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Atari 2600 Sound Circuit Repair

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Atari 2600 six-switch motherboard sound circuit C206 C207 L201
The sound oscillator on a six-switch motherboard: the glass-cased 820 pF polystyrene capacitors C206 and C207, the pink sound coil L201 marked SOUND ADJ, and Q202 (photo: Evan-Amos, public domain)

The Atari 2600 makes its sound in the TIA, which outputs it on pins 12 and 13 (AUD0 and AUD1). The console turns that into a sound carrier with a one-transistor oscillator built round a 2N3563, two 820 pF capacitors and the adjustable sound coil L201, and feeds the carrier to the RF modulator.[1][2] Losing the sound while the picture stays good is a common fault: Best Electronics calls it a very common problem on the 2600 and 2600A, and Atari's tech tip 2 names the two polystyrene capacitors C206 and C207 as the primary suspects.[3][4]

The procedures below are for the six-switch CX2600 and the four-switch CX2600A, the two versions in Atari's domestic (NTSC) field service manual.[5]

Symptoms

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  • Silence or hiss in place of the game sound, with a good picture. Atari's checklists list C206, C207, the L201 adjustment and the oscillator transistor (Q202 on the CX2600, Q201 on the CX2600A), then the RF modulator's adjustment.[6]
  • Weak sound. The checklists add the TIA (A201) and C201 on the CX2600, and the TIA, C208 and R207 on the CX2600A.[6]
  • Sound that comes back when C206 or C207 is pressed (tech tip 2).[4]
  • Only one tone during the test cartridge's audio test, or the wrong sounds: Atari's charts suspect the TIA, or an open circuit at TIA pin 12 or 13.[7]

Lines, grid patterns and a loud hum together on a CX2600A are a supply fault (C241 or C242), not a sound circuit fault; see Atari 2600 Power Supply Circuit Repair.[8]

Tools and parts

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  • Two 820 pF capacitors rated at 50 V for C206 and C207. Atari's part is C010821, a polystyrene capacitor.[2] Vintage Gaming and More's tune-up kits supply small yellow axial capacitors for these positions, which it says are a better type, less prone to failure and not polarised.[9]
  • A multimeter. Atari's procedures also use an oscilloscope and a frequency counter.[10]
  • An adjustment tool that fits the core of L201 snugly. Dirar Abu Kteish found the core of his CX2600A's L201 in pieces when he tried to turn it with a 2 mm hex key; tech tip 16 warns that some cores are frozen or cracked.[11][4]
  • Replacement parts if needed: L201 is Atari C010823 (a variable 0.85 to 1.2 µH inductor on the CX2600A), L202 is C010822 on the CX2600 (2 µH) and C015752 on the CX2600A (1.8 µH), and the oscillator transistor is a 2N3563 or MPS3563.[2]
Sound circuit parts (Atari parts lists and schematic)
Part CX2600 CX2600A
Oscillator capacitors, 820 pF polystyrene C206, C207 C206, C207
Sound coil (adjustable) L201 L201
Oscillator transistor, 2N3563 Q202 Q201 on Atari's schematic and checklist; the parts list gives Q202
Second inductor L202, 2 µH L202, 1.8 µH
First check point for the TIA's square wave C210 C208
Check point for the modulated carrier C211 C211

The CX2600A transistor's designator differs between Atari's documents, so identify it as the 2N3563 next to the two 820 pF capacitors.[1][6][2][12]

Procedure

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Getting to the sound circuit

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Atari 2600 four-switch motherboard with shield top removed
A four-switch board with the shield top off: the red sound coil sits beside the 6532, with the oscillator parts below it (photo: Evan-Amos, public domain)
  • Four-switch CX2600A: open the console as in Atari 2600 Power Supply Circuit Repair#Opening the console, then bend up the four tabs round the metal shield with needle-nose pliers and lift off the shield top. The sound circuit is inside the shield.[9]
  • Six-switch CX2600: open the console and separate the boards as in the same guide, remove the six screws from the bottom of the motherboard's metal case, then the two screws that hold the board to the top of the case.[13]

Quick test

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  1. Run a game with sound and press gently on C206 and then C207 with an insulated tool. If the sound comes back, replace both capacitors; Atari's tip 2 says always replace both.[4]

Tracing the sound signal

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Atari CX-2600A schematic, sound oscillator and luminance network
The sound path on Atari's CX-2600A schematic: TIA pins 12 and 13 at bottom right, the 2N3563 oscillator with its two 820 pF capacitors, the SOUND ADJ coil and the feed to modulator pin 4

Atari's audio flowcharts use the Diagnostic Test Cartridge's audio test, which plays two alternating tones; with the right difficulty switch at B the cartridge alternates two patterns and two tones.[14] Any game that plays steady sound gives a signal to follow with an oscilloscope.

