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Atari 1050 Capacitor Replacement Guide

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Atari 1050 main board; the large power supply filter capacitors are on the right

The Atari 1050 has twelve aluminium electrolytic capacitors on its main board. Three are large filter capacitors in the power supply, which rectifies the adaptor's 9 V AC on the board itself; the others decouple the regulated rails and sit in the spindle motor and read circuits.[1][2] Atari's parts list gives no capacitor values, but its schematic sheets do, and most of them also give the voltage rating. Where Atari's sheets give no rating, the table uses the Sams parts list or Console5's list.[3]

Signs of failure

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  • +12 V missing at TP14 or +5 V missing at TP13. Atari's checklist names C71, with CR15 and CR16, for a dead +12 V doubler.[1]
  • Excess ripple at the regulator inputs. On the Sams schematic Q7's input sits at 9.8 V and Q8's at 17.8 V DC.[2]
  • Bulged, leaking or crusted capacitors around the power supply at the rear of the board.

See Capacitor Failure Symptoms for general signs.

Capacitor list

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Atari 1050 electrolytic capacitors
Ref Value Voltage Where it sits Notes
C67 4,700 µF 35 V (Atari) Input of Q7 (7805), after the bridge rectifier Console5 lists 25 V; the Sams schematic shows 9.8 V DC here
C68 4,700 µF (Atari); 6,800 µF (Sams) 35 V (Atari) Input of Q8 (7812), after the doubler Sams shows 17.8 V DC here
C71 4,700 µF (Atari); 6,800 µF (Sams) 35 V (Atari) Voltage doubler, with CR15 and CR16
C69 47 µF 16 V (Atari) Output of Q7, +5 V
C70 47 µF 16 V (Atari) Output of Q8, +12 V
C41 4.7 µF 16 V (Sams) +5 V supply line Atari's sheet 3 gives no rating
C42 47 µF 16 V (Atari) +12 V supply line
C21 4.7 µF 16 V (Atari) +12 V line, with L4, sheet 1 Console5 lists 35 V
C43 47 µF 16 V (Atari) Collector of Q6 (TIP110), the 12 V feed to the spindle motor switch, after L6
C44 2.2 µF 50 V (Console5) Across the spindle motor connection, J1 pins 1 and 2 Atari's sheet 3 gives no rating
C51 1 µF 50 V (Console5) U5 (LM2917) speed control loop Atari's sheet 3 gives no rating
C2 0.68 µF 35 V (Sams) Read amplifier U22 Not on Console5's list

Values and positions are from Atari's schematic sheets, cross-checked against the Sams schematic; the source of each rating is in brackets.[1][2][3] The Sams sample had 6,800 µF at C68 and C71, with 4,700 µF in brackets, Sams' mark for a value used in some versions.[2] Atari's own figure is 4,700 µF at 35 V for all three large capacitors, and its schematic gives every capacitor a tolerance of +80/-20 %.[1] Check the board for the parts actually fitted, and note the orientation of each capacitor before removing it.

Procedure

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  1. Unplug the adaptor and the SIO cables.
  2. Open the drive and lift out the mechanism as described in the Atari 1050 Maintenance Guide, noting the plug positions.[1]
  3. Unclip the board from the bottom cover, where it sits over three alignment guides.[1]
  4. Replace the capacitors one at a time, matching polarity to the board markings.
  5. Refit the board and mechanism, plugging each connector by the wire colour order in figure 3-1 of the service manual.[1]

Checks after recapping

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Voltages to check
Point Expected Source
TP13 +5 V DC FD100330 p. 4-16[1]
TP14 +12 V DC FD100330 p. 4-16[1]
Q7 input (across C67) 9.8 V DC Sams CD13[2]
Q8 input (across C68) 17.8 V DC Sams CD13[2]

Then run the power-up self test and the diagnostic diskette's automatic tests, described on the Atari 1050 Drive Troubleshooting page.

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Atari, Inc., 1050 Disk Drive Field Service Manual (FD100330): power supply p. 1-8; disassembly and connector polarity pp. 3-1 and 3-2; symptom checklist p. 4-5; power supply waveforms p. 4-16; schematic sheets 1, 3 and 4 of 4 (Tandon), including note 2, "all capacitors are 80% -20%". Hosted here as File:Atari 1050 Disk Drive Field Service Manual.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Howard W. Sams & Co., Computerfacts CD13, Atari Model 1050 Disk Drive (1987): schematic with values and DC voltages; parts list p. 9 (C2 0.68 µF 35 V, C41 4.7 µF 16 V); general note that values in brackets are used in some versions. Hosted here as File:Sams Computerfacts CD13 Atari 1050 Disk Drive.pdf.
  3. ↑ 3.0 3.1 Console5 TechWiki, "Atari 1050", capacitor list (last edited 14 July 2022). wiki.console5.com. Secondary source.


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