A.J. Franzman's Atari VCS Composite Video Modifications

A.J. Franzman's Atari VCS Composite Video Modifications

These instructions assume that you have some basic electronics knowledge including component identification andvalue interpretation, ability to read schematic diagrams and construct circuits from them, have all appropriate toolsand materials, and have skill at soldering and unsoldering components. These modifications have been developedand tested using a 4-switch Atari Video Computer System console with woodgrain front panel. The mainboardsused were Rev. 14; other versions may have components in different locations and even some entirely differentcomponents. These modifications are only for use on NTSC television standard consoles (North America, Japan,and a few other locales). Use these instructions at your own risk.

Each step begins with a general overview, followed (where appropriate) by the specific instructions needed toaccomplish it.

The instructions in the "SIMPLE" column are easiest for electronics novices, and easiest to reverse if someonelater wanted to restore the console to original stock configuration. The "EXPERT" instructions make the greatestchanges to the circuit board and involve removal of several components, which may create enough space to beuseful for adding or relocating other circuitry, as one might wish to do when building a handheld Atari 2600console. There is no need to follow only the "simple" or only the "expert" instructions; for each numbered step,you may choose whichever version you prefer.

Steps marked "OPT" are optional; any or all of them can be omitted or completed as you desire.

As used here, the word "disconnect" can mean either to cut a component's lead(s), or to simply melt the solderusing a soldering iron and pull the lead(s) out of the circuit board hole(s). In either case, the component and lead(s)should be positioned such that no accidental contact to the circuit can take place at the original connection point oranywhere else. Where the word "unsolder" is used, the component lead must not be cut because either thecomponent, or the circuit connection point, must be connected to a new part of the circuit.

COMPONENTS NEEDED-1 or 2 small pieces of perfboard for assembly of circuits, with or without solder pads as desiredTransistor, NPN general purpose: 2N3904 (similar NPN types OK)Capacitor, ceramic disc: 10nF (usually marked "103", possibly with a suffix letter){optional Potentiometer, trimpot: 100kΩ}Resistors, 1/4 watt 5% carbon film (power, tolerance & type not critical): 10Ω, 2 @ 75Ω, 8.2kΩ, 10kΩ. {optionaladditional 10kΩ, 22kΩ, 220kΩ, ~400kΩ}

Mainboard before modification

A.J. Franzman's Atari VCS Composite Video Modifications http://mysite.verizon.net/res0fab4/files/VideoMod.htm

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Step SIMPLE EXPERT

1

Disconnect the audio mixer circuitry. Removing this oscillator helps to keep the sound and video outputsclean, while slightly reducing the total power load of the system.

Disconnect the right-hand leads of R207 (18kΩ) andR208 (9.1kΩ). Disconnect any 2 leads of Q201 orremove it from the board entirely if that's easier (ifyou don't want to lose it, you could remove it, bendone lead sideways, then reattach it by just the bentlead). Disconnect the lower lead of R209 (1.8kΩ).

Remove the following 9 components from the board:C206, C207, C209, L201, L202, Q201, R207, R208,R209.


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2

Disconnect the RF modulator circuitry. As before, removing this eliminates another possible source of A/Vsignal interference and reduces the unit's power consumption a bit more.

Begin by removing the channel 2/3 selector switch. If you have a small soldering iron, this can beaccomplished by heating the pins at one end and the middle simultaneously and pulling that end out a bit,then repeating on the other end & middle and continuing to go back and forth like this until the switchcomes free.

Step 2a, photo

Disconnect the 3rd and 4th pins of the RFmodulator's 5-pin header. These pins are rather stiff,so unsoldering (if you don't want to cut them) willrequire a good small pair of pliers.

