The Video Circuit

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Here we will look at the video circuit. As it comes from the factory, the video is terrible. This problem affects both the Hummer and the DTV Version 2 Joystick. (but does not affect the Version 1 joystick, which has a different video issue) It seems that the factory tried to cut some costs and redesigned the video circuit, but made some mistakes. The information here is the same for Hummer and DTV Joystick, but the pictures shown are only of the hummer. There is a ton of information (with pictures) on the PAL version on Daniel Kahlin's website here: http://www.kahlin.net/daniel/dtv/index.php

All hope is not lost, it is fixable. It isn't easy or for the faint-hearted. As of this writing (January 1, 2006) I have only done the luma-fix to the board. I will describe it here, and shortly I'll be posting a new graphic showing what it should look like when done. Basically, you de-solder 7 surface-mount resistors from the board. They are the four 330 ohm resistors comming from the Luma_0 through Luma_3 and the three 680 ohm resistors below them. You replace the original 330 ohms on top with four 680 ohms, and the bottom three will get 330 ohms. This by itslef will give you greatly improved video quality. Even the hummer game itself will look fantasically better, not to mention your other C64 programs. Also, you may solder an additional 680-ohm resistor from the bottom-right corner of resistor R14 (the furthest-right resistor in the picture) and put theother end to ground. This will also improve your video further but currently it is causing interference on the audio when I do this. However, mine is grounded at the audio circuit and I'm going to try moving it to see if that helps.

Just some tips on soldering. The way I did it was to heat up the resistor with the soldering iron until it just fell off. After removing all 7 resistors, I used desoldering wick to remove excess solder from the pads. New solder will have flux/resin which will help the new resistors stick better. I tapped tiny blobs of solder onto the pad. Then I used tweezers to hold the new resistor over the pad, then held it down with a sharp pointy object. Then I heated the resistor until the solder below melted and the chip seats down properly. Then I remove the iron and wait for the solder to cool before removing my pointy object.

If, after you are done the picture doesn't look right, then one or more of the resistors isn't soldered in place correctly. You may need to use a multi-meter to trace the circuit and see where you are loosing connectivity. Or god forbid you may have put too much solder and be connecting things that aren't supposed to be. When I was done, I had two resistors that needed to be re-soldered before it looked right.

I will be adding a program on here shortly to aid people in diagnosing their video to see if it looks right or not.

This is a schematic of just the video circuit, by Daniel Khalin. This also shows the way it comes from the factory.

This is a diagram I drew in Paint Shop Pro showing the layout of the video system on the board. The traces are not easy to see due to the silk-screened letters. So I used a multi-meter to trace out the traces and draw them here to be studied.

This shows which resistors need to be changed to accomplish the luma-fix. The ones colored with blue are changed from the original. .

This is an example of what this screen looks like before and after the luma-fix. I have not messed with the chroma yet. You can see a dramatic increase in detail on the right picture, which is fixed. However, just so you'll know, the picture looks about 50% better on my composite monitor than it does through the video capture card. So I can't show you exactly how much better it is.

Here you can see the board after I put on the new resistors. You'll notice I bought new 680 ohm resistors and they are larger than the ones that came with the DTV. The larger ones are more common, they fit on the DTV board, and they are actually easier to solder on than the small ones. Granted, these don't look as neatly soldered as the factory, but as long as the work, the looks don't matter.

Soldering a new SMD resistor into place. Jewelry screwdriver on left, soldering iron on right.

Just some tips here on soldering. The first time I did this, I went nuts. It was insanely difficult. But by the second time I had done it, I learned some ways to make it easier. First, before you even begin, I recommend getting some brand-new resistors. You'll have to anyway because you are swapping a set of 3 resistors for a set of 4. Makes sense that you will be short one. So you will have to buy a 680-ohm resistor anyway. I recommend buying all new resistors and get them in the larger SMD package. They are about 4 times bigger than the ones that come on the DTV and they are much easier to work with. Those ones that come on the DTV are about the size of a crumb and if you loose it, it is gone. I actually bought my resistors at Fry's Electronics. They actually carry these things in the store. But if you don't have a Fry's around then just order them from Mouser or Digikey.

Okay, the next thing to realize is that these SMD resistors weigh almost nothing. They will stick to your soldering iron. So you place it down in its little spot and lift on your iron, and the resistor comes with it. Even when you have the resistor down on the circuit board, once that solder is wet the resistor just skates around like it is on ice. You can't control it. The answer? Use a small jewelry screwdrive (like shown in the left in the picture above) and hold the resistor where you want it to be while heating it up with the soldering iron. This way you can maneuver the resistor and hold it down with one hand and apply heat with the other. Be sure to wait a good 4 to 5 seconds after lifting your soldering iron to let the solder dry. Otherwise it might slide off its spot.

It never fails after doing this process that one of the resistors you soldered in are only touching on one side or the other. Therefor not sending any video. So what you will have if you look at the color palette are missing colors (IE: they are black instead of whatever they are supposed to be) It should be possible to write a diagnostic program to show which resistors are and are not working.. but I haven't done so yet. Anyway, if the screen is rolling or otherwise won't sync up, then the problem is likely the resistor on the CSync. So that one is easy to narrow down.


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