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Atari SC1425 Capacitor Replacement Guide

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Atari SC1425 power supply (Philips schematic)
Mains input and reservoir of the SC1425 on the Philips schematic

Electrolytic capacitors in the Atari SC1425 14-inch colour monitor, read from the Philips CM8852/CM8832 circuit diagram that was supplied with an SC1425.[1] No Atari or Philips parts list for the SC1425 has been found, so the values come from the drawing alone.

The SC1425 has a colour CRT and a switch-mode power supply whose primary side the schematic marks "not mains isolated". Everything on that side is at mains potential while the monitor is plugged in, so take live measurements there only with the monitor fed through an isolation transformer. Unplug the monitor, discharge the CRT as described in CRT Discharge Procedure, and check that C112 is discharged before working on the board.[1]

Reading the schematic

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The legend on the drawing marks a capacitor's type with a symbol and a dot code: ▵▵ for ceramic, ●● for film ("flat foil") and a single ● for electrolytic. A letter after the code gives the working voltage: e 16 V, f 25 V, g 40 V, h 63 V, j 100 V, r 250 V, u 400 V, v 500 V, w 630 V.[1] Seventeen of the electrolytics carry no voltage letter. For those, read the rating off the fitted part before ordering, and use at least the voltage of the line the part sits on where the drawing shows it. The component index on the drawing lists every capacitor with its grid position.[1]

Main board

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SC1425 electrolytic capacitors (Philips schematic 4822 727 31481)
Ref µF Voltage Notes
C112 100 Not marked Mains reservoir after the bridge D107-D110. The ●● u marking beside it belongs to C111
C116 47 25 V
C122 1 63 V
C144 47 Not marked
C145 470 40 V
C146 470 25 V
C204 100 16 V SCART red input coupling
C205 100 16 V SCART green input coupling
C206 100 16 V SCART blue input coupling
C214 100 25 V
C216 47 25 V
C232 47 25 V
C233 100 25 V
C234 1 63 V
C318 22 Not marked IC301 (TDA2611A audio)
C323 220 16 V Loudspeaker coupling
C324 1000 Not marked IC301 supply
C334 0.47 63 V
C345 10 Not marked IC302 (TDA2653A vertical)
C346 470 40 V
C347 100 Not marked IC302
C352 680 40 V
C361 47 25 V
C401 2.2 63 V
C412 10 Not marked
C433 100 Not marked
C446 1 Not marked IC402 (TDA2595)
C447 4.7 63 V IC402
C461 2.2 63 V
C462 22 40 V
C473 4.7 Not marked Drawn with the bipolar (non-polarised) electrolytic symbol; fit a bipolar part
C474 1 63 V
C482 1 Not marked
C485 22 Not marked
C491 470 25 V Before IC403 (L7812)
C492 100 16 V After IC403
C493 470 40 V
C494 47 Not marked On the +125 V line
C574 100 25 V IC502 (TDA3505)
C585 4.7 63 V IC502
C634 22 Not marked
C636 22 Not marked
C638 100 Not marked On the -20 V line
C717 4.7 Not marked On the +125 V line to the CRT panel

Pawel Pieczul gives C473 as the high-frequency bipolar capacitor of the Commodore 1084S-D1, which atari-computermuseum.de describes as technically the same monitor as the SC1425. For such positions he fits a polypropylene film capacitor or a bipolar electrolytic made for high-frequency use, not an ordinary electrolytic.[2][3]

That is 44 electrolytics on the main drawing. Two more, C496 (47 µF) and C497 (1.5 µF), are drawn on the schematic but listed as deleted on the CM8832; check whether they are fitted on the board in front of you.[1]

TTL interface board

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The TTL interface (assembly 1108, "Schema B") has one electrolytic, C258, 47 µF 25 V, on its +5 V supply. The other capacitors on that board are C251 to C254 (22 pF), C255 (100 pF) and C256 and C257 (22 nF).[4]

Procedure

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Find the stage the fault is in before replacing parts, and start with any capacitor that is bulging or leaking (see Capacitor Failure Symptoms).

  1. Unplug the monitor and remove the rear cover.
  2. Discharge the CRT anode to the chassis earth (CRT Discharge Procedure).
  3. Measure C112 and discharge it through a resistor if it holds a charge. It is the reservoir on the rectified mains, after the bridge D107 to D110.[1]
  4. Replace one capacitor at a time. Match polarity to the board marking, and fit a bipolar part at C473. Use the voltage in the table where the drawing gives one; where it does not, read it from the part being removed.
  5. Parts marked with the warning triangle on the drawing are safety parts; replace them only with the same type.[1]
  6. Refit the board, reconnect the anode cap, and check the picture before adjusting anything.
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Philips, circuit diagram CM8852/00 and CM8832, 4822 727 31481 (PRS.01474): legend, component index and main diagram, pages 1 and 2 of File:Atari SC1425 Schematic (Philips CM8852 and CM8832).pdf.
  2. ↑ Pawel Pieczul, reply of 1 May 2021 in the comments on "Atari SM124 Monitor", oldcrap.org, https://oldcrap.org/2019/10/20/atari-sm124-monitor/: high-frequency bipolar capacitors at C2524 on the Commodore 1084S-P1 and C473 on the 1084S-D1. Community report.
  3. ↑ "Atari SC1425 RGB Color Monitor", atari-computermuseum.de, https://www.atari-computermuseum.de/st_peri.htm#sc1425. Secondary source for the 1084S-D1 equivalence.
  4. ↑ Philips, "Schema B", TTL interface 1108 (PRS.01496), page 3 of File:Atari SC1425 Schematic (Philips CM8852 and CM8832).pdf.


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