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Sega Master System (Model 1) Video Output Repair

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Sega Master System (Model 1) video output schematic
Sega's video encoder circuit for the PAL board 171-5534. Composite video leaves IC9 on pin 20 and goes two ways: through R27 (75 Ω) and C28 (100 µF) to CN1 pin 3, and through R29 and C30 to the RF modulator

The Sega Master System (Model 1) sends composite video, RGB, sync and audio to its eight-pin DIN socket, CN1, and composite video and audio to the RF modulator. On the PAL board Sega's schematic shows the video encoder IC9 producing composite video on one pin, which then splits: one branch goes through R27 (75 Ω) and the electrolytic C28 (100 µF) to CN1 pin 3, the other through R29 and C30 to the RF modulator's video input.[1] When the picture is missing, the first fault Sega names is "C28, R27 is absent or faulty".[2]

The procedure below follows the composite signal back from the socket towards IC9, using the split to the RF modulator to narrow the fault down, and replaces the failed part.

Symptoms

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  • No picture from the AV socket, with the power LED lit.[2]
  • A picture on RF but not on the AV socket, or the reverse.

Faults that look similar:

  • No LED and no picture: the supply. See Sega Master System (Model 1) Power Regulator Replacement.
  • LED lit, black screen on both RF and AV, no sound either: the console is not running. Check reset and the clocks in the Sega Master System Troubleshooting Guide.
  • Wrong colours, distorted characters or jerky movement: Sega points at the video RAM, IC6 and IC7.[3]
  • A faulty AV lead. Sega's remedy for a video cable with no picture and no sound is to replace the cable.[4] Try another lead before opening the console.

Tools and parts

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  • Oscilloscope to see the video signal, and a multimeter.
  • Phillips screwdriver, soldering iron, desoldering pump or braid.
  • An electrolytic capacitor for C28: 100 µF, 10 V or higher (Sega part 150-0047, 100 µF 10 V).[5]
  • A 75 Ω resistor for R27, if it has failed.[5]

Procedure

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Narrow it down from outside

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  1. Try another AV lead and another input on the television.[4]
  2. Connect the RF output to a television and check for a picture. The two outputs share IC9's composite signal and part company after it, so comparing them shows which side of the split the fault is on.[1]
  3. If there is a picture on RF but not on AV, the encoder is working and the fault lies after the split: R27, C28, CN1 pin 3 and its solder joint.[1]
  4. If there is no picture on either, start at IC9.[1]

Open the console

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Sega Master System M4 Power Base PAL parts location
Sega's PC Main M4 parts-location drawing. CN1 and C28 are at the top edge left of centre, beside IC9; the RF modulator is at the top left
  1. Unplug the adaptor and all leads.
  2. Remove the screws in the bottom case and lift it off; on a West German console remove the bottom shield plate too.[6]
  3. Lift the board out with its top shield so that the area round CN1 and IC9 can be reached from both sides.[6]
  4. Find CN1, C28, R27 and IC9 on Sega's parts-location drawing.[7]

Follow the signal

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With the board powered and a game or the BIOS screen running, work back from the socket:

  1. Measure the DC level on the video line at CN1 with no load. Sega's figure is DC 1.3 V, which it takes at R23; the RGB pins 6, 5 and 8 give DC 1.9 V.[3]
  2. Look for video between CN1 pin 3 and pin 2 (ground).[2][1]
  3. Look for video on the IC9 side of C28, between R27 and C28. Video there and none at CN1 pin 3 means C28, or the track and joint from C28 to CN1.[1]
  4. Look for video at IC9's composite output (pin 20 on the CXA1145P drawn in the schematic). Video there and none between R27 and C28 means R27 or its joints.[1]
  5. If there is nothing at IC9's output, check the RGB signals into IC9 and out of it: Sega's list checks the RGB outputs on pins 23, 22 and 21 and the inputs as pins 1, 2 and 3.[2] Also check IC9's supply pins for 5 V. With good inputs and supply and no output, IC9 has failed.[2]

Repair

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  1. Switch off and unplug.
  2. Check R27 out of circuit: it should read 75 Ω.[5]
  3. Replace C28 with a new 100 µF electrolytic of 10 V or more. Fit it the same way round as the original; on Sega's drawing its positive lead faces R27 and IC9.[5][1]
  4. Reflow CN1's pins if any joint is cracked; Sega lists CN1 and soldering among the causes.[2]
  5. Replace R27 if it is open or far from 75 Ω.[5]

