Atari SM124 Troubleshooting Guide
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Fault-finding for the Atari SM124 monochrome monitor, drawn from Atari's service manuals for the MT-9 and MT-21 chassis, the 1985 owner's manual and owners' repair notes.[1][2][3] The two chassis have different power supplies and horizontal circuits, so work out which one you have first (see Atari SM124 General Maintenance#Identify the chassis).
The tube runs at up to 12 kV, and the MT-21's power supply rectifies the mains directly. Discharge the tube before handling the chassis (CRT Discharge Procedure).[4][5]
Before opening the monitor
[edit | edit source]| Symptom | Check |
|---|---|
| No picture | The monitor is switched on (the on/volume knob turned clockwise, green lamp lit), the mains lead is plugged in, the video cable is pushed fully into the ST's Monitor socket, and brightness and contrast are not turned fully down.[3][6] |
| A program will not run | The attached monitor decides which resolutions the ST offers; with the SM124 that is 640 × 400 only. Software written for the colour modes needs a colour monitor.[7] |
| Picture rolls, vibrates, is distorted or blurred | Atari's owner's manual sends these to a service centre for internal repair or adjustment; the service adjustments are on Atari SM124 General Maintenance#Adjustments.[3] |
| No sound | The SM124 has a speaker; the on/volume knob sets the level.[6] Check the cable, then the sound chart below. |
Atari's troubleshooting charts
[edit | edit source]Both manuals start from the same main chart: check the connection between the monitor and the computer, then follow the power chart if the front lamp does not light, the video chart if the picture is wrong and the sound chart if the sound is wrong. If none of these applies, switch the monitor on, check it again after 15 minutes, and adjust it.[1][2]
MT-9: power chart
[edit | edit source]| Check | Result |
|---|---|
| Lamp lit? | Lit: go to the main chart. Not lit or weak: continue |
| +22 V DC at the input of IC901 | Present: go to the next row. Absent: check the fuse F901 |
| F901 open? | Open: replace it; if the new fuse blows, the fault is in the bridge diodes D901 to D904. Not open: check P5 and P4 on the power board |
| 17 V AC at P5 and P4 | Present: the chart gives "replace power switch". Absent: replace the power transformer |
| +12 V DC at the output of IC901 and pin 2 of connector P901 | Present: go to the main chart. Absent: replace IC901 or Q901 |
MT-21: power chart
[edit | edit source]| Check | Result |
|---|---|
| Lamp lit? | Lit: go to the main chart. Not lit or weak: check D930 |
| +27 V DC (the B+ set by VR901) | Present: check the output of the flyback transformer. Absent: check the fuse F901 |
| F901 open? | Open: replace it; if the new fuse blows, the fault is in D901 or IC901. Not open: check Q702 |
| Q702 (BU806, horizontal output) open? | Yes: replace Q702. No: replace Q901, the 2SC3507 switching transistor |
Video charts
[edit | edit source]If there is a picture but it is wrong, both charts start by adjusting the presets (MT-9: V-hold VR601, H-centre VR701, sub-brightness VR702; MT-21: V-size VR601, H-hold VR701, sub-brightness VR702), then checking the video, horizontal and vertical circuits in turn.[10][11] With no picture at all, work from the input towards the tube. The voltages are peak-to-peak signal levels.
