Had a pre-modded 7800 sent to me with a UAV that had good s-video but was producing some ugly jail bars in composite. But it was ONLY doing this on 2600 games. I at first suspected something in the wiring of the UAV or the UAV itself and first redid all of that and replaced the UAV. No change. It was then I started to look at things more closely and noticed that this '84 main board all socketed chip board appeared to have a TIA I'd not seen before. The company branding on it is IMP and it had a mid year 1988 date stamp on it indicating it was not likely original to the 7800 to begin with. Since I was able to disconnect the TIA color signal and get crystal clear black n white with the jail bars, I knew it had to be in the color signal. And since the UAV is tapping that signal straight from pin#9 off the TIA there was really only one culprit. I had another semi working 7800 with a more normal looking AMI branded TIA on it. Pulled that and put it into the socket on this troublesome 7800. Sure enough the jail bars are gone! I'm guessing this is a late revision TIA and it likely a CMOS part as it doesn't seem to heat up like most TIA chips do and it just feels a bit different. In any event...stay the hell away from these TIAs in and think about replacing them if you find them in your 2600s or 7800s. Especially if you plan to use composite upgrades or a UAV as it does produce jail bars. I also noticed that color trimmer has to be adjusted but a full 180 between the two chips. So there are some obvious internal differences in how they are designed and made. I've attached some pics to show what I was getting and between the two TIA chips.

Here is the section about halfway on the edge of the ColorMatch screen on the Colorbar generator program. You can see the jail bars on the slightly different hue of green on the lower half. Upper half is fine.

Here is the green purity screen with the bad TIA. Again, you can see the jail bars and while this is on green, they are present on all colors pretty much.

Here is the colorbar screen from the bad TIA IMP branded chip. Again you will see that most of the colors exhibit noticed jail bars on them. It is quite distracting and unusual to see from a UAV board.

Here is the actual IMP branded TIA chip that was causing the headaches above:

Now here is the colormatch screen edge with the good AMI branded TIA chip.

Here is the green purity screen with the good working TIA. Quite a difference here.

Here is the colorbar screen on the AMI TIA. While there is still come color bleeding and other artifacts normal with composite on an LCD, you will see that the colors don't have the jail bars as they did with the other TIA chip.

And of course, this is what the common AMI branded TIA looks like in most 7800s I've encountered:

So if you have nasty jail bars from your composite video out signal, you might look at the TIA as the possible cause.

Ivory Tower Collections

View the full article