CX2600A:[7]

  1. Check that the RF modulator is tuned to 61.25 MHz (channel 3). If it cannot be tuned, replace it.
  2. Look for a 2 V peak-to-peak square wave, alternating between two frequencies, on TIA (A201) pin 13. If it is missing, check for +5 V at the end of R206 nearest L201 and for continuity from R206 to A201 pin 13; repair any open, then swap A201.
  3. Look for the square wave at C208. If it is missing, repair the open track between C208 and A201 pins 12 and 13.
  4. Look for a 4.5 MHz carrier, modulated, 1 to 2 V peak to peak, on either side of C211. If it is present but the television is silent, replace C211, then R207.
  5. If there is no carrier, check for +5 V at one end of L201 and repair the track from +5 V if it is missing. Check that L201 has continuity and is not shorted or cracked.
  6. Check and replace in this order: C206 and C207, then L201, then L202. If the sound is still dead, check the tracks round C211 for opens and shorts.

CX2600:[15]

  1. Swap in a known-good switchboard if one is available, to decide which board is at fault.
  2. Look for the 2 V peak-to-peak square wave alternating between two frequencies on TIA (A201) pin 13. If only one frequency appears, swap A201. If there is none, check for +5 V at the top of R208 (repair the track from J201 pin 1 if it is missing) and for continuity from R208 to A201 pin 13.
  3. Look for the square wave at C210. If it is missing, repair the track between C210 and A201 pins 12 and 13.
  4. Look for the 4.5 MHz modulated carrier, 1 to 2 V peak to peak, on either side of C211. If it is present, replace C211, then R216.
  5. If there is no carrier, check for +5 V at one end of L201 and repair the track from VMOD (+5 V) if it is missing, then check that L201 has continuity and is not shorted or cracked.
  6. Check and replace C206 and C207, then Q202, then L202. If the sound is still dead, check the tracks round C211.

On the CX2600 switchboard, Atari's chart for bad or missing sound checks the modulator's tuning and then shorts across L101: if the sound returns, L101 is replaced; if it does not, the modulator is.[16]

Replacing C206 and C207

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  1. Desolder C206 and C207 and remove them.[9][13]
  2. Fit the two new 820 pF capacitors. They are not polarised and can go either way round.[9]
  3. Replace both together, even if only one is suspect.[4]

Adjusting the sound carrier

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Atari's audio test procedure measures the oscillator frequency at the emitter of the oscillator transistor with an oscilloscope or frequency counter and adjusts L201 to within 0.06 MHz of the carrier frequency, looking for a stable reading.[17][12]

  1. Run the audio tone test, or a game with steady sound.
  2. Measure the frequency at the emitter of the oscillator transistor.
  3. Turn the core of L201 slowly until the reading is within 0.06 MHz of 4.5 MHz and stable.
  4. Check that the core is a snug fit in the coil, so that it cannot slip after adjustment (tech tip 16). Replace L201 only if the carrier cannot be brought to 4.5 MHz.[4]

The 2600A chart and tech tip 16 give 4.5 MHz; the six-switch chart on page 4-24 prints 5.5 MHz.[17][12][4]

Testing

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  • Run the Diagnostic Test Cartridge's audio tones test: with the right difficulty switch at B, two patterns alternate on the screen in step with two alternating tones, and both colours and tones should be the same on every cycle. If they are not, Atari swaps the TIA, the 6507 and the 6532 in turn.[14][17]
  • Without the cartridge, play a game with continuous sound through the RF output and listen for clear sound with no hum.
  • Burn the console in for at least two hours.[18]
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References

