Atari 1200XL General Maintenance

Atari 1200XL

This is the maintenance guide for the Atari 1200XL. Much of what is written about the 600XL and 800XL does not apply to it: the 1200XL takes 9 V AC and has its rectifier and regulators on the main board, and its SIO and monitor ports differ from the later XLs.

Designators, part numbers and figures are from Atari's field service manual for the model. The 1200XL was made only from January to July 1983, in an NTSC version for North America, and Karl Heller estimates from serial numbers that fewer than 120,000 were built.

Differences that affect diagnosis

These are the points on which the 1200XL differs from the rest of the family:

• It has two controller ports, not four.
• It has no Memo Pad mode. With no cartridge and no powered drive it boots to the "ATARI" logo screen, and HELP from there starts the self test.
• The monitor socket has no chrominance signal on pin 5.
• Like every XL and XE, the SIO socket has no +12 V on pin 12. The 1200XL is also the only Atari without +5 V on SIO pin 10, because of the current-limit resistor R63.
• Key clicks and system beeps come from the television or monitor speaker, not from a speaker in the case as on the Atari 400 and Atari 800.

Regular cleaning

Opening the case

Atari's sequence:

1. Turn the console upside down and remove the six screws holding the bottom cover to the top cover.
2. Lift out the bottom cover with the board in it and turn it right side up.
3. Remove the six screws holding the board, and lift the board and the cartridge connector out of the bottom cover.
4. Remove the eleven push rivets from the shields, then separate the top and bottom shields and the fish paper, taking care not to damage the board.

When reassembling, keep the adjustment holes in the shields clear.

Board and connectors

• Blow the dust off the board and the shields.
• Clean the cartridge connector J4 (30-pin) and the SIO connector J1 (13-pin, right angle) with 99 % isopropyl alcohol on a lint-free swab.
• Look around the bridge rectifier CR8 and the regulators A1 and A2 for heat discolouration and corrosion.

Case and keyboard

• Clean the aluminium trim and the plastics with a damp microfibre cloth. Do not use abrasives on the brushed aluminium.
• To remove the keyboard, take off the top cover and unplug the keyboard connector and the LED connector, noting the position of the LED connector. Four screws hold the keyboard to the top cover. When refitting, do not twist the LED cable.
• Clean keycaps in mild soapy water and dry them completely. If keys are unresponsive, clean the membrane contacts with isopropyl alcohol without creasing the membrane.

Power supply

Atari's description: "The 1200XL has a linear power supply. The power supply electronics is resident on the motherboard. The power supply accepts a 9VAC input through the power jack. An external step-down transformer accepts 115 VAC from the power line and steps it down to 9VAC. The 9VAC input is rectified by a bridge rectifier and then regulated power outputs from the regulators are bussed on separate power rails to power distinct sections of the motherboard. All power rails have a common ground."

Atari 1200XL power supply specification (Atari, section 1)

Parameter	Value
Input voltage	9 V AC
Input power	31 VA
DC output of each regulator	5 V ±5 % (1.7 A maximum)

5 V ±5 % is 4.75 V to 5.25 V at the regulator outputs.

External transformer

The external transformer is Atari part CA017964 and its output is 9 V AC, not DC. Measure it with the meter on AC volts, and check its cable and plug, the power socket J10 and the socket's solder joints.

On-board regulation

Atari 1200XL power supply parts (Atari parts list)

Designator	Part	Atari part number
J10	Power socket	C014715
CR8	Bridge rectifier, MDA 990-2	C060534
C39	Capacitor, 10,000 µF 16 V	C060603
A1, A2	Voltage regulator, 78M05, 5 V	C014348
Not given	Heatsink	C060531
Not given	Heat sink assembly	CA061135
Not given	Latching fastener (heatsink)	C014796

To check the rails, connect the transformer, switch on, and measure the output of each regulator to ground. Atari's outline drawing gives the regulator pins as input, ground and output; measure between the output and ground pins and expect 4.75–5.25 V.

• C39 is by far the largest capacitor on the board. The parts list does not give its position, but at 10,000 µF it is the supply's filter capacitor, and it holds a charge after power is removed. Discharge it through a resistor before working on the board, and do not short it with a probe.
• Check that the heatsink is properly fixed to A1 and A2 and that its latching fastener is secure.
• The small-signal diodes CR1, CR2, CR6 and CR16–CR21 are 1N4148 (Atari C060607).

