Atari 130XE Video Output Repair

Atari's schematic of the 130XE video circuits: the GTIA (U17) luminance and sync outputs through the CD4050 buffer U20 and the resistor ladder R47 to R51 to the video amplifier Q3; the colour output through L6 and R201 to Q2; and the colour adjustment circuit with Q1 and R38 at the bottom left

The Atari 130XE builds its picture in the GTIA (U17). On Atari's schematic the four luminance outputs (pins 31, 22, 23 and 24) and composite sync (pin 25) pass through the CD4050 buffer U20 and a resistor ladder, R47 to R51, into the video amplifier transistor Q3; the colour output (pin 21) goes through L6 and R201 to Q2. The signals feed the RF modulator and the 5-pin monitor socket J2. When the computer runs but the picture is missing, has no colour or will not hold sync, the fault lies somewhere on that path, and Sams' service data gives a check for each stage.

Symptoms

• No picture on the TV or monitor, while the computer otherwise runs (the keyboard click sounds, a program loads).
• Wrong colours, or no colour at all.
• No vertical or horizontal sync: the picture rolls or tears.
• A machine that shows nothing at all and does not respond is more likely a supply, clock or processor fault; start with Atari 130XE Black Screen Repair.

Safety

• The board runs from the external 5 V pack only. Check it first; see Atari 130XE Black Screen Repair.
• Switch off before removing or fitting chips. ANTIC and GTIA are no longer made.

Tools and parts

• An oscilloscope and a meter; Sams also lists a colour bar generator among its test equipment.
• A non-metallic trimming tool for the colour control R38.
• Replacement parts as found: CD4050B for U20 (Atari C010816), 2N3904 transistors for Q1 to Q5, 1N4148 for CR4, 9.1 kΩ for R49, 750 Ω for R51, and the 500 kΩ colour control R38 (C023030-04).

Procedure

Open the machine

1. Remove the four screws from the bottom of the case, lift the top and disconnect the keyboard cable.
2. Remove the seven screws holding the board and lift out the board and shield; twist the seven tabs to separate the shield halves. Reconnect the keyboard and power for the checks.

Monitor socket

Monitor socket J2 (Atari schematic C070065)

Pin	Signal
1	Composite luminance
2	Ground
3	Audio
4	Composite video
5	Colour

No video

Work back from the output towards the GTIA and stop at the first point where the waveform is missing.

1. If the RF output is in use, check for a video waveform at pin 2 of the RF modulator. If it is present there, the fault is in the RF modulator, the channel select switch S2 or the cable to the TV.
2. If it is missing at the modulator, check for the video waveform at the emitter of the video amplifier Q3.
3. If the emitter has no waveform, check the base of Q3. If the base has the waveform, check Q3 and the parts around it.
4. If the base of Q3 has no waveform, check pin 5 of the buffer U20. If the waveform is present at U20 pin 5, check R49 and U20.
5. If U20 pin 5 has no waveform, check the GTIA (U17) and ANTIC (U7).

ANTIC on a 130XE board. Like the GTIA, it is one of the two chips Sams names when the video is missing at the buffer U20

Wrong colour or no colour

1. Adjust the colour control R38 first (see below).
2. If turning R38 has no effect, measure the voltage at pin 17 of the GTIA while turning R38 from minimum to maximum. Sams gives a range of 0 V to 8.5 V. If the voltage is missing or does not vary, check R38 and the voltages, waveforms and parts around the colour amplifier Q1.
3. If pin 17 varies correctly, check for 3.579545 MHz at pin 28 of the GTIA. If that frequency is wrong, follow the clock chain in Atari 130XE Black Screen Repair; if it is correct, suspect U17.
4. If there is no colour at all, check for the colour waveform at pin 21 of U17. If it is missing, suspect U17. If it is present, check for it at pin 3 of the RF modulator; if it is missing there, check R203 and L32.

On a PAL board the colour circuit inside the dashed box on Atari's schematic (Q6, the 4.433618 MHz crystal Y2 and the 74LS74 U21) also takes part. Sams' figures are for the NTSC machine.

No sync

1. Check for the sync waveform at pin 25 of the GTIA (U17). If it is missing, suspect U17.
2. If it is present, check the buffer U20, the diode CR4 and R51.

The colour control R38 sits between the RAM and the custom chips on Sams' drawing of the board

Adjust the colour

1. The colour control R38 can be reached through a hole in the bottom of the case.
2. Sams' method: switch on and adjust R38 for a blue screen on the monitor.
3. Atari's method, with the XE/XL production test cartridge: in the video test the colour tuning bar at the bottom of the screen should be one solid colour. If another bar shows inside it, adjust the trimmer until the smaller bar disappears. Atari notes that colours not aligning exactly across the screen is acceptable.

Testing

1. Check composite video at J2 pin 4 and luminance at pin 1 with the scope, and the picture on a monitor.
2. Run the self test's audio-visual test: each note of the tune should appear on the staff as it sounds; a tone with no note on screen points at ANTIC or GTIA.
3. If the production test cartridge is available, run its video test and check the twelve players, missiles and playfields, the eight luminance levels and the tuning bar. Missing or overlapping objects, flecks of wrong colour at their edges or gaps in the luminance or colour bands point at the GTIA.

Related pages

• Atari 130XE
• Atari 130XE Troubleshooting Guide
• Atari 130XE General Maintenance
• Atari 130XE Black Screen Repair
• Atari 130XE Sound Repair
• Sams Computerfacts Atari 130XE
• Atari 130XE Parts List and Production Test Instructions