Atari 1200XL Troubleshooting Guide

Atari 1200XL

This is the component-level troubleshooting guide for the Atari 1200XL. Chip positions, pin numbers, the symptom checklist, test procedures and the tech tips are from Atari's field service manual for the model. The hosted scan does not include the schematics, so signal paths below are given only where Atari's pin assignments name them.

Equipment

• Multimeter with AC and DC ranges, continuity and diode test
• Oscilloscope
• Atari SALT 2.05 diagnostic cartridge, which Atari specifies for the 1200XL, and a television
• The 1200XL's own 9 V AC transformer, or a known-good one (Atari CA017964)
• A known-good joystick and cartridge

Power-up checks

The 1200XL takes 9 V AC from its transformer and rectifies and regulates it on the main board. Atari specifies 9 V AC in, 31 VA, and 5 V ±5 % (1.7 A maximum) at the output of each of the two 78M05 regulators, A1 and A2.

Supply checks (Atari figures and pin assignments)

Test point	Expected
Transformer output at the power socket J10, meter on AC	9 V AC
Output of A1 and of A2, to ground	4.75–5.25 V DC
6502 (U21): pin 8 (VCC) to pin 1 or 21 (ground)	+5 V
ANTIC (U20): pin 21 (VCC) to pin 1 (ground)	+5 V
GTIA (U19): pin 27 (VCC) to pin 3 (ground)	+5 V
POKEY (U24): pin 17 (+5 V) to pin 1 (ground)	+5 V
PIA (U23): pin 20 (supply) to pin 1 (ground)	+5 V

If a regulator output is out of range, check the transformer, the power socket, the power switch, the bridge rectifier CR8 (MDA 990-2) and the regulators themselves. These are the parts Atari lists, with L4–L8, ANTIC and the crystal Y1, for a black or grey screen. The large 10,000 µF capacitor C39 holds a charge after power is removed; discharge it before working on the board.

Symptom checklist

Atari's checklist for the 1200XL:

Atari 1200XL symptom checklist (Atari section 4, p. 4-1)

Symptom	Possible causes	Atari's remedy
Snowy screen	Switch box, AC adaptor, channel select switch, RF modulator or cable damage	Adjust or replace
Black or grey screen	A1–A2 (printed "7805"), +5 V DC, L4–L8, ANTIC, CR8, power plug, SW1, Y1	Replace
Green or yellow screen	U27, U1–U9, U12, U13	Replace
Solid blue screen	U24, Q3, Y1, Q2, Q4, U19	Replace
No colour or bad colour bars	VR1 adjustment, U20, U2, U19, Q3, Y1, RF modulator	Adjust or replace
No colour bars on TIA test	U19	Replace
No lights (L1, L2, L3)	LED 3, LED pin 1, +5A, power supply, U23, Q9, Q10, pin 3, pin 4	Replace; if the LED is at fault, replace the LED board
No grey bars	RF modulator, Y1, VR1, Q3, U19	Adjust or replace
Upside-down alphanumerics or player field	U20, U19, Q3	Replace
Some or all keyboard keys fail	U24, keyboard	Replace
Tones missing during sound or games	U24, L11–L12	Adjust (audio) or replace
Console (game) switches do not work	U19, U23	Replace
ROM test failed	U12, U13, U14, U18, U22	Replace
RAM test failed	U1–U9, U7, U14, U22	Replace

In Atari's numbering, U19 is GTIA, U20 ANTIC, U21 the 6502, U22 the DDU-4 delay module, U23 the PIA, U24 POKEY, U12 and U13 the two OS ROMs and U14 the memory management HAL.

Clock and reset

The colour crystal Y1 is 3.579545 MHz (Atari C061090). On an NTSC machine the CPU runs at 1.7897725 MHz, two colour clocks per machine cycle.

Clock and reset pins (Atari figures 1-4 to 1-8)

Chip	Pin	Signal
GTIA (U19)	28	OSC, oscillator input
GTIA (U19)	29	FØ0, clock out
GTIA (U19)	30	Ø2 input
ANTIC (U20)	35	Fast phase 0 clock (FØ0)
ANTIC (U20)	34	Phase 0 clock (Ø0)
ANTIC (U20)	29	Phase 2 clock (Ø2)
ANTIC (U20)	36	Reset
6502 (U21)	37	Phase 0 clock in
6502 (U21)	39	Phase 2 clock out
6502 (U21)	40	Reset
6502 (U21)	35	HALT input
PIA (U23)	34	Reset input

With a black screen and good supplies, check for a clock at 6502 pin 37, then work back to ANTIC pin 34, ANTIC pin 35, GTIA pin 29 and GTIA pin 28. Reset comes from the RESET key through pin 10 of the keyboard connector J8.

Atari's tech tip 19 (15 August 1983) covers 1200XLs that show no picture with the Star Raiders cartridge: the reset timing to the 6502 is wrong for that cartridge, and the cure is a 0.001 µF NPO 50 V capacitor (C014181-01) soldered between pins 3 and 7 of U16.

Display faults

Colour adjustment (VR1)

1. Insert the SALT 2.05 cartridge and run the colour bar test (C, then RETURN).
2. A 15-colour rainbow appears above a reference bar, with a single colour bar below it.
3. Adjust VR1, through the top or bottom cover, until the bars directly above and below the reference bar are the same colour (goldenrod). Minor glitches at the edges of the bars are acceptable.
4. Leave the test running for at least 60 seconds to catch a bar that changes colour or blanks out.

Grey bar test

SALT 2.05's grey bar test (G, then RETURN) checks that GTIA is generating Lum 0, 1 and 2; Lum 3 is not tested. The screen should show eight equal horizontal bars from black at the top to white at the bottom, in even steps, with no colour anywhere, steady for at least 60 seconds. GTIA's luminance outputs are pins 22 (Lum 0), 23 (Lum 1), 24 (Lum 2) and 31 (Lum 3), and its colour output is pin 21.

