Troubleshooting VGA to RGB monitor connection

This document tries to help you to troubleshoot the connection between VGA and RGB monitor.

General information on troubleshooting monitor problems

Monitor will not sync problems

There are wide variety of causes for a monitor that wil not display a stable or properly configured image. First check that the cable connections are correct and secure. Then check that the sync frequencies and polarities you have configured to your graphics card are correct (nor that in quite man modern monitors the sync polarity does not matter).

If those are fine, we need to look at the specifific symptoms:

Color problems

If you have problems with monitor colors, then the most propable reason for color problems are the monitor settings and RGB signal connections.

Double picture on the screen

If you see two equal images on your monitor screen side by side, the reason for this is taht you are sending the picture to the monitor at exactly double the HSYNC rate it can handle. This particular monitor happened to sync to every other of the HSYNC pulses (some monitors sync like that and some don't sync at all in this kind situation.). Two images side by side is caused by the fact that you see two picture scanlines side by side on the monitor screen. This double image problem occurs for example if you try to send a standard VGA (640x480 mode 60 Hz, 31.5 kHz HSYNC) picture to a monitor which can handle 15750 Hz HSYNC rates (some monitors work like that and some do not sync at all).

To silve this problem you have to change the screen monitor setting on your graphics card to halve the HSYNC rate the card sends out so that it maches to the value that your monitor wants.

Measuring tips

Sync signal frequency

Sync signal polarity

Common problems

PROBLEM: My monitor picture looks like a scrambled cable broadcast

Your computer's resolution or refresh rate is not set to a value which that monitor can handle. Another possibility is missing, wrongly connected or otherwise wrong sync signal. Here are most important thing to check in this situation:

PROBLEM: The monitor is not syncing to VGA picture at all

Look the question above.

PROBLEM: No picture at all in monitor

Check that the TV is in AV channels and the drivers are correctly activated in computer. Check that the connections are made correctly and the circuit gets enough power. Make sure that there is +5V open circuit voltage visible at adapter SCART connector pin 16. Make sure also that R8 gets +5V power and has correct resistance value. If this is all correct, the do all the things mentioned in problem solving above.

PROBLEM: I connected the circuit to my monitor and saw that the picture on tv is almost two sreens divided with vertical black line.

You are trying to output video signal with higher horizonal sync frequency than the monitor cna handle. The monitor has synced to every other horizonal sync signal. You can correct this problem bower the screen refresh rate to some valu that your monitor can handle.

PROBLEM: Monitor picture scrolls up/down

You are aither using a vertical sync rate which the montor can't properly sync to or verticla sync signal is missing. Chekc the monitor wiring so that the sync signal really get to the monitor in the right way. If there is no problem in wiring, then try to use a little bit lower or higher vertical sync rate to see if this helps to solve the problem.

PROBLEM: You see the picture "waving" or you see horizonal dark band scrolling over the picture

You have ground loop problem. Connect yout monitor and the computer to same power outlet. The problem should go away.

PROBLEM: Picture looks like the picture below and scrolls

               ---------------------------
             /                          /
            /                          /
           /                          /
          /                          /
         /                          /
        /                          /
       /                          /
       ---------------------------

The problem is clearly related to sync signal produced by the sync converter or the sync signals don't get otherwise properly to the monitor. Other possibility is that you are using a screen refresh rate that is a little bitt of the range that your monitor can properly handle.

PROBLEM: The picture in TV scrolls with a very fast speed from the upperright to the lowerleft and DOS propmpt looks like this:

      /--/------/|
      |C/ C:\> / |
      |/C:\> C/C:|
      |------/---| 

The problem might be caused one of the following reasons:

PROBLEM: Wrong colors

Check the wiring of RED, GREEN and BLUE signals. The might be short circuit or bad connection in one of those signal wires.

PROBLEM: Picture does not sync at all

Possible reasons for this problem:

PROBLEM: The edges of my screen are missing on my RGB monitor.

Adjust the video signal timings and/or monitor picture size controls to make the monitor picture to have right size.

PROBLEM: The picture does not fill the complete monitor screen

Adjust the video signal timings and/or monitor picture size controls to make the monitor picture to have right size.

PROBLEM: Picture has ghosting or is blurred

There is problem in your wiring. You have used too long cable or cable type not suitable for high frequency video signals.

PROBLEM: Radio interference from circuit

Make sure that all wiring is shielded cable and the shield is connected to signal ground and connector shells. Use shielded connector shell and metal box for the circuit. If this does not keep the interference levels acceptable, you have to install small ferrite core around the cable near connector going to VGA card.

What should the sync signals look like on oscilloscope ?

Horizonal sync

The HSYNC output should look like this (negative sync):

 ---_------_-----_-----_-----_-----_-----_---
or this (positive sync):
    _      _     _     _     _     _     _
 --- ------ ----- ----- ----- ----- ----- ---
The time between sync pulses is typically in the range of 15-64 microseconds (15.6-65 kHz horizonal sync rate).

Vertical sync

The VSYNC output should look like this (negative sync):

 ---_------_-----_-----_-----_-----_-----_---
or this (positive sync):
    _      _     _     _     _     _     _
 --- ------ ----- ----- ----- ----- ----- ---
The time between sync pulses is typically in the range of 10-20 milliseconds (50-100 Hz screen refresh rate).

Composite sync

The CSYNC output should look like this:

 ---_------_-----_-----_-----_-----_-----_---
And once for every field like this:
 ---_------______-_____-______-----_------


Tomi Engdahl <then@delta.hut.fi>