Atari 800XL Keyboard Repair

A 600XL/800XL keyboard of the metal-back-plate type opened: the folded mylar on the keyboard body (top) and the back plate (bottom). Photo: Best Electronics.

The keyboard of the Atari 800XL plugs into the 24-pin pinch connector J8 on the main board. POKEY (U22) scans the key matrix through the two CD4051 multiplexers U24 and U25 and reads the keys back on its keyboard response lines; START, SELECT and OPTION go to the GTIA (U17) instead, and RESET goes to the reset circuit. Faults are either in the keyboard itself, in the cable and connector, or in the scanning chips, and Sams' service data gives a check that separates them, including a chart of which keys connect to each J8 pin.

Atari's parts list gives the keyboard as C061983. Best Electronics sells 600XL/800XL keyboards in two versions, a membrane ("mylar") keyboard with a metal back plate and a circuit-board keyboard with mechanical switches, and the FAQ records that five types of 600XL/800XL keyboard have been documented. The keyboard repair below covers the mylar type, following Best Electronics' instructions for fitting its replacement mylar.

Symptoms

• The whole keyboard is dead.
• A group of keys does not work: often a whole row or column, which share one J8 pin (see the chart below).
• One key is erratic.
• START, SELECT or OPTION, or SHIFT, CONTROL or BREAK, do not work.
• A key is permanently pressed and repeats its character. On a mylar keyboard Best traces this to an over-length contact spring.
• Wrong characters appear when a key is pressed.
• No click when a key is pressed, though other sound works: a GTIA or audio fault, covered in Atari 800XL Sound Repair.

Safety

The keyboard's metal back plate can short the pins of the chips under it if the keyboard is laid on the board for testing; Best lays paper over the chips first. Unplug the supply before pulling the keyboard tail from J8.

Tools and parts

• A small and a medium Phillips screwdriver, a small container for the screws, small diagonal cutters (for trimming springs), a torch and a soft brush.
• An ohmmeter, a logic probe and an oscilloscope.
• A replacement mylar for the metal-back-plate keyboard (Best Electronics CB101055-2nd), or a complete keyboard; the 24-pin pinch connector on the board is Atari C061793.
• Spare CD4051B chips (Atari C014336) for U24 and U25.

Procedure

Test the keyboard

1. Hold OPTION while switching on and choose the keyboard test. Each key pressed shows in inverse video on the screen and sounds a tone; CONTROL and SHIFT change only when pressed with another key, and RESET, HELP and BREAK make no tone.
2. Note which keys fail and look them up in the chart below.

Keys on each J8 pin (Sams CC11; routing from Atari's schematic)

J8 pin	Driven from	Keys
10	U24	6, 7, Y, U, N
11	U24	H, J, SPACE
12	U24	5, 8, T, I, G, K, B, M
13	U24	4, 9, R, O, F, L, V, comma
14	U24	3, 0, E, P, D, semicolon, C, full stop
15	U24	2, <, W, −, S, +, X, /
16	U24	1, >, Q, =, A, *, Z, inverse key
17	U24	ESC, BACK S, TAB, RETURN, CAPS
1	U25	BREAK, 7, 8, 9, 0, <, >, BACK S
2	U25	ESC, 1, 2, 3, 4, 5, 6
3	U25	U, I, O, P, −, =, RETURN
4	U25	TAB, Q, W, E, R, T, Y
5	U25	J, K, L, semicolon, +, *, CTRL
6	U25	A, S, D, F, G, H, LOWR
7	U25	N, M, comma, full stop, /, inverse key, SPACE
8	U25	SHIFT, Z, X, C, V, B

Atari's schematic of the keyboard interface: U24 and U25 drive J8 through the resistor networks RN1-RN4; J8 pin 9 is KR2 and pins 20-22 are the console keys

Whole keyboard dead

Sams' sequence:

1. Check J8 for good connections and the ribbon or mylar tail for damage.
2. If they are good, check the waveforms at pins 18 to 23 of POKEY (U22). If they are missing, suspect U22.
3. If POKEY's pins 18 to 23 have waveforms, check the waveform at pin 3 of U24 while pressing any key other than SHIFT, CONTROL, BREAK or the function keys. If it is missing, check U24 and U25. If it is present, check the GTIA (U17) and U22.

A key or group of keys dead

1. Find the J8 pin or pins that the dead keys share in the chart above.
2. With the power off, use the ohmmeter to check the dead keys for continuity; Sams gives about 55 Ω across each key.
3. If the keys measure good, switch on and look with the scope for pulses at J8 while pressing the dead keys. A logic probe may show pulses even when no key is pressed.
4. If the pulses are missing, check the resistor networks between J8 and U24 and U25, and U24 and U25 themselves. Atari's schematic shows which network serves each pin: RN3 and RN4 on the U24 side, RN1 and RN2 on the U25 side.
5. Best Electronics' technician had an owner swap the two keyboard interface chips on the board to see whether the fault moved; it did, and two new chips cured it.

