Atari 800XL

TROUBLESHOOTING (Continued)
KEYBOARD
If the keyboard is dead, check connector J8 for good con¬
nections and check the ribbon cable for defects. If the con¬
nector and cable check good, check the waveforms at pins
18 thru 23 of the POKEY 1C (U22). If the waveforms are miss¬
ing, check 1C U22. If the waveforms are present at pins 18
thru 23 of 1C U22, check the waveform at pin 3 of the
Keyboard Multiplexer 1C (U24) while pressing any key ex¬
cept the Shfit, Control, Break or Function keys. If the
waveform is missing at pin 3 of 1C U24 check IC’s U24 and
U25. If the waveform is present at pin 3 of 1C U24 check the
GTIA IC(U17) and 1C U22.
If the Start, Select or Option keys do not work, check the
logic readings at pins 12,13 and 14 of 1C U17. Pin 12 should
read logic Low when the Start key is pressed, pin 13 should
read logic Low when the Select key is pressed and pin 14
should read logic Low when the Option key is pressed. If
the logic readings are correct check 1C U17. If the logic
readings are not correct check the keyboard connector J8,
ribbon cable and Resistors R134, R135 and R136.
If the Shift, Control or Break keys do not work, check the
waveform at pin 16 of 1C U22 while pressing the Shift, Con¬
trol or Break keys. If the waveform is present, check 1C U22.
If the waveform is missing at pin 16 of 1C U22, check for the
same waveform at pin 5 of Keyboard Multiplexer 1C (U25)
while pressing the Shift, Control or Break keys. If the
waveform is missing, check 1C U25. If the waveform is pres¬
ent at pin 5 of 1C U25, check Resistors R96, R137 and RN14
and check pins 9 and 8 of connector J8 for good connec¬
tions.
If there is no clicking sound when a key is pressed, check
for pulses at pin 15 of 1C U17 while pressing a key. If pulses
are missing, check 1C U17. If pulses are present, check
Capacitor C23 and Resistor R5.
If one key is erratic, clean the key.
If characters come up wrong on the Monitor screen when a
key is pressed, check IC’s U7, U17 and U22.
J8 (Connect to 1C U25)
PIN KEYS
2 ESC, 1, 2, 3, 4, 5, 6
3 U, I, O, P, -, = , RETURN
5 J, K, L, ;, +, \ CTRL
8 SHIFT, Z, X, C, V, B
7 N, M./, □ , SPACE
6 A, S, D, F, G, H, LOWR
1 BREAK, 7, 8, 9, 0, <, >, BACKS
4 TAB, Q, W, E, R, T, Y
If pulses are missing check the Resistors or Resistor Net¬
work that connect connector J8 to IC’s U24 and U25 and
check IC’s U24 and U25.
CASSETTE SAVE AND LOAD
Computer will not save a program on tape. Type a program
or load an existing program into the Computer. Save the
program back on tape, see "Cassette Operation" section of
the General Operating Instructions. While saving the pro¬
gram, check the waveforms on pins 26, 27 and 28 of the
POKEY 1C (U22). If any of the waveforms are missing, check
1C U22 by substitution. If the waveforms are correct, check
the connections at pins 1, 2, and 5 of Jack J1.
Computer will not load a program from tape. Check for
pulses at pin 24 of 1C U22 while loading a program from
tape, see "Cassette Operation" section of the General g*
Operating Instructions. If the pulses are present, check the O
connections at pin 3 of Jack J1. O >
£3 -*
r-
Recorder motor will not start when saving or loading a pro- CD jo
gram. Type POKE 54018,52 and press the RETURN key. © —
Check the logic probe reading on pin 39 of the PIA 1C (U23). *
The reading should go from High to Low to start the
recorder motor. If the reading stays High, check 1C U23 by
substitution. If the reading operates normally, check
Switch Transistor (Q7) and check the connections at pin 8
of Jack Jl. To turn the recorder motor Off, type POKE
54018,60 and press RETURN key.
If one key or group of keys does not work, use the following
chart to determine which pins of Connector J8 the bad keys
connect to and use an ohmmeter to check the keys for con¬
tinuity, each key should measure about 55 ohms. If the
keys check good, turn the Computer On and use a scope to
check for pulses at connector J8 while pressing the bad
keys. Note: A logic probe may indicate pulses even when
no key is pressed.
J8 (Connect to 1C U24)
PIN KEYS
10 6, 7, Y, U, N
11 H, J, SPACE *
13 4, 9, R, O, F, L, V, ,
14 3, 0, E, P, D, C, .
17 ESC, BACKS, TAB, RETURN, CAPS
16 1, >, Q, =, A, *, Z, □
15 2, <, W, - , S, +, X, /
12 5, 8, T, I, G, K, B, M
PADDLES
The following Basic program can be used to check the
operation of the paddles.
10 PRINT, "PADDLE", "BUTTON"
20 FOR P = 0 TO 3
30 PRINT "PADDLE"; P, PADDLE (P), PTRIG (P)
40 NEXT P
50 FOR T = 1 TO 200: NEXT T
60 PRINT: GOTO 10
On the Monitor screen the number under PADDLE should
vary from 228 to 1 as the paddle is varied from MINIMUM to
Maximum. The number 1 under BUTTON should change to
0 when the button is pressed.
If a paddle does not function, use the following chart to
determine which pin of the POKEY 1C (U22) the paddle is
connected to and check the waveform on the pin.