Need repair tips for a 2600 - Color missing

[rigues]

Hi Folks,

 

I'm facing a weird problem while trying to repair my girfriend's 2600, and thought someone here would be able to shed some light on the issue. It's a 4-switch Polyvox machine (from Brazil), made circa 1986. The problem is: some colours on the image are missing. You can see a good representation of the issue here:

 

http://www.caetano.eng.br/rigues/temp/DSC04430.JPG

 

As you can see, green is missing. At first I thought the problem was on the internal transcoding board (NTSC->PAL-M) and removed it, bringing the console back to NTSC (most modern TVs in Brazil show NTSC anyway). Seeing that it had no effect, I turned my attention to the RF module, and did an A/V mod [1] on the machine. Still no deal, so I tried the TIA. The chips are all socketed, and at first I replaced the old worn-out TIA socket with a new one. As a last resort, I spent 4 hours scouring the electronic component shops here in São Paulo looking for a new TIA. I lucked out and found a shop with few pieces of a compatible part (KSC131) in stock and bought two, but even those didn't solve the problem.

 

Does anyone have any idea on what might be going wrong? Any parts that I should check? This console means a LOT to her, and I really want to get it back to a working condition.

 

[1] - The A/V Mod (portuguese): http://sti.br.inter.net/elucas/artigos/Atari-AV/

 

Thanks for your attention!

[russdog]

Hmmm.....I'm surprised that after all these years, no one has helped you. I hope this is not too late, but I'm pretty sure your problem can be fixed simply by adjusting the potentiometer near the voltage regulator. If your Atari has 6 switches on the top panel, this control should be near the power switch (it's probably a large blue dial). If there's only 4 switches on the top panel...well, I don't remember the exact location on that version, but it shouldn't be hard to find. Adjusting this control has fixed EVERY Atari I've ever seen with that problem. But you may want to use some cartridge with something that goes through all the shades of colors (like the Atari logo on the startup screen of Centipede).

Basically, the Atari generates the colors similar to the NES, where it produces a monochrome image and adds in a color carrier. Both the Atari and the Nintendo use phase shifting to determine the hue, but the NES uses a Johnson counter, while the Atari uses a more primative analog delay that has to be calibrated. Yellow is the reference color and the other colors are a result of delaying the yellow signal. If the control is too far off, the delay doesn't work, so you only get the master yellow hue, and every other color lacks the color carrier, making it monochrome. But even if you get the colors to show up, they may still be the wrong colors. Usually, in this case, reds are orange, purples are red, blues are purple, and greens are blue. But yellow is always yellow.

Centipede (and other games that I can't think of right now) have some graphics (as mentioned above) that cycle through all the colors. I recommend running one of those games and fine-tuning the control until you see a smooth transistion between the green and yellow hues. If adusting the potentiometer doesn't work, you could possibly have a bad potentiometer, or perhaps the input pin on the TIA that the potentiometer connects to is burned out (I've never actually seen that, but I have seen a TIA that worked fine except that it wouldn't respond to paddle inputs, due to one bad input pin on the chip, so I'd imagine it's possible for one of the other input pins to go bad too).

[russdog]

Okay, I overlooked a couple of things you mentioned, but if you say you tried various TIAs, then I would definitely suspect the potentiometer. But if it's not an actual Atari 2600, I don't know if it would have the same kind of potentiometer setup. But I'm sure it must have one somewhere on the board, probably near the power supply.

[rigues]

russdog, thank you VERY much!

 

The potentiometer you talked about is located between the power and B&W switches on the Brazilian Atari Polyvox 4-switch board. A quick adjustment brought the colors back, and a REALLY BIG smile to my girlfriend's face. And to think I was planning to "go hardcore" and replace every capacitor and transistor around the TIA hoping to fix the issue

[A.J. Franzman]

I'm glad you were able to fix your console.

 

As a matter of interest, how much did those KSC131 TIA substitute ICs cost? Original TIAs are somewhat expensive and in short supply.

[rigues]

Hi A.J.

 

I know, I found TIAs at Best Electronics listed at US$ 11 each. Here the KSC131 cost R$ 5.00 each, that's US$ 2.50. But the store only had five of them, old stock, and I bought two. A friend on another state (Espirito Santo) says he can find them for R$ 4.00 each (US$ 2.00), but I don't know about availability.

 

However, when searching by the code on Google I found lots of references to the KSC131 on electronic component stores, maybe they are more readily available than we think. Kinda like happened last year, when one member on my MSX (a japanese 8-Bit computer) user's group found a clone (UM3567) for the FM synthesis chip (Yamaha YM2413) used on our MSX machines. The Yamaha chip was almost impossible to find, but would eventually turn up on small quantities at prices like US$ 30 each, minimum order of 50 pieces. The UM3567 could be found at US$ 10.

 

The problem was acquiring them. They were being used as sound ICs on slot machines, which are illegal here in Brazil (albeit being everywhere). When I entered a store to ask for them, the manager came in, gave me a good look and said "boy, boy... why do you want these? who are you working for?". Took me some time to calm him down and convince him that I was not with the police, and just wanted to fix an old computer

[A.J. Franzman]

when searching by the code on Google I found lots of references to the KSC131 on electronic component stores, maybe they are more readily available than we think

I know, I did a Google search on the number also. The problem with all of those old-stock electronic parts distributors is, you can never get a good enough description to verify that the part is the right one before buying, and they won't even respond to your messages if you're not interested in at least USD $250 worth of parts.

 

Anyway, if you can find a source for those KSC131 chips, and can confirm that they are an exact replacement for the Atari CO10444(...) TIA chips at $2 each or so, I'd like to buy 10 to 20 of them.