Atari 130XE Black Screen Repair

Sams' interconnecting diagram for the 130XE: the numbered circles mark where each service check is made, and the power pack plug is shown with 5.0 V between pins 3 and 1

An Atari 130XE that stays on a black screen at switch-on, or does nothing at all, has either lost its 5 V supply or stopped before the 6502C (SALLY, U8) could run the operating system. Sams' service data for the 130XE gives a check for each stage in order: the power pack and switch, the reset circuit, the clock chain from the 14.31818 MHz oscillator through FREDDIE (U6) to the processor, and then activity on the processor's address and data pins. Its frequencies are for the NTSC machine.

Symptoms

• Nothing happens at switch-on: no picture and no response from the keyboard ("computer dead" in Sams' checks).
• The machine powers up but does not come up properly: a blank screen, no READY prompt and no self test.
• A picture with no colour or no sync is a video fault; see Atari 130XE Video Output Repair. Red blocks in the self test's memory test are covered in Atari 130XE RAM Replacement.

Safety

• The 130XE has no regulator of its own: it takes +5 V from the external power pack through the 7-pin socket J7 and splits it into three rails, +5A, +5B and +5C, through the ferrite beads L1, L2 and L3. A supply that reads high reaches every chip.
• The FAQ warns that one version of Atari's 5 V pack, the solid black "Ingot", can damage the computer if it fails and is not switched off quickly, and advises disconnecting it if hum bars appear on the screen.
• Switch off before pulling or fitting any chip. ANTIC, GTIA, POKEY, FREDDIE and the 6502C are no longer made.

Tools and parts

• A digital meter, a logic probe and an oscilloscope or frequency counter, all on Sams' list of test equipment.
• A known-good 5 V power pack. Atari's part is C061982, the same 5 V 1.5 A pack used with the 600XL, 800XL and 65XE. Building or fitting a replacement is covered in Atari 800XL Power Supply Replacement, which uses the same plug and pinout.
• Replacement parts as found: a 555 for U19 (Atari C019748), 0.1 µF for C200, 56 kΩ for R39, 100 Ω for R40, and for Y1 a 14.31818 MHz crystal (C070032) or oscillator module (C070024) on an NTSC board, or a 14.187576 MHz crystal (C070034) on a PAL board.

Procedure

Check the supply

Atari's schematic of the power input: J7 pins 1, 4 and 6 are +5 V, 3, 5 and 7 ground; S1 switches the supply to C1, C2 and the three filtered rails

1. Unplug the power pack from the computer and measure between pins 3 and 1 of its plug. Expect 5.0 V. If it is missing, check the plug and cable for an open circuit; if they are good, the pack has failed.
2. Before plugging in a replacement pack, measure across pins 3 and 1 of the computer's socket J7 with the power switch S1 on. If there is a short, do not plug in a new pack until the short is repaired.
3. With a good pack connected and the computer on, check for 5.0 V on the positive end of the electrolytic C2. If it is missing, check S1.

On Atari's schematic J7 pins 1, 4 and 6 carry +5 V and pins 3, 5 and 7 ground, with pin 2 the shield ground.

Open the machine

1. Remove the four screws from the bottom of the case. Lift the top up and disconnect the keyboard cable; the top can then be removed.
2. Remove the seven screws holding the board to the bottom of the case and lift out the board with its metal shield.
3. To take the shield off, twist the seven tabs that hold it together. Reconnect the keyboard and the power pack for the checks below.

Check the reset circuit

The reset circuit: the 555 timer U19 with R39, C200 and R40; its output, pin 3, drives the RESET input of the CPU, pin 40

1. Put the logic probe on pin 3 of the reset IC, U19. At switch-on, and when the Reset key is pressed, it should read low for a moment and then go high and stay high.
2. If it does not, check C200, R39, R40 and U19. On Atari's schematic U19 is a 555 timer whose output, pin 3, drives the RESET input of the 6502C at pin 40.

Follow the clock chain

FREDDIE (U6) with the MMU, the RAM and, bottom right, the 14.31818 MHz oscillator: the crystal Y1 with Q4 and Q5, or the alternative oscillator module

1. Check for a 1.78977 MHz clock at pin 25 of the PIA, U23. If it is present and on frequency, the clock chain is working; go on to the processor buses.
2. If it is missing or off frequency, check for 14.31818 MHz at pin 2 of FREDDIE (U6). If that is missing or off frequency, check the oscillator Y1.
3. If pin 2 is good, check for 3.579545 MHz at pin 37 of U6. If it is missing, suspect U6.
4. Check for 1.78977 MHz at pin 5 of U6. If it is missing, suspect U6.
5. If pin 5 is good, check for 1.78977 MHz at pin 30 of the GTIA (U17). If it is missing there, suspect U17.

Clock checkpoints on an NTSC 130XE (Sams CC16)

Point	Expected	If missing or wrong
PIA (U23) pin 25	1.78977 MHz	Work through the points below
FREDDIE (U6) pin 2	14.31818 MHz	Oscillator Y1
FREDDIE (U6) pin 37	3.579545 MHz	U6
FREDDIE (U6) pin 5	1.78977 MHz	U6
GTIA (U17) pin 30	1.78977 MHz	U17

On Atari's schematic the 14.31818 MHz source is either the crystal Y1 with the transistors Q4 and Q5 (2N3904) or an oscillator module in its place. A PAL board uses a 14.187576 MHz crystal (C070034), and the processor runs at 1.7734470 MHz instead of the NTSC machine's 1.7897725 MHz.

Check the processor buses

1. With the clocks present, put the logic probe on the address pins of the CPU, U8 (pins 9 to 20 and 22 to 25), and on its data pins (26 to 33). There should be pulses on each.
2. If pulses are missing or a pin appears 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 is held by something else on that line.
3. If pulses are missing with U8 fitted, watch the data pins while switching on or pressing Reset. If pulses appear and then suddenly stop, check the address decoder U2, the memory management unit U3, the BASIC and operating system ROMs (U4 and U5) and the CPU U8.

ANTIC (C021697-01) soldered directly to a 130XE board, with no socket

Isolate the remaining chips

1. Sams' check for a machine that powers up but does not come up properly names the CPU (U8), GTIA (U17), POKEY (U22), ANTIC (U7) and the RAM (U9 to U16).
2. On the board in the photograph ANTIC is soldered in without a socket, so a suspect chip may have to be desoldered before a known-good part can be tried. Fit a socket in its place.
3. Refit the shield and bend its tabs over, refit the board with its seven screws, reconnect the keyboard and refit the top with its four screws.

Testing

1. Switch on with no cartridge and no disk drive; the BASIC READY prompt should appear.
2. Hold OPTION while switching on, or type BYE at the READY prompt, to bring up the self test menu, and run all the tests.
3. On an NTSC machine, check that PIA pin 25 carries 1.78977 MHz.

Related pages

• Atari 130XE
• Atari 130XE Troubleshooting Guide
• Atari 130XE General Maintenance
• Atari 130XE RAM Replacement
• Atari 130XE Video Output Repair
• Atari 800XL Power Supply Replacement
• Sams Computerfacts Atari 130XE
• Atari 130XE Parts List and Production Test Instructions