Atari 7800 Troubleshooting Guide

An NTSC Atari 7800 board with the RF shields removed

Fault-finding on the Atari 7800 ProSystem follows Atari's schematic (drawing C025231-001) and a six-sheet repair flowchart kept with the 7800 development documents, with community findings from ConsoleMods and AtariAge. AtariAge members report that Atari never issued a field service manual for the console. Designators are from the NTSC schematic.

Before you start

• Try a second cartridge and clean the cartridge contacts. ConsoleMods lists dirty contacts first for a black or scrambled screen.
• An NTSC 7800 will not run a PAL 7800 cartridge: the BIOS rejects it because it lacks the signature that NTSC cartridges carry.
• Atari's owner's manual chart for set-up problems: a fuzzy picture or distorted sound means the switch box or the television's fine tuning; no picture means the power switch, a cable, the switch box, the cartridge or the adaptor; interference on channel 3 is cured by switching the console and the television to channel 2.
• Atari made a 7800 diagnostic cartridge, and RevEng has written a utility cartridge for the console; AtariAge members point to both for repairs.

No power

The flowchart's power branch, with the parts from the schematic:

1. Press the power button. If +5 V appears on the chips but the LED stays dark, check the LED and its series resistor (R31, 120 Ω).
2. Check the unregulated DC at the power input. If it is missing, the power adaptor is faulty.
3. Check for 5.1 V from the zener at pin 14 of the 4013 (U10). If it is wrong, check the zener and its resistor (CR5 and R53).
4. Check that U10 pin 3 goes low when the button is pressed. If it does not, the switch connection is open (always high) or shorted (always low).
5. Check that U10 pin 13 toggles at each press. If not, replace the 4013; if a new one does not toggle either, look for a short or open in the R–C network from Q-bar to D.
6. Check that the collector of Q9 (2N3904) toggles with the button. If not, check its connections or replace it.
7. The emitter of Q10 (MJE210) should be at 9 V or more. If not, look for a break between the unregulated supply and Q10.
8. The collector of Q10 should toggle. If it stays low, replace Q10.
9. Check the input, output and ground of the 7805 (VR1); replace it if they are wrong. If they are right, look for a break or short between the regulator and the LED.

ConsoleMods adds three community findings for a dead console:

• The original adaptor has a thin cable that kinks and breaks inside. Measure its output while flexing the cable.
• The power button is a tact switch, like the other three console buttons. A dirty one shows no continuity, or intermittent continuity, when pressed. Replacements are standard 12 × 12 mm switches about 4.5 mm high.
• Look at the 2200 µF capacitor at the corner of the board for leaks or a bulged top, and at the power jack for damage and cracked joints.

Console will not switch off

If the 7800 powers up as soon as it is plugged in and cannot be switched off, ConsoleMods suspects the 4013 (U10) first and then Q10. It notes that the MJE210 is obsolete.

Powers up, nothing on screen

With the power good, the flowchart moves to the clocks and the processor lines:

Clock and control checks (7800 repair flowchart, sheets B to D)

Check	Expected	If wrong
MARIA (U1) pins 3 and 4	Oscillator running	Diagnose the oscillator: Y1 (14.31818 MHz), Q3, Q4
MARIA pin 8	Clock output	Replace MARIA
SALLY (6502, U2) pin 39	Clock	Check the path from MARIA pin 8 through its resistor to SALLY pin 37; if it is good, replace SALLY
Clock distribution	Present at MARIA pin 6, cartridge connector J1 pin 16, TIA pin 26 and 6532 pin 39
SALLY pin 40 (reset)	Low briefly at power-up, then high	Check reset at the 74LS32 output and input and the reset output of U10; repair opens between U10 and the 74LS32, replace the 74LS32 or the 4013, or check the R–C on the 4013's set input
SALLY pin 2 (RDY)	High	Find a short, or replace MARIA or the TIA if they hold it low
SALLY pin 35 (HALT)	High, or mainly high if there is a display	Find the short
SALLY pin 4 (IRQ)	Mainly high	Find the short

The flowchart's first step on this branch is to try Atari's diagnostic cartridge.

ConsoleMods also reports traces cut through by the cartridge port shroud, under its lower right corner, as a cause of a black or scrambled screen.

Picture faults

The 7800 has two sets of video outputs, from MARIA in 7800 mode and from the TIA in 2600 mode. The flowchart's video sheet:

1. Only one set of video lines should be active: MARIA's on pins 42 to 48, or the TIA's on pins 2, 5, 7, 8 and 9. If both are active, the fault is in the "encryption latch": replace the 74LS174 or the 74LS32, or repair the traces to them.
2. With a colour-bar cartridge running, CVIDEO at pin 3 of the RF modulator should show triangle-shaped pulses. If it does and the picture is still bad, replace the RF modulator.
3. The outputs of the 74LS32 in the video mixer (U3) should each equal one of their inputs; if not, replace it.
4. Check the summing resistors for the correct values and connections.

Video summing network (schematic C025231-001)

Signal	Source (MARIA / TIA)	Path to CVIDEO
Sync	MSYNC U1 pin 48 / TSYNC U8 pin 2	U3, then R10 (8.2 kΩ)
Luminance 3 / 2	MLUM3 U1 pin 44 / TLUM2 U8 pin 7	U3, then R13 (10 kΩ)
Luminance 2 / 1	MLUM2 U1 pin 45 / TLUM1 U8 pin 5	U3, then R12 (20 kΩ)
Luminance 1 / 0	MLUM1 U1 pin 47 / TLUM0 U8 pin 8	U3, then R11 (39 kΩ)
Luminance 0 (MARIA only)	MLUM0 U1 pin 42	R15 (82 kΩ)
Colour	MCOL U1 pin 43 / TCOL U8 pin 9	R16 (4.3 kΩ) / R17 (4.7 kΩ), then L1 (47 µH) and C8 (47 pF)

The Brewing Academy sells a mounting board for the UAV video upgrade that includes a CD4050 "to correct for color errors on some 7800 Maria chips". See Atari 7800 Composite Mod.

2600 games that do not run

ConsoleMods reports that C025233-002 boards with the extra timing circuit (around the 74LS02 at U13) do not run some Activision 2600 games, among them The Activision Decathlon, Robot Tank and Space Shuttle, while earlier boards have trouble with Kung-Fu Master and Dark Chambers. Some -002 boards have only a 0 Ω jumper where the circuit would be. A game failing in this way is a compatibility limit of the board revision, not a fault.

No sound

Sound comes from the TIA, mixed with the cartridge audio line, and reaches the modulator through the sound-carrier oscillator around Q1 (2N3563), L2, C4 and C5 (820 pF each). Check the television's tuning first: Atari's owner's manual traces distorted sound, like a fuzzy picture, to the switch box connection or the fine tuning.

Related pages

• Atari 7800
• Atari 7800 General Maintenance
• Atari 7800 Capacitor Replacement Guide
• Atari 7800 Composite Mod
• Atari 7800 Documentation
• Atari 2600 Troubleshooting Guide