The capacitor list is on Atari 1200XL Capacitor Replacement Guide.

Connector care

SIO socket

J1 is a 13-pin right-angle connector. The 1200XL provides neither +12 V on pin 12 nor +5 V on pin 10. A peripheral that takes its power from the SIO socket will not work on a 1200XL without its own supply; this is not a fault in the computer. The FAQ notes that fitting a jumper in place of R63 puts +5 V on pin 10.

Controller jacks

J5 and J6 are 9-pin D connectors. Pin 7 is +5 V and pin 8 is ground.

Cartridge slot

The cartridge connector J4 is on the left side of the case. Clean it with isopropyl alcohol, and clean a cartridge's own contacts before blaming the computer. Check that the cartridge guide (C060097) and connector retainer (C060098) are intact.

LED board

The indicator board carries three LEDs (Atari pilot lamp C014776) and connects to the main board with a five-wire flat cable:

LED board cable (J7)

Pin	Function
1	LED 3, power on
2	No connection
3	LED 2, international character set
4	LED 1, keyboard disabled
5	Ground

L1 lights when the keyboard is disabled and L2 when the international character set is selected; normally both are off. CONTROL+F1 toggles the keyboard and CONTROL+F4 the character set, so a 1200XL with L1 lit and a dead keyboard may simply have its keyboard switched off.

If the LEDs do not light, Atari's checklist names LED 3, LED pin 1, the +5A supply, U23 (PIA), Q9, Q10 and cable pins 3 and 4.

Common failure points

Atari's symptom checklist for the 1200XL:

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

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

Chip map

Atari 1200XL integrated circuits (Atari parts list)

Location	Device	Atari part number
U1–U6, U8, U9	DRAM, 64K × 1	C060612
U7, U10	74LS158 multiplexer	C014345
U11	74LS375	C060613
U12	OS ROM A, 8K × 8	C060617
U13	OS ROM B, 8K × 8	C060616
U14	Memory management unit (HAL)	C060609
U16	74LS14	C016541
U17	74LS08	C017097
U18	74LS138 decoder	C014344
U19	GTIA	C014805
U20	ANTIC	C012296
U21	6502 (modified)	C014806
U22	DDU-4 delay module	C060472
U23	PIA	C014795
U24	POKEY	C012294
U25	74LS51	C060474
U27	CD4050B hex buffer	C010816
A3	RF modulator	C060618

U7 is a multiplexer, not RAM; the eight DRAMs are U1–U6, U8 and U9.

RAM failures

Symptoms are a garbled display, random characters, crashes, failure to boot, or red blocks in the memory test. The built-in memory test is reached by pressing HELP at the logo screen; during the RAM test the LEDs cycle one on and one off, and they are left off when the test exits.

With the SALT 2.05 cartridge, only row D of the RAM test display applies to the 1200XL, and each red 1 in that row identifies one chip:

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

Operating system revisions

The 1200XL shipped with OS revision 10 (26 October 1982), later replaced by revision 11 (23 December 1982). Atari's tech tip 18A calls these Rev. A and Rev. B: the only difference is that Rev. B corrects the function of the System Reset key, and Atari replaced Rev. A only when it had failed or the customer complained about System Reset. The two ROMs (U12, C060617B and U13, C060616B for Rev. B) must be changed as a pair. Early machines have EPROMs in place of the ROMs, and converting them needs jumper changes on the board (W1, W6–W9, W11–W13 and R44).

Faults that are not the computer

• The transformer: measure it on the AC range first.
• An SIO peripheral that relies on power from the SIO socket.
• A dirty cartridge, which can give blank screens and lock-ups that look like board faults.
• A disabled keyboard: check L1 before opening anything.

Adjustments

Atari documents three adjustments, two of which can be made without taking the board out:

• Sound: through the top or bottom cover.
• Colour (VR1): through the top or bottom cover. With SALT 2.05's colour bar test, adjust until the bars directly above and below the reference bar are the same colour.
• RF modulator: the shield must be removed. Adjust for a correct colour bar test screen, or set SW3 to channel 3 and adjust the modulator to 61.25 MHz with a frequency meter on the modulator's RF output (A3).

Related pages

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