Amstrad CTM-644 Maintenance Guide
The Amstrad CTM-644 is a colour monitor with the CPC system's mains power supply built in: it supplies 5 V to the computer and 12 V to the disc drive of a CPC 664 or 6128. The procedures below also apply to the Amstrad CTM-640, which lacks the 12 V supply; its own parts list is in the Amstrad CTM-640 Maintenance Guide. Figures are from the CTM644 section of Amstrad's CPC6128 service manual unless another source is given.[1]
Safety
[edit | edit source]Warning: the CTM644 contains mains voltage on its main board and a CRT charged to several kilovolts, which can hold its charge after the set is unplugged.
- Unplug the monitor from the mains before removing the back. Discharge the CRT as described in CRT Discharge Procedure before touching the board or the anode lead.
- Amstrad's alignment steps are made with the set running and the back off. CPC Amstrad International warns that contacts on the board carry mains voltage during this work.[2] Use one hand, insulated tools and an isolation transformer; Amstrad's later Plus-range monitor manual calls for an isolation transformer for any service on its monitors.[3]
- After any repair, Amstrad's manual specifies a 3 kV flash test between the mains lead (live and neutral joined) and all accessible metal, and an insulation test showing at least 4 MΩ between the same points.[1] Parts marked with a warning triangle on the circuit diagram are critical for safety and must be replaced with the parts listed.
Opening the monitor
[edit | edit source]- Switch off, unplug the mains lead and disconnect the computer.
- Remove the four screws that hold the rear cabinet and lift it off.[2]
- Discharge the CRT before touching anything inside.
- For alignment, reconnect the computer and stand the set so that both the screen and the board are visible.[2]
The cabinet parts list gives the front cabinet (170841), rear cabinet (170313), CRT (170307), deflection yoke (170308), line output transformer (170467), degauss coil (170842), DC cord (170316/A), DIN cord (170317/A), DC jack (170844) and the service/normal switch (900101).[1]
Supply checks
[edit | edit source]| Rail | Expected | Source in the set |
|---|---|---|
| Mains input | 240 V AC 50 Hz through F501 (2 A time-lag) | SW501, F501, line filter L501, bridge D501–D504, reservoir C505 (100 µF 400 V) |
| 5 V to the computer | Exactly 5 V across C518, set with VR501 | IC502 (L78MG); IC502 pin 1 at 18 V, pin 4 at 5.0 V in the voltage chart |
| 12 V to the disc drive | 11.8 V at the regulator output in the circuit diagram | IC503 (µPC78M12), 16.5 V at its input |
| CRT drive | 120 V at the collector of Q802 with brightness at maximum, set with VR402 | Alignment step 7 |
Amstrad's note on the 5 V adjustment says it should not be disturbed under normal conditions.[1] CPC Amstrad International measures the 5 V at the monitor's 5 V supply socket and allows a deviation of no more than −0.1 V.[2] If the 5 V is missing but the screen works, check the 5 V lead and plug before the supply. If the computer runs but the drive does not, measure 12 V at the jack J501.
Voltage chart
[edit | edit source]Amstrad's voltage chart, measured with the set working:[1]
| IC401 LA7800 pin | Voltage | IC401 pin | Voltage |
|---|---|---|---|
| 1 | 6.35 V | 9 | 0.93 V |
| 2 | 6.48 V | 10 | 5.55 V |
| 3 | 0.32 V | 11 | 0.96 V |
| 4 | 0 V | 12 | 11.04 V |
| 5 | 0 V | 13 | 0.89 V |
| 6 | 0.85 V | 14 | 11.18 V |
| 7 | 3.21 V | 15 | 12.43 V |
| 8 | 0.33 V | 16 | 4.11 V |
| Device | Readings |
|---|---|
| IC402 LA7830 / µPC1378 (vertical) | 1: 0 V; 2: 12.74 V; 3: 24.4 V; 4: 0.84 V; 5: 0 V; 6: 24.2 V; 7: 2.5 V |
| IC502 L78MG (printed "LM78M6") | 1: 18 V; 2: 0 V; 3: 5.4 V; 4: 5.0 V |
| Q405 2SD1397 (line output) | E 0 V; B 0.1 V; C 98.8 V |
| Q404 2SC2271 (line driver) | E 0 V; B 0.3 V; C 34.1 V |
The stage names in brackets are from the circuit diagram and parts list, where T401 is the horizontal drive transformer and FB401 the line output transformer.[1]
Alignment
[edit | edit source]Amstrad's procedure needs an oscilloscope, a frequency counter and an AVO meter, with the monitor connected to its computer.[1]
| Step | Function | Method |
|---|---|---|
| 1 | Black and white tracking | Turn the R and B drive controls VR804 and VR805 fully anticlockwise; turn the R, G and B bias controls VR801, VR802 and VR803 fully anticlockwise; set the service/normal switch SW401 to service |
| 2 | Black and white tracking | Set 120 V at the collector of Q802 with the brightness control, on the oscilloscope. Turn the screen control fully anticlockwise and bring it back until a dim line of one colour appears. Turn the other two colours up until the line is dim white. Return SW401 to normal; adjust the colour control if needed |
| 3 | – | If the result is not satisfactory, repeat step 2 |
| 4 | Vertical size | Display a paper-edge pattern and adjust VR406 for a 145 mm paper edge, measured with a non-magnetic ruler |
| 5 | Focus | Adjust the focus control on the line output transformer for the best definition, with brightness and contrast at normal settings |
