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Apple Monitor III Troubleshooting Guide

From RetroTechCollection
Monitor III on an Apple III (Benoît Prieur, Wikimedia Commons, CC0)

Fault-finding on the Apple Monitor III. Apple's dealers did not repair it: their instruction was to exchange the whole monitor.[1] The checks below are the owner's chart from Apple's manual and the circuit-by-circuit troubleshooting in Sams's ComputerFacts for model A3M0039.

Read CRT Discharge Procedure before opening the case, and Apple Monitor III General Maintenance for the cover.

Owner's chart

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From the Monitor III Owner's Manual, p. 5[2]
Problem Cause Remedy
Monitor on but display not working Bad connection to the power socket Push the plug fully in
Display on but no characters Video cable not connected properly; contrast or brightness turned too far anticlockwise Connect the cable; turn contrast, then brightness, clockwise
Display vibrates slightly Video cable, or the input signal Reconnect the cable; otherwise suspect the Apple III
Display rolls up or down Vertical hold misadjusted Turn vertical hold until the picture is stable
Display too high or too low Vertical size misadjusted Turn vertical size

The monitor takes its signal from the Apple III's B/W video output.[3]

Sams symptom guide

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Sams's troubleshooting aid sends each symptom to a circuit.[4]

Sams troubleshooting aid (CMT8-1, p. 4)
Symptom Where to look
No picture, no raster AC power supply, the supplies generated from the horizontal output transformer T702, and the video circuit
No picture, raster present Video circuit
Low or excessive brightness Video circuit
No raster High-voltage rectifier (part of T702) and the horizontal circuit
No vertical deflection Vertical circuit
Poor vertical linearity or foldover Horizontal circuit
Narrow picture Horizontal circuit
Vertical or horizontal off frequency Vertical or horizontal circuit
No vertical or horizontal sync Sync circuit

Circuit checks

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From Sams's troubleshooting section. Voltages are with a digital meter and no signal.[5]

Power supply

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  1. If the AC fuse F901 is open, check the bridge diodes D901-D904 and the electrolytics C901 and C909, and replace any faulty parts.
  2. Check the DC fuse F902. If it is open, check the regulator transistor Q901, the regulator IC901 and its circuit, and for a short at the horizontal output transistor Q701 and diode D701.
  3. Measure 11.6 V at the collector of Q901. If it is wrong, try the B+ control R903, then check the parts around IC901.
  4. Measure 58.5 V at the cathode of rectifier D252. If it is missing, go to the horizontal checks, because this supply comes from the flyback T702.

Horizontal

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  1. No high voltage: measure 19.4 V at the collector of Q701. If it is missing, check B+ at pin 3 of T702; if pin 3 is right, check continuity between pins 3, 4 and 6 of T702.
  2. Look for a horizontal waveform at pin 12 of IC701. If it is missing, check IC701's voltages, waveforms and parts. If it is present, check Q701, D701 and the driver transformer T701.
  3. If 58.5 V at D252 is wrong because of loading, the high-voltage rectifier inside T702 may be at fault.
  4. Poor width, linearity or foldover can come from the deflection yoke DY1, the width coil (part of the yoke), coil L705 and the non-polarised electrolytic C712.

Video

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  1. Check the voltages and waveforms at the CRT and the CRT itself.
  2. Inject a video signal at the input jack P1 and look for it at pin 2 of the CRT. If it is missing, look at the bases of the video output transistor Q252 and the video amplifiers Q202 and Q203.
  3. Retrace lines on the screen: check the vertical blanking transistor Q271.
  4. The beam current limiter Q253 stops the screen burning during long use; check it if the CRT face has become etched.

Vertical

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  1. Look for a vertical waveform at pin 9 of the vertical IC IC601. If it is present, check the parts between IC601 and the yoke DY1. If it is missing, check IC601's voltages and parts.
  2. Vertical linearity or foldover faults can come from the electrolytics C604-C608 and C610. Check the height control R606 and the linearity control R607 first.

Sync

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No vertical or horizontal sync: check the sync waveforms at the emitter of the sync separator Q302, then the sync amplifier Q301 and Q302's circuit.

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References

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  1. ↑ Apple Computer, Monitor III Technical Procedures, Section 1, Monitor III Exchange, rev. October 1988, Introduction, p. 1.3 (PDF p. 8). Hosted here: File:Apple Monitor III Technical Procedures.pdf.
  2. ↑ Apple Computer, Monitor III Owner's Manual, 1981, If There Are Problems, p. 5. Hosted here: File:Apple Monitor III Owner's Manual 1981 (second scan).pdf.
  3. ↑ Apple Computer, Monitor III Owner's Manual, 1981, Identification and Operation of Controls, p. 3. Hosted here: File:Apple Monitor III Owner's Manual 1981.pdf.
  4. ↑ Howard W. Sams & Co., ComputerFacts CMT8-1, Apple Model A3M0039, March 1985, Troubleshooting Aid, p. 4 (PDF p. 7). Hosted here: File:Sams ComputerFacts CMT8 Apple Monitor III A3M0039.pdf.
  5. ↑ Howard W. Sams & Co., ComputerFacts CMT8-1, Troubleshooting and Schematic Notes, p. 5 (PDF p. 8).


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