with the values in the service manual.

I have also replaced the electrolytic caps around the horizontal and vertical circuits.

I have attached a screenshot of the schematic showing the caps I have replaced circled in red.

It will be Monday before I get another chance to work on the monitor.

Thanks again

Dom
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Old 19th Jul 2019, 11:13 am   #9
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Default Re: Commodore 1084-D2 Monitor Repair

Have you changed the cap on the power rail to the video output stages?
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Old 19th Jul 2019, 10:07 pm   #10
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Default Re: Commodore 1084-D2 Monitor Repair

What is happening here is that a narrow pulse is cutting off the CRT beam current of all the three guns at the same time.

So I will make some suggestions how this could happen.

Wherever this pulse is coming from you also know know it was affecting the pal decoder circuit too(because it caused a color phase error when you were feeding the monitor with composite video, of course that is now gone on an RGB signal).

Since that circuit is in a completely different section of the monitor, it now appears likely this pulse is on one or more of the the power supply rails.

To get the three guns of the CRT cut off together requires the pulse at the three cathodes all going positive at the same time. So that means this disturbance is affecting the RG& B channels together, so again likely the power supply to the video output stages, so as suggested by Refugee check the filter capacitor there.

Or the could be an abnormality in the V blanking signals that also are normally common to the three stages.

The other way the three beams could cut off together is if there was a negative pulse on the CRT grids (which are normally common as one control grid in the CRT's gun) . Sometimes, depending on the design, blanking pulses can be introduced there rather than into the video amplifiers.

It would be easy to find where this problem was, with the scope, working backwards from the CRT's cathodes (once the grid circuit was excluded) and finding where it was coming from, either injected into them as a faulty V blanking Signal, or a a large glitch in the power supply rail. To check for noises/pulses on the power supply rail with the scope simply set it on AC coupling so it ignores the large DC offset.
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Old 20th Jul 2019, 9:19 am   #11
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Default Re: Commodore 1084-D2 Monitor Repair

Another left field suggestion - whatever it is is:-

-Periodic, but not synced to any other frequency in the monitor
-Of very short duration
-Powerful enough to disrupt the operation of indirectly related signal paths

Could this be something arcing - fault within the LOPT perhaps, or insulation breaking down across some other component or PCB path? Try getting the unit into a place of complete darkness, let your eyes adjust and see if you can actually see anything flashing over.
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Old 20th Jul 2019, 1:07 pm   #12
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Default Re: Commodore 1084-D2 Monitor Repair

The assumption here is that this is a problem with the repaired monitor, but have you tried another source? Or alternatively feeding the DVD composite output into another display? In other words, are you sure its the monitor and not the composite output on the DVD player that is acting up?
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Old 20th Jul 2019, 2:29 pm   #13
SiriusHardware
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Default Re: Commodore 1084-D2 Monitor Repair

Post #8, OP states he has tried it with RGB output from a computer as an alternative source.
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Old 20th Jul 2019, 5:31 pm   #14
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Default Re: Commodore 1084-D2 Monitor Repair

Quote:
Originally Posted by SiriusHardware View Post
Another left field suggestion - whatever it is is:-

-Periodic, but not synced to any other frequency in the monitor
-Of very short duration
-Powerful enough to disrupt the operation of indirectly related signal paths

Could this be something arcing - fault within the LOPT perhaps, or insulation breaking down across some other component or PCB path? Try getting the unit into a place of complete darkness, let your eyes adjust and see if you can actually see anything flashing over.
As noted in post 7 though, this disturbance it happening at about 5 times the vertical rate. Any sort of LOPT arcing/defect etc would happen at line rate or higher, meaning the disturbances would occur at least once on a scanned line and possibly multiple times along a scanned line, not across a field or frame. So it cannot be the LOPT or any signal the LOPT could produce being coupled into the signal circuits.

It is interesting how sharply and well all CRT beams turn off and on again for the short period of this defect. That makes me wonder if the aberrant signal is in the CRT's blanking circuits. But that theory would not easily explain why on a composite signal it loses color lock immediately afterwards, which suggests the power supply has a large glitch in its output. It would be very easy to solve with a scope.

The other interesting thing is how the time of the disturbance is only about one scanning line or two when it occurs across a frame and the amplitude of it is not disminished over time. For example if there was a coil somewhere ringing at 5 times the vertical rate after the vertical blanking interval, one might expect the disturbance to get less as the vertical scan progressed. So the defect almost looks like something you would expect if you injected a regular narrow rectangular pulse into the signal circuits. It will most likely be on the power supply rail, but there is always an off beat chance a sub-circuit has failed and is oscillating locked to the vertical rate, assuming those black lines are not rolling.

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Old 20th Jul 2019, 10:14 pm   #15
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Default Re: Commodore 1084-D2 Monitor Repair

.....so a good question for the OP would be are those black lines stable in position and size or are they appearing randomly in different places and rolling around vertically ? These things would be obvious from a video of the fault, but a still photo gives less information.
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