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Commodore 1084 Phillips Screen faint lines running off buttons, title bars, etc

It still leaves me with the question as to why three (supposedly) identical circuits are yielding a different result for one channel.

Dave
 
Well not sure if it was the bottom plate shorting something out, but now that it is totally put back together that resistor tests just like the other 2 at 385Ohms while in circuit. Unfortunately it still has the lines on RGB or Composite sources. Additionally the ground strap wire that soldered from the power supply to the neck board also broke in a section that looked somewhat rusted when I took it back apart, but that didn't change anything either when I replaced it. Otherwise everything else looks good, hopefully will have the DIN jacks in the next 7 days, so I will update when that comes in to try another video cable.
 
Okay Din6 plugs arrived yesterday and were super hard to solder to, but finally managed to make a good connection. It looks identical to the original video cable. I had to follow the pinout for the original cable cause when I first started I was referencing this pin out .. https://inanis.net/amiga-500-to-amiga-1084-rgb-cable/ and in the end pin 2 of the DIN6 had to go to Pin 10 of the 23 pin not pin 11. Otherwise the system wouldn't post with a flashing white screen over and over.
The other sync on pin 6 of the DIN wasn't even hooked up on the original cable so I left it disconnected too.
 

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It is hard to know for sure where this subtle problem is occurring, whether at the input where the signal is terminated, in the video stage pre-amps or in the video output stage driving the CRT. The fact that the problem is not really there to any significance with an RBGI input suggests it is happening in the signal processing of the analog rgb prior to the video output stage.

The only way I can see to resolve it is to follow the signal through with the scope, but to resolve this, it would have to be an excellent wideband scope & probes such as the TEk 2465B with a perfect flat response and the P6136 probe with very low input capacitance. And then studying the square wave response via the signal circuits to be able to identify the problem's location.
 
There was a very interesting case of a near identical effect seen on a CRT image on the vintage radio forum. It appeared to not be greatly influenced by the frequency response in the video amplifiers and I'm not sure if it was 100% resolved. However it was not a color CRT and an early type of non-aluminized TV CRT. In these, the screen itself is an electrical insulator. When the phosphor is struck by electrons it charges up and this tends to cancel some of the applied +ve EHT potential, it was once called screen sticking. The electrons simply found their way back to the EHT connection which was connected only to the final anode structure in the gun and to the internal aquadag (graphite like conductive coating), which did not physically or electrically reach the screen phosphor area.

In a color CRT though, aluminium is evaporated over the internal aquadag, so there is a conductive pathway for the electrons and the screen itself doesn't charge up with high beam current, but stays at the final anode potential. The aluminium layer is very thin and the electron's energy is still enough to pass through it and excite the phosphor, that is if the EHT is over about 7kV (its normally about 20kV in a color set)

It raises an interesting quesion though about what happens in a color or aluminized CRT if the internal aquadag and aluminized coating developed a very high resistnce or poor connection to the anode button on the bulb where the EHT is applied. This could posiibly happen with a direct short applied to the CRT's anode button to discharge it (rather than a gentle discharge with an EHT probe, which represents at least a 100 Meg Ohm load). One worry with directly discharging CRT's is damage to the internal coating around the metal anode button, I have mentioned this on other threads. It is not something that I do to CRT's myself, so I have little experience of the possible results, but I know that the peak discharge current into a short and the amount stored energy could easily be enough to damage the thin internal coating coating around the anode button, especially in a color CRT. Then the screen could charge somewhat during peak white levels and when the video signal went to a low white level, the light output would drop a little, producing a dark streak for a while after that. One way to prove it, or exclude that, would be to try a substitute CRT.

Though in this case with the RGBI signal looking basically good, with perhaps only a trace of the effect, it might be an unlikely scenario, but, one thing that masks the effect is when the white or bright areas on the screen fall suddenly to black, if the background is blacked out at that point, you cannot see blacker than black as the beam is already cut off, so it is necessary to lift the background brightness up to see the overshoot effect. The ideal way to see the effect is to have a peak white level that transitions to a low grey level along a horizontal line or group of lines.
 
Thanks Hugo for the detailed info. I am going to try to connect the monitor back with the Digital CGA connector and see if I can find any other programs that would maybe show the video anomaly on a bigger scale. Yes, it seems like white bars against another lighter type color is what I need to find or turn the brightness up.

Unfortunately it sounds like the steps involved to track this down are going to require additional equipment I don't have access to.
 
I really wouldn't worry about damaging the CRT by discharging it too abruptly. Most service manuals even say to SHORT the anode to ground to discharge the CRT. This rumor about possibly damaging the CRT seems to be a somewhat recent Internet myth that has been passed around for the lat few years. I think people not used to working with CRTs see the big loud spark when discharing it and think there's no way that can be harmless. There's just not enough energy there to do any damage to the CRT. I've been working with CRTs for over 30 years, and I've never even heard of a CRT being damaged by discharging it.

If the anode connection was open, then it would be pretty obvious. I expect you'd see frequent and sudden changes in the screen size and brightness as the HV arcs inside the CRT. You could simulate that by holding the HV connector a mm or two from the CRT anode.

I have seen similar symptoms on TVs before. In fact, I think it's normal to some extent on lesser quality video displays. If you're sure it's abnormal for this monitor (have you seen the same model that doesn't do it), then I would be looking at power supply issues, particularly in the video amplifier, and output stages.
 
andy said:
I really wouldn't worry about damaging the CRT by discharging it too abruptly. Most service manuals even say to SHORT the anode to ground to discharge the CRT. This rumor about possibly damaging the CRT seems to be a somewhat recent Internet myth that has been passed around for the lat few years. I think people not used to working with CRTs see the big loud spark when discharing it and think there's no way that can be harmless. There's just not enough energy there to do any damage to the CRT. I've been working with CRTs for over 30 years, and I've never even heard of a CRT being damaged by discharging it.

