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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):

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

  • i'm trying to pass signal directly from machines graphic card monitor socketgraphic card monitor socket bottom of 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
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  • \$\begingroup\$ Difficult to tell since we have no idea what you've done with the field sync. \$\endgroup\$
    – user16324
    Commented Sep 26, 2017 at 9:16
  • \$\begingroup\$ @BrianDrummond Monitor default output had 5 pins going into pcb and a whole side ground(it's 20pin). Three of them I'm sure are r g b, the two are now connected as S and Sv \$\endgroup\$
    – Damian Doman
    Commented Sep 26, 2017 at 9:25
  • \$\begingroup\$ Sv sounds like it may be field sync. I couldn't watch the video but was the "jump" consistently half the picture height? I can't remember EGA but I wonder if there's something odd about field sync to handle interlace? in which case you may have to distinguish between even/odd fields and only pass one of them on. \$\endgroup\$
    – user16324
    Commented Sep 26, 2017 at 9:32
  • \$\begingroup\$ @BrianDrummond yeah it's consistent vertical jump like once or twice a second, about half height. The pins on converter board are r g b s sh Sv grnd grnd so way more then on my signal output \$\endgroup\$
    – Damian Doman
    Commented Sep 26, 2017 at 9:43
  • \$\begingroup\$ How the conversion is done? It seems almost OK, but there can be also other problems like graphics card, noise, the voltage level of the output signals,...think you gonna need a scope. Your card outputs 5 signals + GND, are you sure it is an EGA? \$\endgroup\$
    – Marko Buršič
    Commented Sep 26, 2017 at 12:35

1 Answer 1

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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:-

schematic

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.

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  • \$\begingroup\$ That seems very reasonable, will give it a shot and let you know how it worked. \$\endgroup\$
    – Damian Doman
    Commented Sep 27, 2017 at 4:06
  • \$\begingroup\$ Can I actually set this resistor on vs cable instead of converter pcb? I don't want to interfere in the pcb atm as I still have refund guarantee in case it won't work, and once I make any changes in its circuit structure it'll be lost \$\endgroup\$
    – Damian Doman
    Commented Sep 27, 2017 at 6:14
  • \$\begingroup\$ You can put the resistor anywhere you like, so long as it is in series with the signal. \$\endgroup\$
    – Bruce Abbott
    Commented Sep 27, 2017 at 6:50
  • \$\begingroup\$ Yes!!! This was the perfect solution in this case! Thank you so much, it's not shaking anymore, just a little bit 'zig-zag' flowing, but hopefully i'll manage to get rid of this aswell in similar manner. Thanks! \$\endgroup\$
    – Damian Doman
    Commented Sep 30, 2017 at 14:30
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    \$\begingroup\$ I don’t know how this answer doesn’t have a zillion upvotes. It TOTALLY fixed my issues. Thanks! \$\endgroup\$
    – hernan43
    Commented Nov 28, 2018 at 2:18

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