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Atari 130XE Sound Repair

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Atari 130XE audio amplifier U1 (C070065 schematic)
Atari's schematic of the 130XE audio circuit: the two halves of the LM358 U1, the key-click feed through C23 and R5, the non-polarised capacitors C19, C20 and C22, and the RF modulator A1

The four sound channels of the Atari 130XE come from POKEY (U22). Its audio output, pin 37, is pulled up to +5A by R80 (1 kΩ) and amplified by the LM358 dual op-amp U1, which also mixes in the key click from the GTIA (U17) and the audio inputs from the SIO socket and the expansion connector.[1] The amplified sound goes to the RF modulator and to pin 3 of the monitor socket J2.[2][1] Sams' service data gives a check for each part of the path.[2]

Symptoms

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  • No sound from the TV or monitor. In the self test's audio-visual test, a musical note shown on the staff with no sound is an audio fault.[2]
  • Sound plays but there is no click when a key is pressed.[2]
  • Sound on the RF output but not from the monitor socket.[2]
  • No sound from a cassette recorder's audio track on the SIO socket, or from a device on the expansion connector, while POKEY's own sound works.[2]

If there is no picture either, start with Atari 130XE Black Screen Repair or Atari 130XE Video Output Repair.

Safety

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The board runs only from the external 5 V pack.[1] Check it first; see Atari 130XE Black Screen Repair.

Tools and parts

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  • An oscilloscope, a logic probe and a meter, from Sams' list of test equipment.[2]
  • Replacement parts as found: LM358 for U1 (Atari C061702); 4.7 µF 35 V non-polarised capacitors for C19, C20 and C22 (C061647); 0.047 µF for C23; 470 kΩ for R5; 5.1 kΩ for R8; 1 kΩ for R10.[1][2] See Atari 130XE Capacitor Replacement Guide for the electrolytics.

Procedure

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Open the machine

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  1. Remove the four screws from the bottom of the case, lift the top and disconnect the keyboard cable.[2]
  2. Remove the seven screws holding the board, lift out the board and shield, and twist the seven tabs to separate the shield halves.[2] Reconnect the keyboard and power for the checks.
Atari 130XE board layout, RAM area (C070067)
Atari's board drawing: the audio amplifier U1 (top right of this view) with R1 to R11, C19, C20, C21 and C24 beside the extended RAM

Make a test tone

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Type and run this line in BASIC; it plays a continuous tone on channel 0.[2] 

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

No sound

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Sams' sequence:[2]

  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 (Sams' figure 1). If it is missing, check the voltages and the parts around pins 1 to 8 of U1.
  3. If U1 pin 1 has the waveform, check the RF modulator.
POKEY chip on an Atari 130XE motherboard
POKEY (C012294B-01) on a late 130XE board; its pin 37 is the audio output

Key click missing

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  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]
  2. If they are missing, suspect U17. If they are present, check C23 and R5.[2]

Sound on RF but not on the monitor

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Check the connections at the monitor socket J2 and the resistors R8 and R10.[2] On Atari's schematic the audio leaves U1 for J2 pin 3 through R8 (5.1 kΩ), with R10 (1 kΩ) to ground.[1]

External audio inputs

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  • If the test tone plays but the audio input on pin 5 of the expansion connector J4B is silent, check the electrolytic C22.[2]
  • If the audio input on pin 11 of the SIO socket J1 (a cassette recorder's audio track) is silent, check the electrolytic C19.[2]

Replace the faulty part

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  1. Desolder the faulty part and clear the holes. If U1 is replaced, fit an 8-pin socket.
  2. C19, C20 and C22 are non-polarised; fit bipolar parts there.[1]
  3. Refit the shield and bend its tabs, refit the board with its seven screws, reconnect the keyboard and refit the top.[3]

Testing

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  1. Run the test tone and check the waveform at U1 pin 1 and the sound at the TV or monitor.[2]
  2. Run the self test's audio-visual test: a tune of six tones on POKEY's four channels, each note appearing on the staff as it plays.[2]
  3. If the production test cartridge is available, its audio test should give six pure tones from POKEY followed by a buzz from GTIA.[1]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Atari Corp., 130XE Reference Manual (cover dated March 1990): parts list of 25 June 1985 (U1 LM358 C061702; C19, C20, C22 4.7 µF 35 V non-polarised C061647; C23 0.047 µF; R5 470 kΩ; R8 5.1 kΩ; R10 1 kΩ); XE/XL Production Test Operator's Instructions Rev. 3.4 (audio test); schematic diagram C070065 Rev. B (POKEY pin 37 and R80, U1 and its parts, J2 pin 3). Hosted here as Atari 130XE Parts List and Production Test Instructions.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 Howard W. Sams & Co., Sams Computerfacts CC16, Atari Model 130XE: test equipment and "Self-Test" (PDF page 9); troubleshooting, "Sound" with the BASIC test line and figure 1 (PDF page 10); parts list (C19, C20 and C22 4.7 µF 35 V non-polarised; PDF page 11); preliminary service checks and disassembly instructions (PDF pages 19 to 21). Hosted here as Sams Computerfacts Atari 130XE.
  3. ↑ 130XE Personal Computer Field Service Manual, reproduction by JER/Eightbitter, November 2016, section 4, console assembly. Not an Atari publication. Hosted here as Atari 130XE Field Service Manual.


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