Troubleshooting VGA to TV circuit

The have been quite a few VGA to TV converters built according to my design. The design itself is working well, but sometimes you might have problems in building the circuit which result that the circuit does not work. This document tires to give help to those who are troubleshooting their VGA to TV circuit they have just built.

Common problems

PPROBLEM: My TV and/or monitor picture looks like a scrambled cable broadcast

Your computer's resolution is set too high or you don't have correct driver loaded. The TV absolutely requires a 15625 Hz or 15750 Hz vertican sync frequency. Make the changes necessary so that your video card and driver will produce a usable signal. Check also that the circuit gives out usable composite sync signal.

PROBLEM: TV is not syncing to VGA picture at all

The problem can be either the driver or the circuit. Check the following thing to make sure that there is no driver problems:

Check that the TV is connected to AV channel where it takes the signal from SCART input. Check also that everything is plugged correctly. If this does not help, then you have to really start troubleshooting on the circuit board.

First thing to do is to check that the circuit get steady 5V (+-0.2V) operation voltage. The cehck that the sync signals HSYNC and VSYNC are coming from VGA card to circuit board. Check also that you see those signals at positive polarity at U1 pins 3 and 9. The check that you see TTL level composite sync signal at U1 pin 11.

If this all is correct, the problem might be the sync output section built from resistors and transistors. It is very aeasy to make mistakes in here by connecting one transistor in the wrong way or something else similar simple mistake. Recheck the transistor circuit. That circuit should give around 1..2.5 Vpp open circuit signal voltage at SCART pin 16.

PROBLEM: No picture at all in TV

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: You see disorted VGA picture over normal TV screen

Check the problem troubleshooting above.

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

You are not using any VGA to TV drivers or the driver does not work with your graphics card.

The reason why you saw two pictures is that original VGA has double the horizonal scan frequency compared to normal TV and the TV happened to sync to every other sync pulse coming from the circuit. One VGA scan line is standard VGA modes is 32 microscond long and TV scanline is 64 microseconds, so one VGA scanline fills half of the TV scanline time.

PROBLEM: TV screen scrolls up/down

Check that your TV support the TV standard the driver is written to support. If you have PAL TV use PAL driver and if you have NTSC TV use NTSC driver. NTSC drivers can work well in many modern PAL TVs, but not all.

If the driver is correct, then you must check the hardware to make sure that the VSYNC signal goes well all the way from VGA card to the TV. First check that VSYNC signal goes to U1 pin 4. It goes through one XOR gate and you should see the VSYNC signal at positive polarity at U1 pin 6. Then the signmal is mixed to HSYNC with the following gaes, and finally you should see TTL level composite sync signal at U1 pin 11. Finally the signal goes through transistor amplifier circuit to CSYNC output.

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

You have ground loop problem. Disconnect the TV from antenna network and all other equipments and try again. The problem should go away. If still problem, then check the gound connections in your cabling.

If you have a modern 100 Hz TV or some other digital video equipment then the problem might be that the TV just can't exactly sync to the signal from the VGA to TV converter because it can varies slight from the real TV broadcasts in some timing details. Try with another driver and check if this corrects the problem.

PROBLEM: Picture looks like the picture below and scrolls ?

             /                          /
            /                          /
           /                          /
          /                          /
         /                          /
        /                          /
       /                          /

The problem is clearly related to sync signal producted by the converter. The converter does not produce sync signal because of some fault in the circuit or the TV is not in right operation mode. Make sure that the TV is in AV channel because some TVs can take sync signals from other channel the picture from SCART if the AV channel is not selected.

If the circuit is not producing correct sync signal the TV might use it's default sync frequency which is not exactly the same same as the frequency produced by the drivers (if this is the case usually you see "waving" in the picture because the default frequency in TV is not very stable).

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

You have something wrong in the sync signal electronics or the driver does not operate correctly with your VGA card. The most common problem in sync circuit is that you have put the transistors in the wrong way (happens quite often if you use some other transitors to replace the type ised in the schematic).

PROBLEM: TV shows only gray picture with only some faint details of the computer screne picture

This usually means that the sync signal part of the circuit works nicely, but there is problem in getting the RGB signal to the TV. Check that your TV settings are correct for receiving RGB signals from SCART connector (if your TV has two connectors check that you use the one which has RGB input capability). Check also that the circuit gives control voltage to the SCART pin 16. That voltage should be in 1..3V range when you have the circuit connected to TV.

PROBLEM: The edges of my screen are missing on the TV.

This is due to the overscan built into your TV's design. The edges of your picture ARE being projected onto the CRT, but behind the painted mask, so that you cannot see them. Try another driver because some drivers allow selecting between underscan and overscan mode.

PROBLEM: The picture does not fill the complete TV screen

Normal TV and VGA screen resolutions are not the same. VGA screen has many resolutions and some of them fill more part of the TV screen than other. You can adjust the amount of the TV screen the computer picture fill only by changing the computer resolution (if your TV can handle both PAL and NTSC frequencies you can also try to change the video mode). On horizonal direction some drivers have selection to select between underscan and overscan modes.

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.

If there is still radio interference, try to disconnect the circuit from TV to see if the problem goes away. If this is tha case, then the TV has poor shielding inside it. There is not much you can do to this, but you can try to install another ferrite core to the SCART connector end of the cable.

Tips for troubleshooting the circuit with a multimeter

Oscilloscope would be a very nice tool to check where those sync signals go and where they don't go (connect the circuit to PC and TV and probe with oscilloscope where you see the sync signals).

If you only have a multimeter then you can do still some tests. First don't connect the circuit to anythig (not PC or TV) then you should have the following voltages when the circuit is powered:

If you ghange the state of HSYNC or VSYNC input then there should appear a brief pulse at the output (positive pulse at U1 pin 8 and CSYNC should go to near 0V for some short time).

If this seems to work then you can connect the circuit to the computer and load up the drivers. The sync signals should be now detectable at the circuit CSYNC output and in many other places around U1. The sync signals are in 50Hz to 16 kHz range so you can try to test them acoustically (for example try if you can hear those frequencies with high impedance headphones, audio amplifier or some other suitable equipment to play back sound).

What should he sync signals look like on oscilloscope ?

The CSYNC output should look like this:

And once for every field (once in 1/50 or 1/60 second) like this
The time between the sync pulses should be around 64 microseconds and the lenght around few microseconds if you have VGA to TV drivers loaded and working properly.

The sync signals should look about the same even without the VGA to TV drivers loaded but the timing just varies.

Tomi Engdahl <[email protected]>