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Atari 2600 4 Switch Fault


Dipski

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Hi Folks, I'm a long time reader first time poster.

 

I've been fixing a bunch of Atari 2600's and these days I remove the RF modulator add AV output before switching a system on and that was done to the system I'm currently stuck on also. The machine in question is a 4 Switch PAL unit and when switched on produces a black screen. After warming up a little that black screen has some game colors sprawled horizontally across, and switching on & off can lead to a different toned sound coming through the audio. So the problem is an odd one, the game cartridge isn't loading but the fault changes/improves slightly with warming up with the system producing some isolated horizontally streaked color & occasional random monotone sound.

 

My first thought was that one of the main chips was faulty, so I removed and replaced each of them - no improvement. I checked the 7805 output, power to the chips and data across them which all seem to be ok. I checked the electrolytic capacitors which all tested ok and checked for any cut or damaged traces and found none. I checked for out-of-spec resistors, checked for open/shorted board caps and found none. I replaced the crystals which made no improvement. Thinking it must have related back to power I replaced the 2200uf Cap, 7805 Regulator, Power Socket and 4.7uF Electrolytics. I also replaced the 2N3904 and 2N3906's. No change.

 

There is one point of note on this 2600 in that it doesn't have a piece of circuitry that is present on my other 4 switchers - even those with the same board revision. The A204 chip which sits on the upper right side of the board on my other systems is not present (from the factory) with no provision for its installation. I assume it's just a particular revision of the board that doesn't have the A204 for some reason. This machine also has a second 4.7uF electrolytic next to the red-plastic mounted coil (removed with Q201 as standard for AV mod).

 

I hate to give up on a challenge like this one, but aside from a systematic replacement of every component on the board I'm out of ideas - has anyone come across anything like this before?

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If it's not high tech it is low.

Are the cart slot connectors clean and bent toward the center enough so all make contact?

Are the continuity of the pins to the board all good?

 

I had an Svideo Sega 32X mod that I did turn on all scrambled because the grounding of the pin in the connector was missed.

 

I have broke joystick connectors where the pins break inside the connector making repair impossible, they needed replacement.

 

That's all I can think of at this time.

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