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  1. ↑ 1.0 1.1 Atari, Inc., Video Computer System (Main Board) schematic, CX-2600A, sheet 2 of 4, 30 May 1982: TIA AUD0 and AUD1 (pins 12 and 13), the 2N3563 marked 201, two 820 pF capacitors, the SOUND ADJ coil, 1.8 µH and the feed to modulator pin 4. Hosted as File:Schematic Atari2600A 2000.png.
  2. ↑ 2.0 2.1 2.2 2.3 Atari, Field Service Manual FD100133 Rev. 02, Section 8 parts lists, pp. 8-1 to 8-7: C206 and C207 polystyrene 820 pF 50 V C010821; L201 C010823; L202 C010822 (2600) and C015752 1.8 µH (2600A); 2N3563 or MPS3563.
  3. ↑ Best Electronics, "What to look for when buying a used or third party reconditioned Atari 2600 / 2600A / 2600Jr console", tip 11: loss of sound or weak sound is a very common problem on the 2600 and 2600A.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Atari, Field Service Manual FD100133 Rev. 02, NTSC technical tips at the back of the manual: tip 2 (audio failures, C206 and C207), p. 7-1, and tip 16 (L201 core and the 4.5 MHz carrier), p. 7-4.
  5. ↑ Atari, Inc., Video Computer System Field Service Manual, Domestic Model 2600/2600A, FD100133 Rev. 02, Section 1, pp. 1-1 to 1-5. Hosted as File:Atari 2600 and 2600A Field Service Manual.pdf; see Atari 2600 and 2600A Field Service Manual.
  6. ↑ 6.0 6.1 6.2 Atari, Field Service Manual FD100133 Rev. 02, Section 5, audio failures, pp. 5-3 (2600) and 5-6 (2600A).
  7. ↑ 7.0 7.1 Atari, Field Service Manual FD100133 Rev. 02, "2600A Audio Troubleshooting", pp. 6-24 to 6-26.
  8. ↑ Atari, service bulletin for the 2600A, February 1982: lines, grid distortion and a loud hum caused by C241 or C242. In Field Service Manual FD100133 Rev. 02 (PDF p. 155).
  9. ↑ 9.0 9.1 9.2 9.3 Vintage Gaming and More, "Atari 2600 4 Switch Tune Up Kit Installation Guide": removing the shield and replacing C206 and C207 with non-polarised axial capacitors.
  10. ↑ Atari, Field Service Manual FD100133 Rev. 02, "Equipment Requirements", p. 3-1: 15 MHz oscilloscope, frequency counter, Diagnostic Test Cartridge 2.6.
  11. ↑ Dirar Abu Kteish, "Atari 2600 Static Sound Problem", developer.ps, 15 March 2020: the broken L201 core on a four-switch console and the audio parts he identified next.
  12. ↑ 12.0 12.1 12.2 Atari, Field Service Manual FD100133 Rev. 02, "2600A Audio Test Procedure", p. 6-20: frequency at the emitter of Q201, L201 adjusted to 4.5 MHz within 0.06 MHz.
  13. ↑ 13.0 13.1 Vintage Gaming and More, "Atari 2600 6 Switch Tune Up Kit Installation Guide": getting the six-switch motherboard out of its case and replacing C206 and C207.
  14. ↑ 14.0 14.1 Atari, Field Service Manual FD100133 Rev. 02, "Audio Tones Test", p. 3-13.
  15. ↑ Atari, Field Service Manual FD100133 Rev. 02, "Audio Troubleshooting Procedure, Motherboard", pp. 4-28 and 4-29.
  16. ↑ Atari, Field Service Manual FD100133 Rev. 02, "Bad/No Color, Bad/No Sound, Switchboard", p. 4-13: RF modulator tuned to 61.25 MHz and the L101 short test.
  17. ↑ 17.0 17.1 17.2 Atari, Field Service Manual FD100133 Rev. 02, "Audio Test Procedure", pp. 4-24 and 4-25: frequency at the emitter of Q202, L201 adjustment (printed as 5.5 MHz), tolerance 0.06 MHz, and the pattern and tone checks.
  18. ↑ Atari, Field Service Manual FD100133 Rev. 02, Section 4 introduction, p. 4-1: two-hour burn-in.


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