On SMS Power's forum, a member advising on a Sega Master System II with a CXA1145P encoder reported that replacing the coupling capacitor on the composite line, after the 75 Ω resistor from pin 20, sometimes cures a black screen; the same arrangement is drawn on the Power Base schematic.[8][1]

Reassemble

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Refit the board, the shields and the bottom case in reverse order.[6]

Testing

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  • A stable colour picture from the AV socket on CN1 pin 3, and from RF.[1]
  • DC 1.3 V on the video line and 1.9 V on the RGB pins of CN1 with no load, Sega's figures.[3]

Revision notes

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Sega's documents for the PAL board do not agree on IC9. The parts list gives a V7040 RGB encoder (313-5054), while the schematic draws a CXA1145P.[5][1] The pin numbers in Sega's check list (composite out on pins 19 and 20, RGB in on pins 1, 2 and 3) do not match the CXA1145P drawing, where pin 19 is a supply pin, composite comes out on pin 20 and the RGB inputs are pins 2, 3 and 4.[2][1] Read the part number on your IC9 before probing its pins. The R27, C28 and CN1 pin 3 path is the same in both documents.[5][1]

Some North American and later boards use a CXA1145 at IC9; see Sega Master System (Model 1) for the board list.

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Sega Enterprises, Ltd., Sega Master System Service Manual (Power Base), schematic "IC Board PB PAL VA1", 171-5534. Sheet 1 (p. 13, PDF p. 14): CN1 pins 1 AUDIO, 2 GND, 3 VIDEO, 4 +5 V, 5 G, 6 R, 7 C-SYNC, 8 B. Sheet 2 (p. 14, PDF p. 15): IC9 drawn as a CXA1145P with R in, G in and B in on pins 2, 3 and 4 through C11, C14 and C17 (0.1 µF), Vcc1 on pin 12 and Vcc2 on pin 19, R, G and B out on pins 23, 22 and 21 to CN1, composite video out on pin 20 through R27 (75 Ω) and C28 (100 µF) to CN1 pin 3, and from pin 20 through R29 and C30 and bead FB2 to RF modulator pin 3. Hosted on this wiki as File:Sega Master System (Model 1) Service Manual.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Sega Enterprises, Sega Master System Service Manual, p. 9, "In event picture is not present": checks for video between pins 3 and 2 of CN1, video at pin 3 of the RF modulator, video output from pins 19 and 20 of IC9, RGB output from pins 23, 22 and 21 of IC9, and RGB at pins 1, 2 and 3 of IC9; fault locations C28 and R27 absent or faulty, CN1, soldering, cut track, RF modulator (when picture is present on RF), IC9, and parts around IC9.
  3. ↑ 3.0 3.1 3.2 Sega Enterprises, Sega Master System Service Manual, pp. 4 to 6: test procedures, including CON1 checks of DC 1.3 V at R23 (video) and DC 1.9 V on pins 6, 5 and 8 (RGB) without load, with "In event IC9 is faulty" among the causes; the RF modulator check (+5 V present; channel 3 and 4 both working); and the symptom table by IC.
  4. ↑ 4.0 4.1 Sega Enterprises, Sega Master System Service Manual, p. 11, "Video Cable": no picture, no sound; cable or plug faulty, replace all.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Sega Enterprises, Sega Master System Service Manual, PC Main M4 parts list, "Mark 4 Part List PAL Type", pp. 19 to 21 (PDF pp. 20 to 22): R27 479-0750 75 Ω 1/6 W 5 %; C28 150-0047 100 µF 10 V; R29 1.2 kΩ; C30 47 µF 10 V; R23 2.2 kΩ; IC9 313-5054 "RGB ENCODER V7040"; CON1 212-5106 eight-pin DIN connector.
  6. ↑ 6.0 6.1 6.2 Sega Enterprises, Sega Master System Service Manual, p. 17 (PDF p. 18), exploded view "Assy Power Base", with the parts list on p. 18: bottom case A-2, IC board A-3, top shield A-4, bottom shield A-5 (West Germany only), screws A-18 to A-20.
  7. ↑ Sega Enterprises, Sega Master System Service Manual, p. 15 (PDF p. 16), parts location "PC Main M4", M4 Power Base/PAL. Hosted on this wiki as File:Sega Master System M4 Power Base PAL parts location.png.
  8. ↑ SMS Power! forums, "No video on Master system II", February 2017, reply by Neto: some models have a fuse between Vcc and the CXA1145P encoder; "Sometimes just replacing the coupling capacitor on composite video line connected to pin 20 of CXA1145P after a 75R resistor solve this"; the PAL Master System II model 3006-09 has no composite output and needs an RGB lead.
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