Atari 800XL Video Output Repair

Sams' drawing of the 800XL video circuit: the GTIA outputs through the buffer U20 and resistors R47-R51 to the video amplifier Q3, then to the RF modulator and the monitor socket J2; the colour path through Q2, Q4 and Q5 below

The Atari 800XL builds its video signal from the GTIA (U17). Four luminance outputs and the composite sync output pass through the CD4050 buffer U20 and a resistor network into the video amplifier Q3. The colour signal from GTIA pin 21 is amplified by Q2, Q4 and Q5 and added to Q3's output. The result goes to the RF modulator and, as composite video and composite luminance, to 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 is somewhere along that path. Sams' service data gives a check for each stage, and Atari's manual gives the output levels at J2.

The checks follow the original board in Atari's manual and in Sams. The NTSC and early PAL 800XLs have nothing on pin 5 of the monitor socket; later PAL boards carry the chrominance signal there.

Symptoms

• No picture on a TV or monitor, while the computer otherwise responds (it beeps on the self test's audio test, for example).
• No colour, or the wrong colours. Atari's checklist gives the RF modulator and the colour trimmer R38 first, then U17, U20, Q1, Q3, CR2 and CR3, then the crystal Y1.
• The picture rolls or tears: no vertical or horizontal sync.
• No grey bars, or a missing bar, in a grey-scale test: Atari lists U17 and U20.
• A snowy picture on RF only: the TV switch box, the channel switch, the RF modulator, the RF cable or the supply.

A black screen with no READY prompt and no beeps is a different fault; see Atari 800XL Black Screen Repair. Try the monitor socket before opening the case: it does not go through the RF modulator.

Safety

The board runs only from the external 5 V supply. Check the supply first; see Atari 800XL Power Supply Replacement. If a CRT television or monitor is opened during fault-finding, follow CRT Discharge Procedure.

Tools and parts

• An oscilloscope, a logic probe and a digital meter; Sams' engineers also used a frequency counter.
• A monitor with composite input and a cable for the 5-pin DIN socket, and a TV for the RF output.
• Replacement parts as found: CD4050B (U20, Atari C010816), 2N3904 transistors (Q1-Q5), 1N4148 diodes (CR1-CR5), the 500 kΩ colour trimmer R38 (19-411504), and the RF modulator (CA061619 on domestic boards).

Procedure

Open the machine

1. Remove the six screws from the bottom cover, tip the top cover up from the left and disconnect the keyboard ground strap and the keyboard cable.
2. Remove the four board screws and lift the board and shield out of the bottom case. The shield is held together by ten screws.
3. Reconnect the keyboard and power for the checks below, keeping the board clear of the case.

Measure the monitor socket

Monitor socket J2 (Atari schematic and figure levels)

Pin	Signal	Level
1	Composite luminance	1.2-1.5 V p-p
2	Ground	–
3	Audio	600-750 mV p-p
4	Composite video	750 mV-1 V p-p
5	Not connected on the original board	–

1. Run a program that shows a stable screen, such as the READY prompt, and put the scope on J2 pin 4 and then pin 1. Compare them with the levels in the table.
2. If J2 is good and the RF picture is bad, the fault is after Q3 in the RF path: check pin 1 of the RF modulator, then the modulator, the channel switch S2 and the cable.
3. If the RF picture is good but J2 has no signal, check the joints at J2. On the schematic, pin 1 is fed from Q3's output through R55 (75 Ω) and the bead L9, and pin 4 through the bead L7 from the junction of R56 (75 Ω, the luminance) and R67 (75 Ω, the colour).

No video

Sams' sequence, from the RF modulator back to the GTIA:

1. Check for a video waveform at pin 1 of the RF modulator. If it is there, check the modulator, the channel switch S2 and the cable to the TV.
2. If it is missing at pin 1, check for the waveform at the emitter of the video amplifier Q3. If it is there, check the coil L6 and the resistor R54 between Q3 and the modulator.
3. If it is missing at the emitter of Q3, check the base of Q3. If the base has the waveform, check Q3 and the parts around it.
4. If the base has no waveform, check pin 5 of the buffer U20. If pin 5 has it, check R49 and U20.
5. If pin 5 of U20 has no waveform, check the GTIA (U17) and ANTIC (U7).

Luminance and sync inputs to the video amplifier (Sams CC11 schematic)

GTIA (U17) pin	U20 input	U20 output	Series part
31	7	6	R47, 36 kΩ
22	11	12	R48, 18 kΩ
23	5	4	R49, 9.1 kΩ
24	3	2	R50, 4.7 kΩ
25 (composite sync)	14	15	CR4, then R51, 750 Ω

No colour or wrong colour

Atari's schematic of the colour adjustment circuit: U18 drives Q1, and the 500 kΩ trimmer R38 sets the level passed on to the GTIA

1. Switch on and adjust the colour control R38 for a blue screen on the monitor, as Sams' adjustment procedure specifies.
2. If turning R38 has no effect, measure the voltage at pin 17 of the GTIA while turning R38 from one end to the other. Sams gives a range of 0 V to 8.5 V. If it does not vary, check R38 and the voltages, waveforms and parts around the colour amplifier Q1.
3. If the voltage at pin 17 varies correctly, check for 3.579545 MHz at pin 28 of the GTIA. If the frequency is wrong, check the crystal Y1 and the oscillator transistors Q8 and Q9; if it is correct, suspect the GTIA. On a PAL board Y1 is the 3.546894 MHz part.
4. With no colour at all, check the waveform at pin 21 of the GTIA. If it is missing, suspect the GTIA. If it is present, check the emitter of the colour amplifier Q5.
5. If the waveform is missing at Q5's emitter, check the voltages and parts around Q2, Q4 and Q5. If it is present there, check C54, C55, R67 and R68, which carry the colour into the composite signal.

The video buffer U20, the colour amplifiers Q2, Q4 and Q5 and, at the top right, the audio amplifier U1 on a PAL 800XL board

No sync

1. If the picture has no vertical or horizontal sync, check the waveform at pin 25 of the GTIA. If it is missing, suspect the GTIA.
2. If it is present, check the buffer U20, the diode CR4 and the resistor R51.

Missing grey levels

Atari's checklist gives U17 and U20 for no grey bars or a missing bar. Check each of the four luminance paths in the table above, from the GTIA pin through U20 to its series resistor.

Testing

1. Check J2 pins 1 and 4 against Atari's levels.
2. Run the self test's audio-visual test. A tone with no note on the staff points at a video fault in ANTIC or GTIA, and a note with no tone at an audio fault.
3. Readjust R38 for a blue screen, and on RF set the channel switch and the modulator's channel coil L201 for the best picture with the TV's AFC off.

Related pages

• Atari 800XL
• Atari 800XL Troubleshooting Guide
• Atari 800XL Black Screen Repair
• Atari 800XL Power Supply Replacement
• Sams Computerfacts Atari 800XL
• Atari 800XL Computer Field Service Manual