| MT-9 check | If absent | MT-21 check | If absent |
|---|---|---|---|
| 1.6 V p-p at pin 3 of P301 | Signal cable | 1.0 V p-p at P301 | Signal cable |
| 5 V p-p at pin 5 or 9 of IC302 | IC301 or Q304 | 3.3 V p-p at the collector of Q301 | Q301 |
| 4.8 V p-p at pin 8 or 6 of IC302 | IC302 or Q301 | 7.1 V p-p at the collector of Q302 | Q302 |
| 28 V p-p at the cathode on the CRT board | Check Q303 and Q302; if they are faulty, replace them | 30 V p-p at the cathode on the CRT board | Check Q304 and Q305; if they are faulty, replace them |
| Flyback circuit normal | Flyback transformer | Flyback circuit normal | Flyback transformer |
| All of the above normal | Picture tube | All of the above normal | Picture tube |
Sound charts
[edit | edit source]With the power on, check the connectors in order: P901, P402 and P401 on the MT-9, P602, P402 and P401 on the MT-21. If all are good, the MT-9 chart blames IC401 (AN5265) and C403 (470 µF 16 V), and the MT-21 chart blames Q401.[12][13][14]
Voltage charts
[edit | edit source]DC voltages from the service manuals, measured with no signal and with a 1 V p-p video signal.[15][16]
| Transistor | Type | Function | B (no signal) | E (no signal) | C (no signal) | B (signal) | E (signal) | C (signal) |
|---|---|---|---|---|---|---|---|---|
| Q301 | KTC1959 | Brightness/contrast | 3.87 | 3.25 | 11.94 | 3.92 | 3.28 | 11.91 |
| Q302 | BSX20 | Video output | 0.26 | 0 | 5.58 | 1.93 | 1.37 | 5.52 |
| Q303 | KTC2229 | Video output | 5.0 | 5.0 | 7.2 | 5.88 | 5.51 | 51.0 |
| Q304 | BSX20 | Video input | 0.02 | 0 | 5.01 | 0.50 | 0 | 2.25 |
| Q701 | KTC562 | H frequency detector | 3.94 | 0.01 | 0 | 0.58 | −0.14 | 0 |
| Q702 | KTC1959 | Monostable | 0.04 | 0 | 9.0 | 0.41 | 0 | 5.38 |
| Q703 | KTC1959 | Monostable | 0.69 | 0 | 0.04 | −0.17 | 0 | 2.47 |
| Q704 | KTC1959 | Astable | −0.25 | 0 | 2.82 | −0.07 | 0 | 2.46 |
| Q705 | KTC1959 | Astable | −0.78 | 0 | 4.2 | −0.75 | 0 | 4.00 |
| Q706 | KTC1959 | H inverter | 0.48 | 0 | 0.33 | 0.46 | 0 | 0.35 |
| Q707 | KTC1959 | H drive | 0.33 | 0 | 2.93 | 0.33 | 0 | 2.71 |
| Q708 | BU806 | H output | −0.26 | 0 | 29.7 | 0.04 | 0 | 28.9 |
| Transistor | Type | Function | B (no signal) | E (no signal) | C (no signal) | B (signal) | E (signal) | C (signal) |
|---|---|---|---|---|---|---|---|---|
| Q301 | 2N2369 | Video input | 0 | 0 | 3.8 | 0.8 | 0 | 3.4 |
| Q302 | 2N2369 | Video amplifier | 0.8 | 0 | 0.1 | 0.8 | 0 | 7.2 |
| Q303 | KTC1959 | Brightness/contrast | 8 | 7 | 12 | 8 | 7 | 12 |
| Q304 | 2N3904 | Video drive | 0.3 | 0 | 12 | 7.2 | 6.7 | 13 |
| Q305 | 2SC3503 | Video output | 12 | 12 | 60 | 12 | 13 | 57 |
| Q701 | KTC200 | Horizontal drive | 6.8 | 6.85 | 6.8 | 6.8 | 7 | 6.8 |
| Q702 | BU806 | Horizontal output | 6.4 | 0 | 180 | 6.4 | 0 | 180 |
| Q901 | 2SC3507 | Power switching | 1.6 | 0 | 510 | 1.6 | 0 | 520 |
| Q401 | KTC1959 | Audio input | 0.7 | 0 | 0 | 0.7 | 0 | 0 |
| Q402 | KTC1959 | Audio amplifier | 6.6 | 1 | 12 | 6.6 | 1 | 12 |
| Q403 | KTC562 | Audio amplifier | 2.6 | 6 | 0 | 2.6 | 6 | 0 |
Q901 on the MT-21 sits on the rectified mains, about 500 V on its collector. Do not probe it without an isolation transformer and a meter rated for the voltage.[16][5] The MT-9 manual types Q303 as KTC2229 in its voltage chart and video amplifier figure, and as KTC2249 in the parts list.[15][14]
Faults reported by owners
[edit | edit source]| Symptom | Cause and fix |
|---|---|
| A single vertical line on the screen | The bipolar capacitor C716 (2.2 µF 50 V) in the horizontal deflection circuit. Pawel Pieczul of oldcrap.org names it as the part most likely to fail on the SM124. It is C716 in the MT-21 parts list and C714 in the MT-9 list.[17][18][14] |
| Monitor will not switch on from the knob | One owner reported a faulty switch on the combined on/volume control.[19] Atari's MT-9 parts list gives the switch as an EVV-G6LF20B14.[20] |
Self-inflicted faults
[edit | edit source]- Do not replace C716 with an ordinary electrolytic capacitor of the same rating. Pieczul reports that it blows up; he fits a polypropylene (MKP) film capacitor, which another owner also used.[17][19]
- Do not trust a single capacitor list for the CRT board. An owner recapping a revision 1.2 board found that C302 and C303 were 100 µF 10 V; oldcrap.org's list had first given them as 10 µF 100 V and was corrected after his report.[19]
- CRT Database warns that stretching the raster with the internal presets to fill more of the screen may cause premature failure of internal components.[21]
Related pages
[edit | edit source]- Atari SM124
- Atari SM124 General Maintenance
- Atari SM124 Capacitor Replacement Guide
- Atari SM124 Documentation
- CRT Discharge Procedure
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.0 1.1 SM124 Service Manual (MT-9), Troubleshooting Chart, main chart, p. 16.
