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alex_79

Atari 2600 Pal - B&W picture

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Ciao!

I need help with a 6 switch Atari 2600 I've bought on ebay.

 

I'm from Italy and the console is a PAL unit.

The problem is that there's no color on the TV screen.

 

I've another light sixer that works perfectly and I tried swapping components between the two units:

 

The TIA chip is ok (swapping the chips still produce no color image on the "bad" unit, while the other works correctly)

 

I've also swapped the board with the switches between the units and nothing changed, so I think the problem is on the board with the the TIA, RIOT and 6507 chips on it.

 

Adjusting the pot for color regolation while the console is on doesn't seem to have any effect.

I've also cleaned it with a contact cleaner and I tested with a multimeter (It ranges from 0 to about 470 K-Ohms; on the functioning unit the range is about 0-410 K-Ohms).

 

 

I also tried different rf cables and power supply from other functioning atari units, but nothing changed.

 

Thanks in advance for any suggestion (and sorry for my bad english!)

Ciao!

 

Alessandro.

post-10599-1160399979_thumb.jpg

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Is the game from the U.K.? If it is, wasn't the RF modulator different for their "PAL" TVs. What happens if you run an AV mod and feed your TV from composite video?

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I don't know if the console is from UK, the label on the bottom says "MODEL NO. CX-2600 P" but there are no indications like "pal-B" or "pal-I".

post-10599-1160474979_thumb.jpg

Anyway, I mounted the board with the switches (see pic) on the functioning console and it gives a perfect color image.

post-10599-1160474999_thumb.jpg

The rf-modulator is on that board, together with the voltage-regolator and the channel-select switch, so i presume all these components are ok and the console is compatible with my TV set.

 

At the moment, I've very scarce experience in soldering and electronics in general, so I'm not able to perform an A/V mod on my Atari (I'd like to do it in the future, however).

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Compare the voltage between pin 1 and pin 10 of the TIA IC (CO11903) on both of your boards. Pin 1 (nearest the green capacitor) is ground. Pin 10 is the color adjust input and should be at ~ 4.25 volts (varies slightly from one unit to the next to get proper colors). Turning the color adjustment pot changes this voltage.

Edited by A.J. Franzman

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Compare the voltage between pin 1 and pin 10 of the TIA IC (CO11903) on both of your boards. Pin 1 (nearest the green capacitor) is ground. Pin 10 is the color adjust input and should be at ~ 4.25 volts (varies slightly from one unit to the next to get proper colors). Turning the color adjustment pot changes this voltage.

 

I adjusted the pot on the "good" console to match the 4.25 Volts between pin 1 and 10, and the colors are correct. (I used z26 emulator with the same game for comparison). On the broken one, tuning the pot varies the voltage from ~ 0.0 to ~ 7.9 V, without jitter, but the image remains B&W throughout that range.

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OK, so now we have eliminated that part of the circuit as well as the TIA itself as being the cause of the problem. You can set the pin 10 voltage to 4.25 volts and leave it there for now. The next likely part of the circuit is the color subcarrier frequency generator. Unfortunately, testing it will require something that can detect a 4.43 MHz signal. Possibly a logic probe, lab bench frequency counter, or oscilloscope. Even a few of the better handheld multimeters now have frequency counter functions built in. Do you have access to anything like this?

 

See Atari 2600 Schematics - PAL for the schematic. The area of interest is at the bottom of the drawing, pins 8 and 12 of IC A201 (the PAL TIA) are labeled PALI and PALS. These connect to several resistors, 3 capacitors, an inductor, a 2N3904 transistor, and the important part, the 4.43 MHz crystal (one of the two big metal cans on the board; yours is marked 4433.618 KC). Pin 8 carries the transistor amplified and buffered signal into the TIA, while pin 12 provides the pulse to the crystal that keeps it oscillating. Any bad connections or components in this area will cause loss of color. In your first image above, I see what looks like a burned spot on the board beside the bottom end of R239. Check carefully on both sides of the board in this area, and test that resistor as well as capacitor C243 beside it for proper values (if you can).

Edited by A.J. Franzman

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I haven't the equipment (nor the skills) to test that circuit and I fear I can damage something. I want to learn more about electronics and practise with soldering iron before trying again.

I've set pin 10 voltage to 4.25 V as you suggested and I reassembled the console for now.

Thank you very much for your help.

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