Atari 130XE RAM Replacement

Atari's board drawing of the 130XE RAM area: the main bank U9 to U16 on the left, each with its decoupling capacitor (C34 to C41), and the extended bank U26 to U33 on the right (C113 to C120), with the gate array U34 below

The Atari 130XE holds its 128 KB in sixteen 64K × 1 dynamic RAM chips (Atari part C060612, 150 or 200 ns): a main bank, U9 to U16, and an extended bank, U26 to U33. Each chip stores one bit of every byte. On Atari's schematic U9 and U26 carry data line D0 and U16 and U33 carry D7, with each chip's data in (pin 2) and data out (pin 14) tied to its bit. The extended bank is reached through a 16 KB window at 16384 to 32767, switched by memory location 54017. A failed chip shows as red blocks in the self test's memory test, as a failure in Atari's production RAM test, or as a machine that does not come up at all. The repair is to find the chip that holds the failing bit and replace it with a 4164-type part.

The cost-reduced CA200519 board replaces the sixteen 4164s with four 64K × 4 chips; the bit-per-chip method below applies only to the sixteen-chip board.

Symptoms

• Red blocks in the self test's memory test. Sams describes 48 squares for the main RAM, each representing 1 KB of U9 to U16; when the self test is entered with BYE, only 40 squares appear.
• "RAM BAD AT LOCATION" from Sams' extended-memory test program (below), with the main memory passing.
• A failure in the RAM test of Atari's XE/XL production test cartridge, or its error codes 04, 05, 07, 08 or 10 (page 0 and 1 RAM, data lines, refresh and error storage).
• A blank screen at switch-on. Sams lists the RAM at U9 to U16 among the parts to check when the computer does not come up properly; work through Atari 130XE Black Screen Repair first.

Safety

• Check the 5 V supply before replacing RAM; see Atari 130XE Black Screen Repair. The board has no regulator of its own.
• Switch off before fitting or removing any chip, including for a piggyback test.

Tools and parts

• Replacement chips: 64K × 1 DRAM, 16-pin DIP. Sams' 130XE had Micron MT4264-15 parts at U9 to U15 and an NEC D4164C-15 at U16, and gives NTE4164 and ECG2164 as replacements.
• 16-pin sockets.
• A desoldering station or pump, and a logic probe.
• Optionally, Atari's XE/XL production test cartridge with its loop-back plugs, which names the failing bit.

Procedure

Open the machine

1. Remove the four screws from the bottom of the case, lift the top and disconnect the keyboard cable.
2. Remove the seven screws holding the board and lift out the board and shield. Twist the seven tabs to separate the shield halves.

Find the failing bank

1. Hold OPTION while switching on, or type BYE at the READY prompt, select MEMORY with SELECT and start it with START. A square under test is white; it turns green if that block is good and red if it is bad.
2. To test the extended bank, type and run Sams' program in BASIC. It selects each of the four 16 KB extended banks in turn by writing to location 54017 (normal value 253; 225, 229, 233 and 237 select the first to fourth extended banks), then writes 0 and 255 to every location from 16384 to 32767 and reads them back. It takes about 34 minutes.

10 PRINT "TESTING RAM"
20 POKE 54017, 225
30 GOSUB 120
40 POKE 54017, 229
50 GOSUB 120
60 POKE 54017, 233
70 GOSUB 120
80 POKE 54017, 237
90 GOSUB 120
100 PRINT "RAM CHECKS GOOD"
110 END
120 FOR X = 16384 TO 32767
130 POKE X,0
140 IF PEEK(X)=0 THEN 160
150 GOTO 210
160 POKE X,255
170 IF PEEK(X) = 255 THEN 190
180 GOTO 210
190 NEXT X
200 RETURN
210 PRINT "RAM BAD AT LOCATION"; X

1. A failure in the self test points at U9 to U16; a failure only in this program points at U26 to U33.

Find the failing chip

The data bit of each chip on Atari's schematic: U9 to U16 carry D0 to D7 in the main bank, and U26 to U33 carry D0 to D7 in the extended bank

1. With the production test cartridge, the RAM test halts on an error and shows the test number (1 to 4), the failing address, the data read, the data expected, and the bit numbers 1 to 8, one for each of the eight RAM chips, in red for a bad bit and green for a good one. Atari's instructions do not say whether bit 1 is D0 or D7; compare the data read with the data expected to find which bit differs.
2. Use the schematic to turn the bit into a chip position: in the main bank D0 is U9 and D7 is U16; in the extended bank D0 is U26 and D7 is U33.

Data bit to chip position (Atari schematic C070065)

Data bit	D0	D1	D2	D3	D4	D5	D6	D7
Main bank	U9	U10	U11	U12	U13	U14	U15	U16
Extended bank	U26	U27	U28	U29	U30	U31	U32	U33
Decoupling capacitor	C34 / C113	C35 / C114	C36 / C115	C37 / C116	C38 / C117	C39 / C118	C40 / C119	C41 / C120

1. Without the cartridge, the piggyback test described in Atari 800XL RAM Replacement works the same way on these 4164-type chips: with a known-good chip pressed on top of a suspect, the memory test passes when it sits on the failed one.
2. If the extended bank fails as a whole while the main bank passes, check the parts the extended bank does not share with the main bank. On Atari's schematic the two banks share FREDDIE's (U6) address, RAS and write lines, but each bank has its own CAS line, generated through the gate array U34 (outputs CASMAN and CASBANK) and the 33 Ω resistors R110 and R111. The bank itself is selected through location 54017.

Replace the chip

1. Check the supply pins of the suspect before removing it: pin 8 is +5 V and pin 16 ground on every RAM chip.
2. Desolder the failed chip and clear each hole. Fit a 16-pin socket, notch matching the board drawing, and fit the new 4164-type chip.
3. Refit the shield and bend its tabs, refit the board with its seven screws, reconnect the keyboard and refit the top.

Testing

1. Run the self test's memory test: every square should turn green.
2. Run Sams' extended-memory program: it should print RAM CHECKS GOOD.
3. If the production test cartridge is available, run its RAM test; it should report PASS after about 15 seconds.

Related pages

• Atari 130XE
• Atari 130XE Troubleshooting Guide
• Atari 130XE Black Screen Repair
• Atari 800XL RAM Replacement
• Sams Computerfacts Atari 130XE
• Atari 130XE Parts List and Production Test Instructions
• Atari 130XE Field Service Manual