Remove the RF modulator assembly from the board.Step 2b, expert, pictorial


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3

Build the Composite Video Amplifier on a separate piece of perfboard. See schematic below. Connectionpoints: +5V is available in many locations, the bottom end of C213 was convenient for me. Ground is alsoavailable in many locations, the top end of C213 may be a good place for you to get it. Video signal comesfrom the top end of C215 - connect a wire from there to "Video In" on the amplifier. Video output comesfrom the location indicated on the amplifier schematic and connects to the center conductor of your RCAvideo output jack or cable. Audio output comes from the top end of C208 (opposite from the junction ofC208 and R206) on the main board and connects directly to the center conductor of your RCA audio outputjack or cable. Don't forget to ground the "shields" (outer conductors or terminals) of the RCA jacks orcables!

OPTA

Add the horizontal blanking signal. This may improve picture stability and/or color saturation in somecases.

Add the 10kΩ and 220kΩ resistors indicated as "Optional" on the Composite Video Amplifier schematicabove. If your mainboard has an 820Ω resistor between the 6th and 9th pins from the lower left of IC A201(TIA/Stella, part no. CO10444 - check the bottom of the board too), disconnect either lead or remove theresistor. Add a wire to connect the blanking signal from IC A201 pin 6 to the point indicated as "Blank" onthe schematic of the Composite Video Amplifier.

OPTB

Change a video signal attenuator into just a chroma attenuator. This may improve image sharpness in somecases.

Unsolder and remove R222 (15kΩ) and C215 (150pF). The board position for C215 should have 2 sets ofholes, this is to allow for the 2 different styles of capacitors that were used. If your video signal wire cameoff with the capacitor, disconnect the wire from the capacitor and solder the wire back into the topmost ofthe 4 holes at the original C215 location. Insert one lead of the resistor removed from the R222 locationshallowly into the second hole from the bottom edge of the board in the C215 position - you want to havejust enough lead protruding on the bottom of the board to solder securely. Now take the other end of this15kΩ resistor and stretch it up diagonally to the top end of R210 (6.8kΩ) and solder it there, avoidingshort-circuiting to the upper lead of capacitor C211, which it will need to reach over or around. Solder the150pF capacitor that was removed from the C215 position across the top of the 15kΩ resistor that you justmoved.


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OPTC

Replace the 2600's chroma phase adjust pot (color tint control) with one offering more precise adjustment,and not requiring a separate unregulated supply voltage.

This option is intended more for people hacking their Ataris into portables. The original chroma phase pot,R213 (500kΩ; the only potentiometer on the board), is wired to a couple of diodes and a resistor andthence to the 2600's *unregulated* DC supply (~ +9VDC) and ground, with output going to TIA pin 10. Alittle research reveals that the specified voltage level at TIA pin 10 is only 4.23 volts. In other words,there's no need for the pot to be powered from the +9V supply! (Which is a big pain to people makingportables that run from lower voltages).

Unsolder R213 from the board. Build the Chroma Pot Replacement on a separate piece of perfboard. Seeschematic below. Connect the ~ 400kΩ resistor to ground (if you're keeping the full original board, you canconnect to ground at the lower left hole where R213 was.) Connect the 22kΩ resistor to +5V (you can get+5V from the right end of diode CR201, which BTW is no longer needed and can be removed). Nowconnect the "wiper" or center terminal of the new pot to the circuit track leading to IC A201 pin 10 (ifkeeping the full circuit board this is at the upper right hole where R213 was.) Now to set the color controlto roughly the correct level, insert a cartridge and power on the unit (for this step, you do not need toconnect a TV or video monitor). Connect a voltmeter between circuit ground, such as the silver box aroundthe central area of the board, and IC A201 pin 10. Adjust the new pot until this reads as close as you like to4.23 volts. This should be very easy to get right on the money with this circuit, whereas if you tried it withthe original pot it would be touchy and difficult. You can now connect a TV to the unit and tweak the colora bit to your liking if desired; I use Pitfall! and adjust so the green background between the tree trunks isnot too yellowish or blueish, and maximize the contrast between that and Pitfall Harry's shirt color. If youhave the Color Bar Generator or other appropriate diagnostic cart you could use that instead.


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Have you installed this mod in any of your consoles (or attempted to)? Has it worked well for you? Please send mefeedback:


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A.J. Franzman's Atari VCS Composite Video Modifications

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