If the anode connection was open, then it would be pretty obvious. I expect you'd see frequent and sudden changes in the screen size and brightness as the HV arcs inside the CRT. You could simulate that by holding the HV connector a mm or two from the CRT anode.

I have seen similar symptoms on TVs before. In fact, I think it's normal to some extent on lesser quality video displays. If you're sure it's abnormal for this monitor (have you seen the same model that doesn't do it), then I would be looking at power supply issues, particularly in the video amplifier, and output stages.
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It is not an internet myth.

I have seen a case of it with a large color CRT, the stored energy in those approached 1J and that can definitely cause damage. With a small CRT around 5 to 9" the energy is in the 100mJ or less vicinity and there is a lot less chance of damage, still I don't take the risk. To give you an idea about energy, it takes about 2 to 5mJ to punch a hole in a piece of Ceran wrap. Shorting CRT's out only became popular in the last 20 or 30 years as inexperienced repairers followed internet myths about the dangers of stored energy. Sony warned against it, with direct shorts, as early as 1963.

But its not just the CRT that is at risk of damage. When the CRT anode is shorted to earth (somewhere) the peak current is many thousands of amps (albeit extremely briefly). Even if the discharge pathway is 10 Ohms, the peak initial current is 2,000 amps for a CRT charged to 20kV The spark that you see before physical contact may as well be a direct short, because it drops a fixed voltage in Air of around 500v and behaves as a negative resistance and doesn't limit the current and the 500V drop is insignificant compared to the voltage on the bulb. This current spike can easily destroy semiconductors/IC's on the pcb, depending on where the earth of the discharge wire has been connected. Obviously its better on the extrnal dag but not everbody connects it there.

The sensible thing to discharge the CRT, if it is ever required (and seldom is) is to do it with an EHT probe, which limits the peak discharge current to 20,000volts / 100,000,000 Ohms or around 200uA, even the cheap EHT probes use 100MOhm resistors, better ones are 1G Ohm.

Also, it is a general principle never to directly short any charged capacitor regardless of its internal construction (unless it is specifically designed for it) a current limiting resistance should always be employed. A charged CRT is just another physical form of a capacitor and the same precautions to prevent damage from the connections to its "plates" (anode button to internal aquadag in this case) should be observed.
 
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You obviously have to be very careful about where you discharge a CRT to. The service manuals generally say to discharge to the CRT shield, or external coating. Even using a HV probe grounded to the wrong spot is likely to cause damage to the electronics. Some early color TVs even had an interlock that shorted out the HV supply when you removed the back cover. Show me even one confirmed case of a CRT that was damaged by discharging it.

I agree that it's a bad idea to short out other types of capacitors, particularly large electrolytics.
 
andy said:
You obviously have to be very careful about where you discharge a CRT to. The service manuals generally say to discharge to the CRT shield, or external coating. Even using a HV probe grounded to the wrong spot is likely to cause damage to the electronics. Some early color TVs even had an interlock that shorted out the HV supply when you removed the back cover. Show me even one confirmed case of a CRT that was damaged by discharging it.

I agree that it's a bad idea to short out other types of capacitors, particularly large electrolytics.
Click to expand...
With the EHT probe, it can be connected practically anywhere to ground because the peak current is limited to a few hundred uA.
As mentioned though the CRT is only one of the worries.

Tek performed an interesting series of tests on ESD resistance of families of devices. The model they used (I would have to check it) involved charging a 1000pF capacitor to only around 500 or 1000v. That is similar to the bulb capacitance on color CRT's but the stored energy was only a fraction of the CRT charged to 20kv, because of the V^2 stored energy relation.

Also, the method to use the EHT probe to discharge the CRT is better anyway, as there is less chance of coming into contact with the charge, because most EHT probes are designed with a long smooth tapered tip to slip under the rubber anode cap to get to the connection.

Really, all people who work on TV's & VDU's should have an EHT probe, they are not expensive, especially the early ones with the integral analog meter. The EHT can be checked and the CRT discharged safely with the same probe. However people often persist with repairs without the appropriate tools.

As you are aware, in color VDU's & TV's the focus and screen voltage bleeder chain is usually derived from the EHT, so the tube self discharges within a minute or two of being de-powered anyway. This is why after turning the set off and on after a minute or two you can hear the tube recharge with small surface electrostatic crackles.

The main VDU's that store CRT charge for a longer period are the monochrome type, where the only reverse leakage pathway is the silicon EHT rectifier, mostly with those after a day or two, they are self discharged too. The only time I deliberately discharge a CRT is before removing it from a set and I carry it across the room. And I always use the EHT probe to do it. It is generally not required for VDU servicing because the charge remains safely trapped in the bulb, so if you don't go under the anode cap, or remove the anode cap, its impossible to come into contact with the charge, if any is still there.

Back onto the topic of potential causes for the streaking in the video, if it turned out that it was not present in the actual video drive signal; could possibly be gas in the CRT.

In most cases though, this causes beam defocus, however, the overshoot effect in the video level after a peak white, represents a forms of charge storage, and this could possibly happen with enough gas affecting the grid-cathode in the CRT. When gas is there though, one often sees a violet glow goming from the gun area near the base. It could be worth looking for a violet glow from the gun in a dark room, just in case, or at least to help rule that out.
 
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