Atari 800XL RAM Replacement

The eight 64K × 1 RAM chips on a PAL 800XL board, soldered in and marked U9, U10, U11 and U12 from the top, then U16, U15, U14 and U13. The two 74LS158 address multiplexers U27 and U26 are to their right.

The Atari 800XL holds its 64 KB of RAM in eight 64K × 1 dynamic RAM chips, U9 to U16 (Atari part C060612). Each chip stores one bit of every byte: on Atari's schematic U9 is data line D0 and U16 is D7. A failed chip shows as red blocks in the self test's memory test, or stops the machine starting. Terry Stewart's 800XL had two failed chips, both killed by a faulty power supply that was delivering 7.6 V. The repair is to find the chip that holds the failing bit and replace it with a 4164-type 64K × 1 DRAM.

Atari's parts list gives sockets for U9 to U16, but on some boards the chips are soldered in. Both the PAL board photographed by Multicherry and Terry Stewart's machine have Micron MT4264-15 chips soldered directly to the board.

Symptoms

• Red blocks in the RAM part of the self-test memory test. Two long bars at the top stand for the 16 KB operating system ROM and the 48 blocks below for the RAM, 1 KB each; a block turns light green if it passes and red if it fails.
• On Terry Stewart's 800XL the memory test screen with a failed block appeared at switch-on.
• Atari's symptom checklist gives the causes of a RAM test failure as U2, U3, U9 to U16, U18, U19 and U26 to U30, and says to check that ANTIC is revision E. U26 and U27 are the 74LS158 multiplexers whose outputs drive the address inputs of all eight chips; U28 is a 74LS375 latch, U29 the delay module and U30 a 74LS51.

The memory test does not test the top 16 KB of RAM. A RAM fault can also stop the machine starting at all: Atari's list of causes for a black or grey screen runs from U7 to U20, which takes in the RAM, and Sams lists the RAM among the parts to check when the computer does not come up properly. For a red or brown screen Atari lists the ROMs and decoding logic, not the RAM. Start with the Atari 800XL Troubleshooting Guide if the symptom does not match.

Safety

• Test the power supply before powering the board again. A supply that delivers more than 5 V kills RAM chips, and Terry Stewart's second chip failed when he tried the machine's original supply after the first repair. See Atari 800XL Power Supply Replacement.
• Desoldering a 16-pin chip takes heat. Use a temperature-controlled iron and a desoldering tool, and do not lift the pads; see Recommended Tools.

Tools and parts

• Replacement chips: 64K × 1 DRAM, 16-pin DIP. Sams lists the fitted part as an HM4864P-2 (Atari C060612) and gives General Electric 4164-15, NTE4164 and ECG2164 as replacements. Terry Stewart replaced a failed Micron MT4264-15 with an NEC D4164C and the machine ran BASIC programs normally. Check the maker's prefix: the Matsushita MN4264 is a 16K × 4 part, not a 64K × 1 part like the Micron MT4264.
• A known-good chip of the same type for the piggyback test.
• 16-pin sockets for boards whose RAM is soldered in.
• A multimeter; a desoldering iron or pump and braid.
• Optional: a diagnostic that reports the failing data, such as Atari's SuperSALT cartridge (FD100335) used in the field service manual, or the XE/XL production test.

Procedure

Check the supply

1. Unplug the supply from the 800XL and measure it at its plug: pins 1, 4 and 6 are +5 V and pins 3, 5 and 7 ground, and Atari's limits are 4.90 to 5.10 V.
2. Do not power the board from a supply that reads high.

Open the machine

Atari's disassembly sequence:

1. Turn the unit upside down and remove the six screws from the bottom cover.
2. Turn it upright and tip the top cover by raising its left side. Disconnect the keyboard ground strap and pull the keyboard cable out of its socket, then set the top aside.
3. Remove the four screws holding the board.
4. Lift the front of the board while pushing out the right side of the bottom housing, where the controller ports are, until the ports clear it, and lift the board out.
5. Keep each machine's top and bottom housings together; Atari warns that they are not interchangeable between units.

The board sits between a top and a bottom shield (C024467-001 and C024468-001). Sams' instructions are to remove the ten screws that hold the shield together and lift it off the board.

Find the failed chip

Atari's schematic of the RAM: each chip's data pins 2 and 14 go to one data line, U9 to D0 through U16 to D7

Data line held by each RAM chip (FD100740 schematic)

Data line	D0	D1	D2	D3	D4	D5	D6	D7
Chip	U9	U10	U11	U12	U13	U14	U15	U16
Decoupling capacitor	C34	C35	C36	C37	C38	C39	C40	C41

Use whichever of these suits the machine.

From a diagnostic's error report

1. A diagnostic that halts on a RAM error shows the failing address, the data read and the data expected. Atari's XE/XL production test prints them as, for example, "7E45=34 35".
2. Compare the two values bit by bit. In that example 34 and 35 differ only in the lowest bit, so the failing data line is D0.
3. Read the chip for that data line from the table above: D0 is U9. If several bits fail at once, suspect the shared parts in Atari's list first: U2, U3, U18, U19 and U26 to U30.

Piggyback test

Terry Stewart's piggyback test: a known-good NEC 4164 pressed on top of the chip at U14

Terry Stewart found his failed chip this way. With a known-good 4164 piggybacked over one of the original chips, the machine passed its memory test and started; that chip was the failed one. He found that the method often works on the 800XL but not always, and another owner reported that piggybacking made his machine stop working altogether. A 16-pin chip cannot be piggybacked on an 18-pin 64K × 4 part.

1. Switch off. Press the known-good chip onto the first RAM chip, with its notch at the same end, so that every pin touches the pin below it.
2. Switch on and run the memory test (hold OPTION at switch-on, then select Memory).
3. Switch off and move the good chip to the next RAM chip. Stop at the chip whose piggyback lets the memory test pass.

Replace the chip

1. With the board out, desolder the failed chip. Clear each hole. Terry Stewart cut his failed chip out before soldering in a socket.
2. On a board with soldered RAM, fit a 16-pin socket in its place so the next replacement needs no desoldering.
3. Fit the new chip with its notch at the same end as the others. On Atari's schematic pin 8 is the supply and pin 16 ground; check that pin 8 reads +5 V to pin 16 once the board is powered.
4. Inspect the joints under magnification.
5. Refit the shields and the board in reverse order, reconnecting the keyboard cable and the keyboard ground strap.

Testing

1. Run the self test's memory test and leave it for several passes; every RAM block should turn light green. Do not leave a self-test screen up for more than about seven minutes, because the self test has no attract mode.
2. The self test does not test the top 16 KB of RAM. A RAM test such as SuperSALT's or the XE/XL production test, which reports PASS or FAIL after about 15 seconds, covers the whole 64 KB.
3. If the failure moves to another chip after a few hours, check the supply again; Terry Stewart's second failure was caused by his supply.

Revision notes

• Atari's manual describes one board, a domestic/U.K. version with the U.K. additions shown in dotted boxes. The later PAL "800XLF" board uses the FREDDIE memory controller, and the FAQ lists a separate SECAM board; no Atari schematic for either is hosted here, so read the chip designators off the board before relying on the table above.
• The Atari 600XL has two 16K × 4 chips at U11 and U12 instead; see Atari 600XL Troubleshooting Guide.

Related pages

• Atari 800XL
• Atari 800XL Troubleshooting Guide
• Atari 800XL Power Supply Replacement
• Atari 800XL General Maintenance
• Atari 800XL Computer Field Service Manual