Atari SC1224 Troubleshooting Guide

Test points, fuses and presets on the Version 2 boards (Atari service manual)

Fault-finding for the Atari SC1224 colour monitor, drawn from Atari's two surviving service manuals: the CA-3 manual for the Goldstar-built set and the SC1224 Version 2 Field Service Manual for the JVC-built set. The two versions share nothing below the case, so first work out which one you have (see Atari SC1224 Capacitor Replacement Guide#Which version is it?).

The monitor has a colour CRT with an anode supply of 22 to 23 kV and mains-connected primary circuits. Discharge the CRT before handling the chassis (CRT Discharge Procedure). Goldstar's manual requires an isolation transformer between the monitor and the mains when the chassis is run open, and the Version 2 rectifies the mains directly to make its 125 V B1 supply.

Before opening the monitor

First checks

Symptom	Check
Picture rolls or tears	Set the vertical and horizontal hold controls on the back panel. On the CA-3 these are V-HOLD (R603) and H-HOLD (R716).
No picture with a working ST	The SC1224 shows only the ST's low and medium resolutions. A program that needs high resolution needs the monochrome monitor.
Colour patches after moving the monitor	The degaussing coil runs for about one second at switch-on. Leave the monitor off for at least ten minutes, then switch on again.
Monitor shuts down and stays dead	The CA-3's X-ray protection circuit latches the horizontal oscillator off. Switch off for at least 30 seconds before trying again, then look for the cause of the over-voltage.
Picture missing or one colour missing	Check the signal cable and the 13-pin plug; both Atari charts start from the input signal.

Goldstar CA-3 chassis

Atari's CA-3 manual gives flowcharts for eleven symptoms. The tables follow their order of checks; the waveform and voltage figures are the manual's.

No power (chart B)

Check	If wrong
115 V at the power board connector P962	Continue with the checks below
Fuse F901 (3.15 A)	Fuse open: find the short before fitting a new one
DC at the bridge BD901	Bridge KBL06 faulty
IC950 pins 1 and 3	STR41090 or the transformer T950 faulty
Q950 (KTC2120)	Faulty over-current protection transistor

No raster (chart C)

Check	If wrong
CRT heater glowing	Measure about 6.3 V rms between H1 and H2; if absent, suspect the flyback or R516, and go to chart C-1
Whole screen lights with brightness at maximum	If not, check the G2 (screen) voltage, which the chart gives as about 450 V; if low, the flyback
115 V at point A10	Connector P707 or R363
Screen lights when SUB BRIGHT (R742) is turned clockwise	If not, no high voltage: the flyback

Horizontal output stage (chart C-1)

Check	If wrong
115 V at flyback pin 7	Check R730; if it is open, replace it, and if the new one fails, Q703 is short
Power supply normal	Go to chart B
Waveform at the base of Q703	If the drive is present, the flyback is faulty
2.8 V p-p square wave at the base of Q702	If present, check for 23 V at C713: present means Q702 or T701, absent means Q701 or R704
12 V at IC701 pin 10	D702 or the power supply
Waveform at IC701 pin 12 with pin 13 shorted to ground	Present: the high-voltage or shut-down circuit is tripping. Absent: IC701

No picture, raster present (chart A)

Check	Result
CRT cathode signals KR, KG, KB, about 45 V p-p	Present: fault round the CRT socket
Signals at connector P555 pins 2, 3 and 5, about 2.5 V p-p	Present: check 115 V at A10 (else P707 or R363) and 12 V at A9 (else P305), then the video output transistors Q501 to Q506
Signals at TP1, TP2 and TP3 on the main board, about 2.5 V p-p	Present: P304 disconnected
12 V at C332	Present: video drive circuit Q301 to Q324. Absent: R732 or D709

One colour missing (chart D)

Work from the input to the CRT: the input signal at P301 pins 1, 2 and 3 (absent means the cable or the computer), then the video drive circuit Q301 to Q324, the connector P304, the signals at P555 pins 2, 3 and 5, the collector voltages of Q501, Q503 and Q505 (wrong means the 115 V line or the video output transistors Q501 to Q506), the CRT cathode voltages, and R510 to R512. If all are right and the G1 and G2 voltages are right, the chart ends at the flyback or the CRT. The chart's G1 and G2 figures are hard to read in the surviving scan; the chart C figure for G2 is about 450 V.

Sync and deflection (charts E, F, I, J)

Symptom	Checks
No horizontal sync	Sync at P301 pin 5, 3 to 5 V p-p (else the cable or the computer); adjust H-HOLD R716; waveforms at IC701 pins 1 to 3 (else IC701); about 3.2 V p-p at IC701 pin 4 (else R712 or C742)
No vertical sync	Sync at P301 pin 6, 3 to 5 V p-p; adjust V-HOLD R603; waveforms at IC701 pins 16 to 19 (else IC701); waveforms at IC601 pins 2 and 4 (else IC601)
Pincushion distortion	Adjust the side PCC control R621; if that does not cure it, the fault is in the pincushion circuit
Single horizontal line	Waveform at IC601 pin 4; 12 V at IC701 pin 20 (else the power supply); 24 V at IC601 pin 6 (else R731, D708 or the flyback); waveform at IC601 pin 2 (if good, the deflection yoke may be open)

No sound (chart K)

Turn up the volume (R409). Check for 12 V at pin 1 of the sound IC (else R732, D709 or the flyback), 14 V at pin 9 (else the power supply), signal at pin 2 (else the signal cable), then connector P404 and finally IC401 (AN5265).

