TI-99/4a Washed-out colors?

[jmetal88]

I have a TI-99/4a that seems to have a video problem. I got it a few years ago for free, but the video ram was shot. I replaced it all, and it worked fine afterwards, then I put it in my closet and forgot about it. Today I happened to pick up a few games from a local shop for it, so I hooked the machine up to test them out, and now for some reason the video colors are all wrong! It seems like most (although not all, black in particular seems to be unaffected) of the colors are too light, or washed-out looking. No amount of adjustment on my TV alleviates the issue, even the color saturation adjustment. Does anyone have any idea what might be causing this? I'll take a picture of the output on my TV soon and I'll post it if it accurately captures what I'm seeing.

[jmetal88]

Here is what it looks like. To my eyes, it's worse than what's in this photo, but you can see a bit of what's going on. The background blue looks almost grey on the screen, the square that's supposed to be green second from the left side of the screen is almost white (which makes the Munch Man title screen almost impossible to read), the reds are generally pink-ish, and the other colors just look a few shades too white.

 

EDIT: Forgot to attach the photo.

Edited April 23, 2014 by jmetal88

[jmetal88]

Okay, more weirdness has ensued.

 

I went back through and checked the video RAM again. All the chips I had in there seem to be good, but here's the wacky thing. If I replace the first chip in the bank (well, I'm not sure if it's electrically the first chip, but it's the chip closest to the middle of the board) with one of my known bad chips, it makes the colors correct! It garbles the text, but it gets me correct colors. What's going on here?

[jmetal88]

Alright, I solved it. When originally replacing the RAM, I had done a modification that allowed me to use 4164 chips instead of 4116 chips (because 4164s were easier/cheaper for me to get at the time). I didn't remember the colors being this messed up when I first replaced the RAM, but maybe they were. Anyway, I discovered that the 'bad' chip I was subbing in was a 4116, and I decided to measure the voltages on its power supply pins when it was inserted versus when I had a 4164 in its place. There was one difference. The pin that was originally -5V for the 4116, which I had simply disconnected from the -5V supply, was being pulled down to nearly 0V when the 4116 was inserted, but was floating around 2.5V when the 4164 was inserted. So I decided to try shorting that pin to ground with all 4164s in the machine, and that brought all the right colors back! I put a little solder bridge between the previous -5V pin and the ground pin on the motherboard and it now seems to be doing fine. Now I just need to troubleshoot my keyboard and figure out why it thinks I'm holding down the letter P all the time, I guess. I have a spare keyboard mech back at my parents' place that I could use if I need to, but if memory serves, it was actually in worse shape than this one, which is why I swapped them.

[jmetal88]

I think the keyboard issue turned out to be part of the RF shielding shorting something out. Anyway, I ended up accidentally ripping off the whole ribbon cable coming from the keyboard when I opened the machine up to investigate. Had to re-solder the whole thing, but everything seems fine, now.