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Hi
I'm new on the forum
I have a PAL dragon made in Swansea, I like it very much and everything works except that it has a display problem: as soon as there is color, characters or graphics oscillate quickly from bottom to top, vertically and are not sometimes filled, so they are difficult to understand and painful to look at
I made a good video cable
I cleaned the cards and all the elements
I carefully set the two 10K resistors relating to luminance and synchro
I tried on CRT and LCD screen, the same
I checked all the tensions according to the schemas, it's ok
I tested the capacitors on the power supply, it's ok
Is this a known problem? How to solve it? Quartz, capacitor, logic circuit, mod, other?

Hi,

Unfortunately this is a normal 'feature' of a Dragon. It's just dot crawl resulting from a lack of fixed relationship between the pixel and chroma clocks.

Later versions have a 2-22pF trimmer cap in the position of C7. This can be adjusted for best appearance but it will drift due to temperature.

Older machines have a fixed capacitor in this position. I've replaced this with a trimmer cap in a couple of my machines.

Hope this helps!

Interestingly (FSVO), it turns out the PAL CoCos keep the video in lockstep with the colour crystal line by line. First by using a 14.23753MHz crystal for the SAM, which theoretically yields an exact number of colour cycles per scanline, then by having explicit circuitry to delay clock at the end of each line for when it inevitably drifts a bit.

(This second point only became clear thanks to Torsten Dittel scanning some old PAL CoCo 1 schematics - in the CoCo 2, such features are packed up into custom chips, I think.)

All very dependent on that very unobtainable crystal frequency though, so not really something we can mod our Dragons for.

thanks for your advices
However I made a video mod for my dragon which is completely acceptable:
This mod consists in recovering a variable capacitor from a small FM radio, manage to sold it to a support and build an axis in order to add a rotary button. Put in serie a 22PF capacitor with the variable capacitor and it is very important to connect the module to the motherboard with coaxial cable before installing the module on the dragon enclosure. It works very well.

Just popped in to say I love this fix! In my time as a collector of Dragons I have seen so many hacks to try and improve the video quality, but this is a new one on me

yes, and it's ok!