- ↑ 2.0 2.1 Atari, SM124 High-Resolution Monochrome Monitor Service Manual, chassis MT-21, P/N 483-351A, Troubleshooting Chart, main chart, p. 16. File:Atari SM124 Service Manual (MT-21 chassis).pdf.
- ↑ 3.0 3.1 3.2 Atari Corp., SM124 Owner's Manual, 1985, p. 4, "Troubleshooting". File:Atari SM124 Owner's Manual (1985).pdf.
- ↑ Atari, SM124 High-Resolution Monochrome Monitor Service Manual, chassis MT-9, P/N 483-303A, Safety Precautions, p. 2. File:Atari SM124 Service Manual (MT-9 chassis).pdf.
- ↑ 5.0 5.1 SM124 Service Manual (MT-21), Theory of Operation, power supply, pp. 7-8.
- ↑ 6.0 6.1 SM124 Owner's Manual, p. 3, "The On/Volume Control".
- ↑ "Atari ST", Wikipedia, section "Monitor". https://en.wikipedia.org/wiki/Atari_ST
- ↑ SM124 Service Manual (MT-9), Power Chart, p. 17.
- ↑ SM124 Service Manual (MT-21), Power Chart, p. 17.
- ↑ 10.0 10.1 SM124 Service Manual (MT-9), Video Chart, p. 18.
- ↑ 11.0 11.1 SM124 Service Manual (MT-21), Video Chart, p. 18.
- ↑ SM124 Service Manual (MT-9), Sound Chart, p. 19.
- ↑ SM124 Service Manual (MT-21), Sound Chart, p. 19.
- ↑ 14.0 14.1 14.2 SM124 Service Manual (MT-9), Replacement Parts List, pp. 26-27.
- ↑ 15.0 15.1 15.2 SM124 Service Manual (MT-9), Voltage Chart, p. 11, and Fig. 3, Video Amplifier, p. 5.
- ↑ 16.0 16.1 16.2 SM124 Service Manual (MT-21), Voltage Chart, p. 12.
- ↑ 17.0 17.1 Pawel Pieczul, "Atari SM124 Monitor", oldcrap.org, 20 October 2019, https://oldcrap.org/2019/10/20/atari-sm124-monitor/. Community source: the note on C716 under "Board".
- ↑ SM124 Service Manual (MT-21), Replacement Parts List, p. 21.
- ↑ 19.0 19.1 19.2 Comments on "Atari SM124 Monitor", oldcrap.org: a reader's question about a faulty power switch (27 April 2022) and krs's recapping report (8 and 12 October 2023). https://oldcrap.org/2019/10/20/atari-sm124-monitor/. Community reports.
- ↑ SM124 Service Manual (MT-9), Replacement Parts List, p. 28: power switch 180-291A, EVV-G6LF20B14.
- ↑ "Atari SM124", CRT Database, https://crtdatabase.com/crts/atari/atari-sm124-mt-9. Community source (CC BY-NC-SA 4.0), notes section.