Atari 2600 - 4 Switch

[Trent555]

Quick question:

 

Why is there a 620 Ohm resistor on the TIA chip on the backside of the Atari motherboard? Its the only 1 there and it seems kinda odd?

 

What is its function/purpose??

[A.J. Franzman]

From the Field Service Manual:

 

 

This provides the lowest "blacker-than-black" level of the horizontal blanking interval, that is left unconnected on some VCS models. Although it doesn't actually have anything to do with color, apparently it does improve saturation on some CRT TVs. I found that on a handheld LCD TV screen, it made the picture worse, as only one of "black" or "blacker-than-black" can actually appear as black at any given time, with the other appearing gray.

Edited October 21, 2010 by A.J. Franzman

[BigO]

If you're in the habit of hooking up the console to different TV's at different times, would it be worthwhile to switch that resistor in and out of the circuit or install a pot (probably with some series fixed resistor to keep from going too low) in its place for fine tuning? I don't know how drastic a difference this can make.

[Trent555]

A.J. thanks for posting that link. Interesting.

 

Did you remove the resistor to accomodate your LCD? Perhaps i will try this as well. With it on and off to see if there are any noticeable differences.

[A.J. Franzman]

The LCD and the VCS board in question are part of a handheld project that is now on the back shelf. But since the unit will have output to connect to a TV to use as alternative to the built-in screen, I do plan to install a switch which will enable or disable the blanking signal.

 

BigO, IMO a pot would be a waste. The resistor should either be installed or left out (open circuit). But, changing the value with, say, a 2K pot might work as a poor man's saturation control with some TVs, but it could also mess up the picture if turned too low. I recommend putting a series resistor of at least 330 ohms in series with the pot.

 

My composite mod as it exists now actually has a resistor of a different value in a different location for the same function, but disabling it is the same; simply open the circuit.

Edited October 22, 2010 by A.J. Franzman