Atari 800XL Black Screen Repair

Atari's schematic of the 800XL clock oscillator (Q8, Q9, Y1, C109) on the right and the power-on reset circuit (C49, R39, CR1, U19) on the left

An Atari 800XL that powers up with a black or grey screen and never reaches the READY prompt or the self test has stopped before the 6502C could run the operating system. Atari's symptom checklist gives the causes in order: first the clock circuit, Y1, Q8, Q9 and C109, then the ICs U2, U7 to U20, U22, U23 and U26 to U30. Sams' service data for the 800XL works through the same faults stage by stage: the supply, the reset circuit, the clock chain from the crystal to the processor, and then activity on the processor's address and data pins. Both documents describe the original board, and the frequencies Sams gives are for the NTSC machine.

Symptoms

• A black or grey screen at switch-on, with or without the power light, and no response to SYSTEM RESET.
• Other screen colours point elsewhere in Atari's checklist: a red or brown screen at U2 to U5, U18, U19, U28 and U30; a solid blue screen at U2; a yellow screen at U28; a snowy screen at the RF path or the supply.
• A picture with no sync, or no colour, is a video fault; see Atari 800XL Video Output Repair. Red blocks in the memory test are covered in Atari 800XL RAM Replacement.

Safety

• Check the supply first and do not run the board from a supply that reads high; nothing on the board regulates it. See Atari 800XL Power Supply Replacement.
• Switch off before pulling or fitting any chip. The custom chips (ANTIC, GTIA, POKEY and the 6502C) are no longer made.

Tools and parts

• A digital meter, a logic probe and an oscilloscope or frequency counter: Sams lists all of these among the test equipment its engineers used.
• Replacement parts as found: 2N3906 transistors for Q8 and Q9, a 3.579545 MHz crystal for Y1 on NTSC boards (Atari C061090), 0.1 µF for C109 and 47 µF for C49.
• For swapping: known-good custom chips from a working XL, if available.

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. The checks below can be made with the board in the bottom housing. To lift it out, remove the four board screws and lift the front of the board while pushing out the right side of the housing until the controller ports clear.
3. The shield is held together by ten screws.

Check the supply

1. With the power pack unplugged from the computer, measure 5.0 V from pin 3 to pin 1 of its plug.
2. With the pack connected and the computer switched on, measure 5.0 V on the positive end of C2. If it is missing, check the power switch S1.
3. Atari's limits for the supply are +5 V ±2 %, 4.90 to 5.10 V.

Check the reset circuit

1. Put the logic probe on pin 8 of U19 (74LS14) and switch on. It should read low for a moment and then go high and stay high. Pressing SYSTEM RESET should repeat that.
2. If it does not, check C49, CR1, R39, R40 and U19. On Atari's schematic C49 (47 µF) charges from +5A through R39 (56 kΩ), CR1 discharges it, and its voltage passes through two sections of U19, pins 11 to 10 and 9 to 8.

Follow the clock chain

Sams' drawing of the oscillator with its DC readings: 2.5 V at the collector of Q8 and 4.4 V at both emitters

Work from the crystal towards the processor and stop at the first point where the signal is missing.

1. Check for a 3.579545 MHz waveform at the collector of the oscillator transistor Q8. If it is missing, check the voltages and the parts around Q8 and Q9; Sams' readings are 2.5 V at the collector of Q8 and 4.4 V at the emitters of Q8 and Q9. If the oscillator runs but is off frequency, check the crystal Y1.
2. If Q8's collector is good, check for the waveform at pin 29 of the GTIA (U17). If it is missing there, suspect U17.
3. If GTIA pin 29 is good, check for 1.78977 MHz at pin 34 of ANTIC (U7). If it is missing, suspect U7.
4. If ANTIC pin 34 is good, check for clock pulses at pin 6 of U18 (74LS08). If they are missing, suspect U18.
5. Next check for pulses at pins 3 and 39 of the CPU (U8). If either is missing, suspect U8.
6. Check for pulses at pin 11 of U18. If they are missing while pin 13 reads high, suspect U18.
7. Check for pulses at pin 2 of U19, then at pins 4, 6, 8, 10 and 12 of the delay line U29. A missing output points at the chip feeding it.

Clock checkpoints on an NTSC 800XL (Sams CC11)

Point	Expected
Collector of Q8	3.579545 MHz waveform
GTIA (U17) pin 29	Waveform present
ANTIC (U7) pin 34	1.78977 MHz
U18 pin 6	Clock pulses
CPU (U8) pins 3 and 39	Clock pulses
U18 pin 11	1.78977 MHz clock (pin 13 high)
U19 pin 2	Pulses
Delay line U29 pins 4, 6, 8, 10, 12	Pulses

On a PAL board the crystal is the 3.546894 MHz part shown on Atari's schematic, and the processor runs at 1.7734470 MHz instead of the NTSC machine's 1.7897725 MHz.

The oscillator area of a PAL 800XL board, with Q8, R109, C109 and the jumper W2 marked on the silkscreen

Check the processor buses

1. With the clocks present, put the logic probe on the CPU's address pins (pins 9 to 20 and 22 to 25) and data pins (26 to 33). There should be pulses on each.
2. If pulses are missing, or a pin is stuck high or low, switch off, remove U8 and switch on again. With the CPU out, the address pins should read open and the data pins high. A pin that still reads stuck points at whatever else drives that line.
3. If the data pins show pulses at switch-on or after pressing Reset and then stop, check the address decoder U2, the memory management unit U3, the operating system and BASIC ROMs U4 and U5, and the CPU U8.

Isolate the remaining chips

1. Sams lists the CPU (U8), GTIA (U17), POKEY (U22), ANTIC (U7) and the RAM (U9 to U16) as the parts to check when the computer does not come up properly.
2. Atari's parts list gives sockets for U7, U8, U17, U22 and U23 among others, which makes swapping a suspect for a known-good chip practical. Switch off for every swap.
3. Replace the failed part, then refit the board and the covers, reconnecting the keyboard cable and ground strap.

Testing

1. Switch on with no cartridge and no disk drive: the READY prompt should appear in BASIC.
2. Hold OPTION while switching on to run the self test, and run all three tests.
3. On an NTSC machine, check that ANTIC pin 34 carries 1.78977 MHz.

Related pages

• Atari 800XL
• Atari 800XL Troubleshooting Guide
• Atari 800XL Power Supply Replacement
• Atari 800XL RAM Replacement
• Sams Computerfacts Atari 800XL
• Atari 800XL Computer Field Service Manual