START, SELECT, OPTION, SHIFT, CONTROL or BREAK dead

1. For START, SELECT and OPTION, check the logic levels at pins 12, 13 and 14 of the GTIA. Each should read low while its key is pressed: pin 12 START, pin 13 SELECT, pin 14 OPTION. If they do, suspect U17; if not, check J8, the cable and R134, R135 and R136. On the schematic these lines are J8 pins 20, 21 and 22.
2. For SHIFT, CONTROL and BREAK, check the waveform at pin 16 of POKEY while pressing them. If it is present, suspect U22. If not, check pin 5 of U25; if that is missing, suspect U25; if it is present, check R96, R137 and RN14 (a section of RN1), and the connections at J8 pins 9 and 8.

One key erratic, or wrong characters

• Clean an erratic key.
• If wrong characters appear, check ANTIC (U7), GTIA (U17) and POKEY (U22).

Open a mylar keyboard

Best Electronics' procedure for the metal-back-plate keyboard:

1. Remove the screws from the bottom case and turn the computer upright.
2. Lift the top case about 2 to 3 inches. Reach in from the right-hand end and, holding the mylar tail as close to the connector as possible with fingers of both hands, pull it out of the 24-pin pinch connector evenly.
3. Turn the top case over and remove the four screws that hold the keyboard to it.
4. Lay the keyboard face up on white paper, mylar tail to the right. Note the hump in the mylar between the main keyboard and the separate function-key module, and the two silver pads under the LED; they are references for reassembly.
5. Turn the keyboard over. Remove the four or six small screws that hold the function-key module and lift it away with its springs facing up; tipping it lets its five small springs fall out.
6. Remove the remaining small screws from the metal back plate and lift the plate off. The mylar usually stays on the keyboard body.
7. Check that the back plate is flat. A plate bent by a knock can leave rows of keys dead.

Inside the keyboard

The keyboard body with the mylar removed. Best's red circle marks a contact spring standing higher than the rest, which keeps its key pressed.

Each key has a white plunger with two springs: a large one that returns the key, and a small one that passes through the body and presses the two halves of the mylar together through a hole in a clear spacer sheet. The mylar is one sheet folded over on itself with the spacer between the folds.

1. Brush loose dirt off the body a small area at a time, without pulling springs out of their holes.
2. Look along the body at eye level. A small spring standing higher than the rest makes its key stick on; reseat it, and if it still stands high, trim one turn at a time with the cutters, fitting the cut end into the body so it cannot cut the mylar.
3. A key that does not work may have a short or missing small spring; stretch the spring a little at a time until the key works.
4. Do the same on the function-key module.

Fit the new mylar and reassemble

1. Fit the function-key module under the function end of the new mylar first, lining up its screw holes, and fit the back plate over it with its small screws.
2. Slide the main body under the mylar and back plate from the left, align the mylar on the three large locating studs of the body, then let the plate drop onto the studs.
3. Check with the torch that the mylar's holes line up under all the screw holes, then fit the small screws in a staggered order to spread the pressure.
4. Before fitting the keyboard in the top case, lay paper over the board's chips, rest the keyboard on it and push the tail into the pinch connector until it bottoms out. Test every key.
5. Fit the keyboard to the top case with its four screws, check that no key binds against the case opening, then tighten them.
6. Reconnect the keyboard ground strap, close the case, and keep the tail from being pinched between the case halves.

Function-key LED

If the red LED in the function-key module does not light or flickers, check the small screw under the LED socket, check that the flat side of the LED faces right, and work the LED in and out of its socket to clean the socket contacts.

On the 600XL

The Atari 600XL uses the same keyboard assembly, Atari part CA061983, so the keyboard procedures above apply to it. Some 600XL boards take the keyboard tail through a small adapter board carrying the keyboard resistors, which has to be removed and refitted with the tail; the 800XL has those resistors on its main board. The board-side parts differ: for some or all keys failing, Atari's 600XL checklist names POKEY (U20), the CD4051 multiplexers U22 and U23, the keyboard and the cable, and for the console keys the GTIA (U17), the switches and the cable. All of these chips are socketed on the 600XL.

Testing

Run the self test's keyboard test and press every key; each should show in inverse video and sound a tone.

An 800XL with the other type of keyboard: a brown circuit board with mechanical switches, seen from inside the top case

Revision notes

• Best Electronics found two versions of the metal-back-plate keyboard, dated 1983 and 1984, with the same part number and revision but mylars that are not interchangeable; the function-key module is held by four or six screws depending on the version.
• The circuit-board keyboard (Best: "PCB / mechanical switch version") is not covered by the mylar procedure. The Sams continuity and J8 checks above apply to both types.
• The Atari 600XL uses the same keyboard; on some 600XL boards the tail plugs into a small adapter board carrying the keyboard resistors, which the 800XL has on its main board.

Related pages

• Atari 800XL
• Atari 800XL Troubleshooting Guide
• Atari 600XL
• Atari 800XL General Maintenance
• Atari 800XL Sound Repair
• Sams Computerfacts Atari 800XL