Focus and convergence (charts G, H)

Bad focus: adjust the focus control on the flyback; if that fails, the CRT or the flyback's focus section. Bad convergence: static convergence magnets for the centre, the deflection yoke position for the edges. The procedures are in Atari SC1224 General Maintenance.

JVC Version 2

Atari's Version 2 manual gives five fault charts and annotated circuit pages that name the symptom each part produces when it fails.

Atari's fault charts

Symptom	Atari's cause and checks
No raster, no sound, B1 normal	Horizontal deflection fault, so no focus voltage, no 120 V HB and no 15 V B2. Check for 0 V at pin 9 of IC1501 (printed IC501; if not, D1701 and D1702) and 11.2 V at the emitter of the starter Q1501. No 120 V HB: the flyback T01. Collector voltage of Q1522 abnormal: check Q1522, C1526, C1534, C1535, C1529, D1523 and D1524 for shorts or opens. About 0.8 V AC across the base and emitter of Q1522: horizontal output circuit. 0 V there: measure AC from the collector of Q1521 to ground; over 5 V means T1521 is open, 0 V means the T1521 primary, Q1521, R1503, R1506, R1572 or IC1501
No picture, normal sound	IC1001, IC1801 and their parts, the CRT heater circuit and the high voltage. Check 12 V at IC1001 (else IC1002 and IC1001), then IC1801 with R1805, R1807, R1808 and R1810, or Q1801 with R1803 and R1804
No sound, normal picture	11.8 V at IC1601 pin 7 (else R1603, D1601, D1552) and 14.1 V at pin 9 (else R1607, D1551); then R1601, R1604 and the speaker
Single horizontal line, normal sound	25.3 V at IC1421 pin 3 (else D1421 and IC1421); 11.6 V at IC1501 pin 6 (else R1461 and C1403); 1.4 V at IC1501 pin 2 (else IC1501); otherwise R1405, R1410, C1424 and IC1421. R1434 open halves the vertical size; R1452 open blacks out the top of the screen
Horizontal sync faulty	IC1501 or the AFC: C1501, R1501, R1502, R1504, C1506, IC1801
Vertical sync faulty	Adjust V HOLD (R1432); 0.6 V at the base of Q1802 (else R1801 open or D1801 short); otherwise Q1802, C1801, R1802, R1403, R1404 and IC1501

Part failures named on the circuit pages

Symptom	Parts Atari names
No raster, no sound	Q1522 or Q1521 open or short; T1521 open or short; C1507 open; IC1501 not oscillating
No raster	D1523 or D1524 (dampers) open; R1536 open; C1701 open or D1702 short (B1 over-voltage detection); D1503 open; D1510 short; R1513 open; Q1501 open or base-emitter short; D1522 or R1531 open (15 V B2 rectifier); C1010 short; Q1002 or Q1003 shorted (beam limiter)
Small raster	D1906, R1907, R1904 or R1908 open; R1434 open
Narrow picture	C1526, C1534 or C1535 open (retrace capacitors)
Single vertical line	C1529 open (S-correction)
Single horizontal line	C1401, C1403 or C1428 short; C1402 open or short; R1412, R1413 or D1421 open
Faulty vertical sync	R1404, R1403 or C1404 open; C1404 or C1405 short; R1422 open; D1801 short
Faulty horizontal sync	C1506 open; R1502 open; C1505 short; C1504 or R1510 open (top half only)
Picture shifted	R1505 or R1507 open: to the left. C1541 short: to the right
Side pin distortion	C1427, C1429 or D1404 short; R1435 or R1436 open
Poor vertical linearity	R1409 open
No red, or a red raster	No red: C1002, C1151, Q1102 or Q1103 open or short, R1158 open, C1106 short, Q1104 shorted. Red raster: C1105 open or short, C1106 open, Q1104 open. The green and blue channels use the same circuit with 12xx and 13xx numbers
Bright or dark picture	D1001 short: bright. Q1001 base-emitter short: dark; other shorts or open: whitish
No sound	D1551 open (16 V rectifier); C1601 or C1603 short; C1604 open; R1609 open
No picture	D1802 short; Q1801 open or short

Faults reported by owners

• A Goldstar-type power board that smoked and killed picture and sound had three leaking electrolytics, including the large reservoir, and heat damage on C956; the power light still worked. See Atari SC1224 Capacitor Replacement Guide.
• A Goldstar SC1224 that became garbled and jittery after warming up was attributed on Atari-Forum to a weak electrolytic in the vertical deflection circuit; the thread does not record a repair.
• A Goldstar SC1224 whose picture could not be centred vertically with R626 at its limit: the advice was to replace the electrolytics first, and to keep the flyback and the horizontal deflection transistor (2SD1453) in mind as the other parts that fail on these monitors.

Self-inflicted faults

• The hold-down preset R725 on the CA-3 sets the X-ray protection threshold. Goldstar sets it to give 10.25 V at the cathode of D704 and fixes it with epoxy; do not turn it to cure a shut-down.
• There is no high-voltage adjustment on the CA-3. The 115 V supply sets the high voltage, so a wrong 115 V line gives a wrong anode voltage.
• On the Version 2, keep the CRT earth wire connected between the CRT socket board and the main board whenever the set is powered with the board out.

Related pages

• Atari SC1224
• Atari SC1224 General Maintenance
• Atari SC1224 Capacitor Replacement Guide
• Atari SC1224 Documentation
• CRT Discharge Procedure
• Capacitor Failure Symptoms