Atari 800XL Technical Service Data

TROUBLESHOOTING (Continued)
CLOCK AND DIVIDERS
Check for a 3.579545MHz waveform at the collector of
Oscillator Transistor (Q8). If the waveform is missing check
the voltages and components associated with Oscillator
Transistors Q8 and Q9. If the oscillator is off frequency
check Crystal Y1. If the waveform is good at the collector of
Transistor Q8, check the waveform at pin 29 of the GTIA 1C
(U17). If the waveform is missing check 1C U17. If the
waveform is present at pin 29 of 1C U17, check for a
1.78977MHz waveform at pin 34 of the ANTIC 1C (U7). If the
waveform is missing at pin 34 of 1C U7, check 1C U7. If the
waveform is good at pin 34 of 1C U7, check for clock pulses
at pin 6 of 1C U18. If clock pulses are missing at pin 6 of 1C
U18, check 1C U18. If clock pulses are present at pin 6 of 1C
U18, check for clock pulses at pins 3 and 39 of the CPU 1C
(U8). If pulses are missing at pin 3 or 39 of 1C U8, check 1C
U8. If pulses are present at pins 3 and 39 of 1C U8, check for
clock pulses at pin 11 of 1C U18. If pulses are missing at pin
11 of 1C U18 and pin 13 reads a logic High, check 1C U18. If
pulses are present at pin 11 of 1C U18, check for pulses at
pin 2 of 1C U19. If pulses are missing at pin 2 of 1C U19,
check 1C U19. If pulses are present at pin 2 of 1C U19, check
for pulses at pins 4,6,8,10 and 12 of the Delay Line 1C (U29).
If pulses are missing at any pin, check 1C U29.
VIDEO
If there is no video on the Monitor screen and the RF
Modulator output is being used, check for a video waveform
at pin 1 of the RF Modulator. If the waveform is present,
check the RF Modulator, Channel Select Switch (S2) and
cable going to the Monitor. If the waveform is missing at
pin 1 of the RF Modulator, check for a video waveform at the
emitter of the Video Amp Transistor (Q3). If the waveform is
present at Transistor Q3 check Coil L6 and Resistor R54. If
the waveform is missing at the emitter of Transistor Q3,
check the waveform at the base of Transistor Q3. If the
waveform is present at the base of Transistor Q3, check
Transistor Q3 and the components associated with Tran¬
sistor Q3. If the waveform is missing at the base of Tran¬
sistor Q3, check the waveform at pin 5 of the Buffer 1C (U20).
If the waveform is present at pin 5 of 1C U20, check Resistor
R49 and 1C U20. If the waveform is missing at pin 5 of 1C
U20, check the GITA 1C (U17) and ANTIC 1C (U7).
COLOR
If the color is not correct, check the adjustment of the Color
Control {R38), see "Miscellaneous Adjustments". If ad¬
justing R38 has no affect, check for a voltage range of 0V to
8.5V at pin 17 of the GTIA 1C (U17) while turning R38 from
minimum to maximum. If the voltage is missing or does not
vary, check Control R38 and the voltages, waveforms and
components associated with Color Amp Transistor (Q1). If
the voltage range is correct at pin 17 of 1C U17, check for a
frequency of 3.579545MHz at pin 28 of 1C U17. If the fre¬
quency is not correct check Crystal Y1 and the voltages and
components associated with Oscillator Transistors (Q8 and
Q9). If the frequency is correct, check 1C U17.
If there is no color, check the waveform at pin 21 of 1C U17.
If the waveform is missing check 1C U17. If the waveform is
present at pin 21 of 1C U17, check the waveform at the emit¬
ter of the Color Amp Transistor (Q5). If the waveform is
missing, check the voltages and components associated
with the Color Amp Transistors (Q2, Q4 and Q5). If the
waveform is present at the emitter of Transistor Q5, check
Capacitors C54 and C55 and Resistors R67 and R68.
VERTICAL AND HORIZONTAL SYNC
If there is no vertical or horizontal sync, check the waveform
at pin 25 of the GTIA 1C (U17). If the waveform is missing
check 1C U17. If the waveform is present check Buffer 1C
(U20), Diode CR4 and Resistor R51.
SOUND
No sound. Type in and run the following program in Basic.
10 SOUND 0, 100, 10, 15: GOTO 10
Check for pulses at pin 37 of the POKEY 1C (U22). If pulses
are missing check 1C U22. If pulses are present at pin 37,
check the waveform at pin 1 of the Audio Amp 1C (U1), see
Figure 1. If the waveform is missing check the voltages and
components associated with pins 1 thru 8 of 1C U1. If the
waveform is present at pin 1 of 1C U1 check the adjustment
of the Sound Coil (L202) on the RF Modulator and check the
RF Modulator.
2V
2mSec
,8V REF
If there is sound when running the above program, but there
is no clicking sound when a key is pressed, check for
pulses at pin 15 of the GTIA 1C (U17) while pressing a key. If
pulses are missing check 1C U17. If pulses are present
check Capacitor C23 and Resistor R5.
Sound works when RF modulator is used, but not when us¬
ing a Video/Audio Monitor connected to Jack J2. Check the
connections at Jack J2 and check Resistors R8 and R10.
Sound works when above program is run, but not when the
audio input of Jack J9 pin 49 is used. Check Electrolytic
C22.
Sound works when above program is run, but not when the
audio input of Jack J1 pin 11 is used. Check Electrolytic
C19 and Capacitor C110.

Atari 800XL Specifications

General

Manufacturer	Atari
Type	Home computer
Release year	1983
CPU	MOS Technology 6502C
RAM	64 KB
ROM	24 KB
Operating System	Atari OS
CPU Speed	1.79 MHz (NTSC) / 1.77 MHz (PAL)
Graphics	ANTIC and GTIA chips
Display resolution	160×192 to 320×192 pixels
Sound	POKEY chip
Storage	Cassette tape, floppy disk
Ports	Serial, parallel, monitor, joystick ports