Video sync (tech tip 12)

Atari's tech tip 12 (16 June 1983) covers loss of video sync on some televisions and monitors: on high-contrast screens the characters look wavy, torn or ghosted and the picture seems to enlarge. The fix, for affected machines only, is to change R22 from 4.3 kΩ to 6.2 kΩ, replace R188 (910 Ω) with a 1N914 diode with its cathode towards the bridge rectifier, change R163 from 100 Ω to 150 Ω, and fit an 18 kΩ resistor at the unused position R145.

RAM and ROM faults

The 1200XL has eight 64K × 1 DRAMs, one per data line, at U1–U6, U8 and U9; U7 is a multiplexer.

Built-in memory test

1. Power on with no cartridge and press HELP when the logo appears.
2. Select MEMORY and press START.
3. Two long bars at the top represent the two 8K ROMs; three rows of sixteen blocks below represent the RAM, one block per kilobyte.
4. A segment under test is white. It turns light green if it passes and red if it fails, and a failed segment is not tested again.
5. The test loops until HELP or RESET is pressed. During the RAM test the LEDs cycle one on and one off.

SALT 2.05 RAM test

1. Insert the SALT 2.05 cartridge and switch on. On a 1200XL, "ERR" in the ROM row of the SALT header and a red screen are not failures.
2. Type D, then E, then C for continuous testing, then R to run the RAM test.
3. Failures appear as red 1s. Only row D applies to the 1200XL.

Defective RAM identifier (Atari section 3, p. 3-10)

Column	Chip
D0	U9
D1	U8
D2	U6
D3	U5
D4	U4
D5	U3
D6	U2
D7	U1

OS ROMs (tech tips 18 and 18A)

Early 1200XLs have the operating system in EPROMs at U12 and U13. Atari's tech tip 18A (29 September 1983) replaces Rev. A EPROMs or ROMs with Rev. B ROMs (U12 C060617B, U13 C060616B) if they have failed or if the customer reports the SYSTEM RESET key not working properly; Rev. B differs from Rev. A only in that fix. Both chips must be changed together. On an EPROM board, remove jumpers W11, W12 and W13 and fit W6, W7, W8 and W9 before inserting the ROMs, with pin 1 of each ROM in pin position 3 of its socket.

Custom chip pins

The pin assignments below are Atari's (figures 1-5 to 1-8).

ANTIC (U20)

Pins	Signals
1 / 21	Ground / +5 V
2, 3, 5	AN0–AN2 alphanumeric data to GTIA
4	Light pen
6, 7	Interrupt input (RNMI), interrupt output (NMI)
8	Refresh
9	HALT
14, 15	Read/write, ready
10–13, 16–20, 22–28	Address bus
30–33, 37–40	Data bus

GTIA (U19)

Pins	Signals
3 / 27	Ground / +5 V
8–11	Triggers 0–3
12, 13, 14	S0, S1, S2: console key inputs
15	Keyboard click
18–20	AN0–AN2 from ANTIC
21	Colour output
22, 23, 24, 31	Luminance 0, 1, 2, 3
25	Composite sync
28, 29, 30	Oscillator in, clock out, Ø2 in

POKEY (U24)

Pins	Signals
1 / 17	Ground / +5 V
8–15	Pot scan (paddle inputs)
16, 25	Keyboard response
18–23	Keyboard scan
24	Serial input data
26, 27, 28	Bidirectional clock, serial output clock, serial output data
29	Interrupt request
37	Audio out

PIA (U23)

Pins	Signals
1 / 20	Ground / +5 V
2–9	PA0–PA7, controller inputs
10	PB0, enable/disable output
12, 13	PB2 and PB3, outputs to LED 1 and LED 2
17	PB7, self test output
18, 19	CB1 control line, CB2 command output
34	Reset input
39	CA2, motor control output

Keyboard faults

Atari's checklist gives U24 (POKEY) and the keyboard for both some and all keys failing. Before opening the machine, check L1: CONTROL+F1 disables the keyboard and lights L1.

Keyboard connector J8, 15-pin (Atari figure 1-10)

Pin	Signal	Pin	Signal
1	KR2	9	K5
2	No connection	10	Reset
3	KR1	11	S0
4	K0	12	S1
5	K1	13	S2
6	K2	14	Ground
7	K3	15	+5 V DC
8	K4

K0–K5 are POKEY's scan lines and KR1 and KR2 its response lines; S0–S2 are the console key (START, SELECT, OPTION) inputs to GTIA. If a whole row or column of keys is dead, check the cable and connector first.

Sound

For missing tones Atari's checklist names U24 and L11–L12, with the audio adjustment. L11 is the adjustable inductor in the parts list; the adjustment can be reached through the top or bottom cover. If the sound drifts and needs constant readjustment, Atari's tech tip 15 replaces the ceramic 820 pF capacitors C96 and C97 in the audio oscillator with polystyrene parts; see Atari 1200XL Capacitor Replacement Guide.

RF modulator

Atari specifies the modulator output into 75 Ω as 1–2 mV, with the sound carrier at 4.5 MHz, on channel 3 at 61.25 MHz ±0.5 MHz or channel 2 at 55.25 MHz. To adjust it:

1. Take the board out and remove the RF shield.
2. Adjust the modulator (A3) for a correct SALT colour bar screen, or set SW3 to channel 3 and adjust for 61.25 MHz with a frequency meter on the modulator's RF output.
3. Refit the shield.

Related pages

• Atari 1200XL
• Atari 1200XL General Maintenance
• Atari 1200XL Capacitor Replacement Guide
• Atari 800XL Troubleshooting Guide