Atari SM125 Troubleshooting Guide

Atari SM125 (atari-computermuseum.de)

Fault-finding for the Atari SM125 monochrome monitor, from the service notes, flowcharts and voltage tables in Atari's SM125 Service's Manual (P/N C026408). The manual has no schematic, so the checks name the parts its text and charts name.

The picture tube runs at several thousand volts and the power supply holds about 300 V DC on its input capacitors. Discharge the tube before handling the chassis (CRT Discharge Procedure).

Where to start

Atari's general procedure: turn the brightness control to maximum and switch on.

First diagnosis

What you find	Next step
No high-frequency whistle from the line circuits	No high voltage is being made: use the horizontal flowchart below
Whistle, and a visible raster	Use the regular diagnostic flowchart
Whistle, but no raster	Probably a video fault: use the video flowchart
Completely dead	One or both fuses have probably blown. The board layout marks the mains fuse 1 A 250 V.

Regular diagnostic flowchart

Regular diagnostic chart (Fig. 2)

Check	Result
Information on the screen?	No: check for 40 V DC at C406. If it is missing, the usual cause is a broken track on the board. Yes: continue
Picture in sync?	Yes: the unit should be working; if not, start again. No: decide which way it is out of sync
Vertical sync lost	IC201 (TDA1170N) or the sync input
Horizontal sync lost	IC301 (SN74LS221) or the sync input

Video flowchart

Video diagnostic chart (Fig. 3)

Check	If absent
Video input signal present	Connect the video signal; check the cable and connector
Waveform at the emitter of Q402 (2.8 V p-p)	IC401 (SN7406) or Q401, or a passive part round them
Waveform at the collector of Q402 (30 V p-p)	Q402 or the video B+ supply
Both present	The picture tube is probably faulty

Horizontal flowchart

Atari's method is to start at the output and work back.

Horizontal diagnostic chart (Fig. 4)

Check	Result
Pulse at the collector of Q302 (about 195 V p-p)	Present but no high voltage: usually a broken track round the flyback transformer, or a faulty flyback or high-voltage component
Drive waveform at the base of Q302	Present: Q302 or the flyback transformer T301 is faulty
Waveform at the base of Q301	Present: replace Q301. Absent: replace IC301, then check the other parts round it

Vertical and high voltage

Nearly all of the vertical circuit is inside IC201. A single horizontal line or a badly distorted vertical picture means a shorted deflection yoke, a faulty IC201 or a faulty passive part; Atari notes that very few problems have been seen in this area.

If the horizontal stage works, the set is normally making both the bootstrap and the aquadag voltages. If not, look for broken tracks or faulty diodes. The rectifier for the anode voltage is inside the flyback transformer and cannot be replaced on its own; replace T301.

Voltages

DC voltages from Atari's waveform pages. IC201 is printed 56A157-1 on these pages and named TDA1170N in the text; IC301 is printed 56A74LS-221.

IC201 (vertical) pin voltages

Pin	1	2	3	4	5	6	7	8	9	10	11	12
DC (V)	3.1	12	6.23	6.3	11.6	6.59	6.69	0	2.85	2.1	0.69	2.4

IC301 (horizontal one-shot) pin voltages

Pin	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
DC (V)	0.63	5.18	5.18	1	0.63	4.5	0.56	0	0	4.41	5.18	NC	NC	1.7	5.18	5.18

Transistor voltages and waveforms

Transistor	Base	Collector	Emitter	Waveform
Q402 (video output)	4.35 V	22.7 V	3.86 V	2.8 V p-p at the emitter, 30 V p-p at the collector
Q300	0.6 V	3.5 V		4.4 V p-p at the collector
Q302 (horizontal output)	0.32 V			4 V p-p at the base; 195 V p-p pulse, 6.5 µs wide, at the collector

The horizontal yoke current is 5 A p-p and the vertical yoke current 0.7 A p-p. The waveform pages call the horizontal driver Q300, where the text and flowchart call it Q301.

Related pages

• Atari SM125
• Atari SM125 General Maintenance
• Atari SM125 Documentation
• Atari SM124 Troubleshooting Guide
• CRT Discharge Procedure