Brand: TANDY
Category: Color Monitor
Manual Type: User Manual
Language: English
Manual for CM-4, 25-1021 - Troubleshooting and error codes + PDF Download
Uploaded: March 8, 2025, 11:51 p.m.
Warranty Summary
No warranty information is included in this service manual.
The Tandy CM-4 Color Monitor utilizes an RGB drive system for video output. Signals are buffered and inverted to set voltage levels for RGB-Amp transistors. Brightness and sub-brightness are controlled via bias adjustments. Contrast is adjusted to manage intensity signal levels.
Video output combines color and brightness signals, amplified to drive cathodes. Horizontal output pulses supply voltage to collectors of output transistors. Blanking signals modulate emitters for brightness. Color signals drive transistor bases. Bias controls are used for picture tube cut-off adjustment, and drive controls for white balance.
Deflection circuits include vertical and horizontal oscillators, AFC, and drive stages. Vertical sync signals control the vertical oscillator, while horizontal sync and flyback pulses manage the horizontal oscillator and AFC. Horizontal drive voltage is amplified and coupled to the horizontal output stage, generating scan and high voltage.
Troubleshooting and error codes
| Problem | Possible Cause |
|---|---|
| NO POWER | SW701, D704, F701, T702, F702, T701, D701, R701, D702, C701, D703, D709, Q701 |
| NO RASTER APPEARS | Power/horizontal sync/protector circuit, Connections between monitor and computer, CRT R872 T602 C859, Input circuit/computer, Q401, Q402 and its related circuits |
| NO PICTURE APPEARS | Check video input circuit and its related circuits, Check Q402 and its related circuits, Check Q451, Q851 and its related circuits, Input cable input socket computer IC491 IC492 IC493 IC494 |
| NO SYNCHRONIZATION CAN BE PROVIDED | IC601, Q423, C606, C604, IC601, R605, R620, R606, D602, R607, Q423 |
| VERTICAL SYNCHRONIZATION CANNOT BE OBTAINED | IC601 R502 C501 IC601 R514 C504, IC601 R513 R510 R514 R511 C503 R512 C505 |
| VERTICAL SWEEP DOES NOT OCCUR | IC601 R508 R511 R504 R509 R512 R505 R510 R513 R506 C503 R514 R507 C506 R515, IC601 R505 R506 R519 Q501 D502 R507 R520 Q502 D503 R508 C509 DY601 R503 R516 C510 D504 R504 R518 C512 R524 |
| NO SPECIFIC COLOR APPEARS | Check the red video circuit (Q451, Q851), Check the green video circuit (Q461, Q861), Check the blue video circuit (Q471, Q871) and related components listed in manual |
Troubleshooting and error codes
| Problem | Possible Cause |
|---|---|
| NO POWER | SW701, D704, F701, T702, F702, T701, D701, R701, D702, C701, D703, D709, Q701 |
| NO RASTER APPEARS | Power/horizontal sync/protector circuit, Connections between monitor and computer, CRT R872 T602 C859, Input circuit/computer, Q401, Q402 and its related circuits |
| NO PICTURE APPEARS | Check video input circuit and its related circuits, Check Q402 and its related circuits, Check Q451, Q851 and its related circuits, Input cable input socket computer IC491 IC492 IC493 IC494 |
| NO SYNCHRONIZATION CAN BE PROVIDED | IC601, Q423, C606, C604, IC601, R605, R620, R606, D602, R607, Q423 |
| VERTICAL SYNCHRONIZATION CANNOT BE OBTAINED | IC601 R502 C501 IC601 R514 C504, IC601 R513 R510 R514 R511 C503 R512 C505 |
| VERTICAL SWEEP DOES NOT OCCUR | IC601 R508 R511 R504 R509 R512 R505 R510 R513 R506 C503 R514 R507 C506 R515, IC601 R505 R506 R519 Q501 D502 R507 R520 Q502 D503 R508 C509 DY601 R503 R516 C510 D504 R504 R518 C512 R524 |
| NO SPECIFIC COLOR APPEARS | Check the red video circuit (Q451, Q851), Check the green video circuit (Q461, Q861), Check the blue video circuit (Q471, Q871) and related components listed in manual |