| 6 | 5 V | Connect the meter across C518 and adjust VR501 for exactly 5 V. Not to be disturbed under normal conditions |
| 7 | Sub-brightness | Meter on the collector of Q802; with the brightness control at maximum, adjust VR402 for 120 V |
| 8 | Sub H. hold and H. hold | Turn the H. hold fully anticlockwise; with the frequency counter across the CRT heater and earth, adjust VR404 for 14,500 Hz, then VR403 for 15,625 Hz |
The amendment manual's CTM644 alignment is the same except step 7, which sets 123–124 V at the Q802 collector.[4]
CPC Amstrad International gives a home version of the same procedure, using BASIC programs in place of Amstrad's test patterns: a circle for vertical size (VR406), the focus and screen controls on the line output transformer, VR402 for 120 V at Q802 with the DIN plug removed, white balance with VR801–VR805 and SW401 after ten minutes' warm-up on a white screen, and a grid for convergence with the magnet rings on the CRT neck. It advises marking every preset's position before turning it. The deflection yoke and its rubber wedges are glued to the neck and are not normally moved.[2]
Capacitors
[edit | edit source]| Value | Circuit reference | Amstrad part no. |
|---|---|---|
| 2200 µF / 25 V | C424 | 170425 |
| 470 µF / 35 V | C402, C522 | 1422262 |
| 470 µF / 25 V | C435, C519 | 20044 |
| 470 µF / 10 V | C518 | 170424 |
| 220 µF / 160 V | C515 | 170851 |
| 220 µF / 35 V | C507 | 20055 |
| 100 µF / 400 V | C505 | 170423 |
| 100 µF / 160 V | C515 (printed; see below) | 1400246 |
| 100 µF / 35 V | C425 | 1422157 |
| 100 µF / 16 V | C412, C443, C523 | 20028 |
| 47 µF / 160 V | C401 | 170422 |
| 47 µF / 50 V | C512 | 170421 |
| 47 µF / 16 V | C405, C418 | 1400244 |
| 47 µF / 10 V | C436 | 170420 |
| 22 µF / 250 V | C430 | 170419 |
| 22 µF / 10 V | C437 | 170418 |
| 10 µF / 16 V | C520 | 20024 |
| 4.7 µF / 50 V | C407, C420 | 1400240 |
| 1 µF / 250 V | C506 | 1422152 |
| 1 µF / 160 V | C419 | 1422151 |
| 1 µF / 50 V | C414 | 20062 |
The list gives C515 twice, as 100 µF 160 V and as 220 µF 160 V. The circuit diagram and the amendment list both give 220 µF 160 V.[1][4] The circuit diagram also shows C517 (10 µF 25 V) in the 5 V control circuit, which the parts list omits.[1]
The amendment's list for the later chassis gives only the larger electrolytics, and several differ: C405 470 µF 16 V, C512 100 µF 50 V and C518 1000 µF 10 V.[4] Read the value and voltage printed on each part before ordering, and never fit a lower voltage rating. C505 sits on the rectified mains (about 340 V DC from 240 V AC) and must be a 400 V part.
The film capacitors around the line output stage are high-voltage parts: C431 (0.012 µF 1600 V polypropylene), C432 (270 pF 2 kV), C513 (2200 pF 4 kV), C502–C504 (0.0022 µF 2 kV) and others.[1] The amendment gives C431 as 0.82 µF 1.6 kV, a large difference from the original list; check the part fitted before replacing it.[4]
Faults
[edit | edit source]Amstrad's basic hardware analysis for the CPC 6128 handles the monitor first:[1]
- No power LED on the computer: check the monitor's mains fuse, then whether 5 V is present on the cable from the monitor. No 5 V means a faulty cable or plug from the monitor.
- Computer starts but no picture: check the RGB connector and turn the brightness up. Try the computer on another monitor; if it works there, the fault is in this monitor's cable termination, internal connectors or brightness control.
For the electrolytics, see Capacitor Failure Symptoms.
Related pages
[edit | edit source]- Amstrad CTM-644 · Amstrad CTM-640 Maintenance Guide
- Amstrad CPC 6128 Maintenance Guide · Amstrad CPC 664 Maintenance Guide
- Amstrad CPC 6128 Service Manual
- CRT Discharge Procedure · Capacitor Failure Symptoms · Recommended Tools
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Amstrad Consumer Electronics plc, CPC6128 Micro Computer / CTM644 Colour Monitor / GT65 Green Monitor Service Manual. Described at Amstrad CPC 6128 Service Manual. Safety test p. 2; basic hardware analysis p. 4; CTM644 cabinet drawing and parts list p. 26; electrical parts list and alignment instructions p. 27; parts list and voltages p. 28; circuit diagram pp. 29–30; main PCB pp. 31–32.
- ↑ 2.0 2.1 2.2 2.3 2.4 Jörg Gurowski, "Hinter Glas – Ein Blick in den Farbmonitor des CPC", CPC Amstrad International (1992). Transcribed at CPCrulez. Retrieved 2026-10-01. In German. Opening the set, the home alignment procedure with BASIC test programs, the 5 V tolerance and the convergence magnets.
- ↑ Amstrad plc, 464 PLUS / 6128 PLUS / GX4000 / MM 12 / CM 14 Service Manual, MM12 and CM14 electrical adjustments, pp. 37 and 44. Described at Amstrad 464 Plus / 6128 Plus / GX4000 Service Manual.
- ↑ 4.0 4.1 4.2 4.3 Amstrad plc, CPC464 / CPC6128 / GT65 / CTM644 / MP3 / CT1 Amendment Service Manual, CTM644 alignment instructions and electrical parts list, p. 27. Described at Amstrad CPC 464 / CPC 6128 / GT65 / CTM644 / MP3 / CT1 Service Manual Amendment.