Atari 800XL Sound Repair

Sams' drawing of the 800XL audio circuit: the two halves of the LM358 U1, the key-click feed through C23 and R5, the SIO and expansion audio inputs through C19 and C22, and the output to J2 pin 3

All four sound channels of the Atari 800XL come from POKEY (U22). Its audio output, pin 37, is pulled up to +5A by R80 (1 kΩ) and feeds the LM358 dual op-amp U1 through C17 and R4. The first half of U1 (pins 5, 6 and 7) passes the signal through R2, C21, C20 and R7 to the second half (pins 1, 2 and 3), which also mixes in the GTIA's key click from pin 15 (through C23 and R5), the SIO audio input on J1 pin 11 (through C19) and the expansion connector's audio input (through C22). The output at U1 pin 1 goes to pin 3 of the RF modulator and, through R8 with R10 to ground, to pin 3 of the monitor socket J2. Sams' service data gives a check for each part of that path, and Atari's manual gives the output level at J2.

Symptoms

• No sound from the TV or monitor, or a tone missing in the self test's audio-visual test. Atari's checklist gives POKEY (U22), U1, the 4.5 MHz audio carrier and the TV volume. In the self test, a note shown on the staff with no sound is an audio fault.
• Sound but no key click.
• Sound on the RF output but not from the monitor socket.
• No sound from a cassette recorder's audio track or from a device on the expansion connector, while POKEY's own sound works.

If there is no picture either, start with Atari 800XL Black Screen Repair or Atari 800XL Video Output Repair.

Safety

The board runs only from the external 5 V supply. Check it first; see Atari 800XL Power Supply Replacement.

Tools and parts

• An oscilloscope, a logic probe and a meter.
• A non-metallic alignment tool for the RF modulator's sound coil L202; Sams names GC Electronics 9440.
• Replacement parts as found: LM358 for U1 (Atari C061702), 4.7 µF 35 V non-polarised capacitors for C19, C20 and C22 (C061647), 10 µF for C24. See Atari 800XL Capacitor Replacement Guide for the electrolytics.

Procedure

Open the machine

1. Remove the six screws from the bottom cover, tip the top cover up from the left and disconnect the keyboard ground strap and the keyboard cable.
2. Remove the four board screws and lift the board and shield out; the shield is held together by ten screws.
3. Reconnect the keyboard and power for the checks below.

Make a test tone

Type and run this line in BASIC; it plays a continuous tone on channel 0.

10 SOUND 0,100,10,15:GOTO 10

No sound

Sams' sequence:

1. Check for pulses at pin 37 of POKEY (U22). If they are missing, suspect U22.
2. If pin 37 has pulses, check for the audio waveform at pin 1 of U1. If it is missing, check the voltages and parts around pins 1 to 8 of U1.
3. If U1 pin 1 has the waveform, check the adjustment of the sound coil L202 on the RF modulator, then the modulator itself.

Audio path test points (Sams CC11 and Atari schematic)

Point	What to see
POKEY (U22) pin 37	Pulses while the test tone plays
U1 pins 5, 6, 7 and 1, 2, 3	About 1.7 V DC on Sams' unit
U1 pin 8 (supply)	4.3 V on Sams' unit
U1 pin 1	Audio waveform (Sams' figure 1)
J2 pin 3	600-750 mV p-p

Key click missing

1. If the test tone plays but pressing a key makes no click, check for pulses at pin 15 of the GTIA (U17) while pressing a key.
2. If they are missing, suspect U17. If they are present, check C23 and R5.

Sound on RF but not on the monitor

Check the connections at J2 and the resistors R8 and R10. R8 (5.1 kΩ) runs from U1 pin 1 to J2 pin 3, and R10 (1 kΩ) from J2 pin 3 to ground.

External audio inputs

• If the test tone plays but a device on the expansion connector's audio input (J9 pin 49) is silent, check C22.
• If the audio from a cassette recorder on J1 pin 11 is silent, check C19 and C110.

The LM358 audio amplifier U1 (top right) with C24, C17 and C23 on a PAL 800XL board

Replace the faulty part

1. Desolder or unplug the faulty part; Atari's parts list gives an 8-pin socket for U1.
2. Fit the replacement. C19, C20 and C22 are non-polarised; fit bipolar parts there.
3. Refit the shield, the board and the covers, reconnecting the keyboard cable and ground strap.

Testing

1. Run the test tone and check J2 pin 3 against Atari's 600-750 mV p-p.
2. On RF, run the same program and adjust the sound coil L202 for maximum sound with minimum noise.
3. Run the self test's audio-visual test: six tones on each of the four channels, each with its note on the staff.

Related pages

• Atari 800XL
• Atari 800XL Troubleshooting Guide
• Atari 800XL Capacitor Replacement Guide
• Atari 800XL Video Output Repair
• Sams Computerfacts Atari 800XL
• Atari 800XL Computer Field Service Manual