Display issue

[srowe]

I have a spare motherboard which I use for general testing that has a fault with the picture it displays. On power up it shows the correct blue text on a white background but the border is a mixture of pink and cyan stripes, this isn't stable and a few of the stripes flicker between the two colours.

I'm guessing the VIC chip is failing, other functions (sound, ADC) seem to work fine.

[norm8332]

Of course I suggest swapping chips to confirm. You could also adjust the clock to see if that makes a difference. It's weird that it's just the borders.

[eslapion]

I have a spare motherboard which I use for general testing that has a fault with the picture it displays. On power up it shows the correct blue text on a white background but the border is a mixture of pink and cyan stripes, this isn't stable and a few of the stripes flicker between the two colours.

I'm guessing the VIC chip is failing, other functions (sound, ADC) seem to work fine.

I don't think the VIC-I can do that. I suspect external components involved in video signal amplification is much more likely the culprit.

Perhaps an electrolytic capacitor or the setting of variable resistors.

[srowe]

Of course I suggest swapping chips to confirm. You could also adjust the clock to see if that makes a difference. It's weird that it's just the borders.

I'm reluctant to pull the VIC from my functioning systems, I've some more on order so I'll try that when they arrive. It has other intermittent faults, sometimes bits are dropped from the characters displayed too.

[srowe]

I don't think the VIC-I can do that. I suspect external components involved in video signal amplification is much more likely the culprit.

Perhaps an electrolytic capacitor or the setting of variable resistors.

That was my first thought but I've adjusted the variable resistor at the top right but that made no significant difference.

There's no visible leaks to any of the electrolyics, I've probably got some with the correct values so I try replacing them.

[eslapion]

There's no visible leaks to any of the electrolyics, I've probably got some with the correct values so I try replacing them.

I suspect C17 and C15 are the most likely culprit. No leaks are necessary for electrolytic capacitors to have problems.

In fact, leaks are usually problems associated with the capacitor plague affecting computers made mostly between 2002 and 2005. That's not something I have ever seen in Commodore 8 bit or Amiga computers.

[Radical Brad]

Are you running this on a glass tube CRT or LCD monitor?

The reason I ask is because some LCD systems will put a very noticeable banding across your video, depending on how the auto-adjust first calculates the pixel clock and phase for the source.

If it is a modern LCD, try tweaking the pixel clock and phase to see what happens.

On the DELL monitor I use for my VIC, which has a composite input, I have to carefully adjust phase, or my screen has banding that is so bad that it could be mistaken for a failing video chip. A few nudges of the phase, and the video is perfect again.

Brad

[srowe]

It's an LCD TV, connected to the composite input. The picture from my old two pin machines has a faithful colour representation.

[srowe]

I suspect C17 and C15 are the most likely culprit. No leaks are necessary for electrolytic capacitors to have problems.

I've a spare 47uF cap so I'll start with that.

Well, swapping that didn't improve the picture, I don't have a spare 10uF.

[Radical Brad]

Can you post a photo of the video anomaly?
Just curious.

I have 3 restored VICs (all new caps), and on one of them, I have to tweak the phase on my monitor or there are serious vertical lines on certain colors due to the banding effect of the monitor. The effect of banding on a color like light blue creates lines that are actually as dark as if they were really there (drawn by the VIC). Add that NTSC noise, and it does look like a bad chip at times. A quick tweak of the phase instantly clears this up perfectly every time.

Seems a nanosecond or two on the clock can put the frame out of sync enough to confuse the phase if the monitor does an auto adjust.
I have seen this as well on many of my other video projects. LCD having an exact number of horizontal pixels has to do some sneaky magic in order to show something that does not match. My current project shoves an 800x600 screen onto a 1200x1024 monitor, so I always have to manually tweak the phase as well as the pixel clock.

This is probably not the problem you are having, but I am curious to see what you are describing.

Brad

[srowe]

This first picture is my old two pin system

This is the CR system

They are both PAL.

[Radical Brad]

Ah, horizontal lines!
For some reason I was thinking vertical lines.

I wonder if your 4.436MHz color carrier is off a bit?
In PAL, every second scan line has the color phase inverted as an attempt to reduce errors.
Seems you are getting one good scanline, and one bad one.

I only have NTSC VICs, but I know you can slightly "tweak" the input clock.
On NTSC, the base clock is 14.318, and then divided internally by the 6560.
The 6561 uses the PAL color carrier frequency directly as the input clock.
So your clock is 4.436MHz, or at least it should be.

On an NTSC system, this puts the color out pf phase, working like a "hue" adjustment.
On a PAL system, it may account for what you are seeing there.

Have you probed pins 38, and 39 on the VIC chip to see what the input clock frequency is?

Brad

[srowe]

I wonder if your 4.436MHz color carrier is off a bit?
In PAL, every second scan line has the color phase inverted as an attempt to reduce errors.
Seems you are getting one good scanline, and one bad one.

Is this adjusted by the white plastic component in the middle? I've tried changing that, at one extreme I get various artefacts on screen but no significant change in colour.

Have you probed pins 38, and 39 on the VIC chip to see what the input clock frequency is?

I don't have a scope.

[eslapion]

I don't have a scope.

Many good multimeters have good frequency counters. My old Meterman 37XR can take up to 10MHz.

[Radical Brad]

The clock fine tune is marked C35 in the PAL schematic I looked at...



This is a variable capacitor, not a variable resistor like the other ones you adjusted.
Most likely, it also has some form of loctite on the top to avoid being turned easily.

Before you try tweaking C35, note that you will need a scope or good frequency counter to set this correctly, or could make things worse in a hurry.

This may not actually be the issue, but it does seem like some "slight" issue with every second scanline, which leads me to believe that it could be a slight maladjustment of the color burst frequency.

If you can round up a nerd buddy, just point him to the VIC schematic on Zimmers and tell him to adjust the crystal controlled clock oscillator to the proper 8.867 MHz frequency.

With a scope and a plastic screwdriver, it is an easy thing to try before purchasing any new components at least.

Good luck!
Brad

I wonder if your 4.436MHz color carrier is off a bit?
In PAL, every second scan line has the color phase inverted as an attempt to reduce errors.
Seems you are getting one good scanline, and one bad one.

Is this adjusted by the white plastic component in the middle? I've tried changing that, at one extreme I get various artefacts on screen but no significant change in colour.

Have you probed pins 38, and 39 on the VIC chip to see what the input clock frequency is?

I don't have a scope.