Atari SC1425 Capacitor Replacement Guide

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. 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". Unplug the monitor, discharge the CRT as described in CRT Discharge Procedure, and check that C112 is discharged before working on the board.

Reading the schematic

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. 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.

Main board

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
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

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.

TTL interface board

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).

Procedure

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.
4. Replace one capacitor at a time. Match polarity to the board marking. 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.
6. Refit the board, reconnect the anode cap, and check the picture before adjusting anything.

Related pages

• Atari SC1425
• Atari SC1425 Documentation
• Atari SC1435 Capacitor Replacement Guide
• Capacitor Failure Symptoms
• CRT Discharge Procedure