conversion - EGA display converting to VGA - screen's shaking

I've recently tried to make conversion from EGA (probably, couldn't get scope to verify) to VGA to replace an old factory CRT monitor to LCD.

Screen displays fine but it appears to shake vertically in a terrible manner that make it hell to read information from it. Video presentig the issue is linked below. What may be possible cause of this issue?

vimeo .com/235485641

EDIT: Some informations that might appear to be useful (most have been asked in comments section):

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  i'm using GBS-8200 CGA/EGA to VGA converter (manual for this one specifically is here: arcadeworlduk.com/content/CGA-VGA-Manual.pdf),

  
  


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  i'm trying to pass signal directly from machines graphic card monitor socket I've soldered 6 cables to the bottom of monitor graphic card and connected them to the converter's P11 as follows:

  
  

(GROUND->black, GROUND->none, VS->white, HS->none, S->yellow, B->blue, G->green, R->red)

• frequencies on yellow/white pins are: yellow-15-17Hz, white-18-21Hz

Answer

The GBS8200 upscaler chip runs on 3.3V and doesn't like 5V TTL inputs. Typical symptoms of excessive input voltage are vertical shaking and intermittent freezing of the display.

The GBS8200's input circuitry looks like this:-

^{simulate this circuit – Schematic created using CircuitLab}

The 1k resistor in series with the Vertical Sync input may or may not be enough to limit injected current and drop signal voltage to a safe value. You could try adding another 4.7kΩ in series to get the upscaler's input below 3.3V.

The horizontal/composite sync input has no series resistance so the full input voltage is applied. I have found that adding 680Ω in series was sufficient to get it working properly. In combination with R34 this should divide a 5V input down to ~3V.