Jump to content

Sega Mega Drive / Genesis (Model 1) Power Regulator Replacement

From RetroTechCollection
Sega Mega Drive Model 1 regulators IC15 and IC17
The power corner of a PAL Mega Drive "IC BD M5 PAL" board: the two 7805 regulators IC15 and IC17 against the heat sink strip on the left edge, the reservoir C1, FB1, FB2, the DC input lead connector CN11 and the slide switch SW1 (photo: Bill Bertram, CC BY-SA 3.0, cropped)

The Sega Mega Drive / Genesis (Model 1) has two 7805 linear regulators on its main board. IC15 supplies the rail Sega calls Vcc1 and IC17 supplies Vcc2, and each is screwed to Sega's heat sink M5-02.[1][2] Sega's repair procedure for a console whose power LED does not light lists a faulty IC17 among the causes, together with cracked tracks at the DIN connector, under the heat sink and at the power switch.[3] Sega's PAL-G maintenance manual documents two boards with this layout, VA4 and VA6.5. Check that your board has two regulators at IC15 and IC17 before ordering parts.

Symptoms

[edit | edit source]
  • No power LED. On Sega's VA4 schematic the LED connector CN9 is fed from Vcc2 through a 120 Ω resistor, so the LED stays dark when IC17 has no output.[1] Sega's list of causes for no power LED is a crack or bad solder at the DIN connector, a track broken by a crack or corrosion under the heat sink ("radiation portion"), a track broken by a crack or corrosion at the power switch, and a defective IC17.[3]
  • LED lit but no picture, with one rail missing. Sega's flow chart checks that Vcc1 and Vcc2 are both +5 V before it checks the crystal and reset circuits.[4]

A low voltage at the regulator inputs points to the adaptor, the power switch, the DC jack or the tracks before the regulators, not the regulators. In a Racketboy forum thread about a dead Model 1, the regulator inputs measured about 6 V and the moderator advised tracing the path from the DC jack before replacing the regulators.[5] Work through the Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide first if the fault is not clearly in the supply.

Safety

[edit | edit source]

Unplug the adaptor before desoldering, and let the regulators and the heat sink cool before handling them. In a Racketboy forum thread, the owner of a faulty console found the heat sink very hot with the power on.[5]

Tools and parts

[edit | edit source]
  • A multimeter, a temperature-controlled soldering iron, desoldering braid or a desoldering pump, a cross-head screwdriver and isopropyl alcohol.
  • Thermal compound. Sega fits thermal joint compound (601-0076) between each regulator and the heat sink.[2]
  • A replacement 7805 in a TO-220 package. Sega's parts lists give two alternatives for each position: the NEC MA7805UC (313-0092) and the Motorola MC7805CT (313-5092).[2][6] The MC7805CT is in onsemi's current MC7800 datasheet.[7]
Regulator parts on the VA6.5 board (837-8779), from Sega's parts list[2]
Item Part Sega part number
IC15, IC17 7805 regulator, MA7805UC or MC7805CT 313-0092 or 313-5092
Heat sink Heat sink M5-02 530-5038-02
Fixing screw, one per regulator Deltite screw M3 × 6 029-0227
Thermal compound, IC15 and IC17 Thermal joint compound 601-0076
Screw lock, IC15 and IC17 Screw lock 090-0012

The VA4 list has the same heat sink and the same two 7805 types, with thermal compound at two positions and M3 × 6 Deltite screws at four.[6]

7805 pins, TO-220 package[7]
Pin Function
1 Input
2 Ground; the metal tab is connected to pin 2
3 Output, +5 V

The supply circuit

[edit | edit source]
Sega Mega Drive M5 PAL VA4 power input schematic
Power input on Sega's VA4 schematic: DC jack lead (CN11), FB1, power switch SW1, the two 1SR35-100A diodes, C1, and the regulators IC15 (Vcc1) and IC17 (Vcc2), each drawn with its heat sink

On Sega's VA4 schematic the DC input from the jack on its sub board comes in at CN11 and passes through ferrite bead FB1, the power switch SW1 and two 1SR35-100A diodes to the reservoir capacitor C1 and the inputs of IC15 and IC17. The ground return passes through FB2. IC15's output is Vcc1 and IC17's output is Vcc2.[1] Sega's parts list gives C1 as a 220 µF 16 V electrolytic and the diodes as D1 and D3.[8][2] The PAL-G adaptor supplied with the console is Sega 400-5122A or 400-5122B, AC 220 V in and DC 10 V 1.2 A out.[9]

Procedure

[edit | edit source]

Opening the console

[edit | edit source]

Sega's disassembly procedure for the PAL-G console. Screw 201 is a 3 × 10 mm tapping screw, 202 a 3 × 8 mm tapping screw, 203 a 3 × 6 mm Deltite screw and 204 a black 3 × 12 mm tapping screw.[10]

  1. Turn the console over and remove the six screws (201) from the bottom case.[10]
  2. Lift the top case and unplug the two-pin power LED connector from the underside of the top case.[10]
  3. Remove seven screws (202) and one screw (203), and lift off the shield plate.[10]

Testing the supply

[edit | edit source]
  1. With the board still in the bottom case, connect the adaptor and switch on. Measure the voltage at the power switch. If it is missing, check the tracks between the DC input and the switch, especially for board cracks.[4]
  2. Measure each regulator with the meter's negative probe on the centre pin (pin 2, ground). Pin 1 should read the input voltage and pin 3 should read 5 V.[5][7] onsemi specifies 4.8 to 5.2 V for the MC7805C at 25 °C, and 4.75 to 5.25 V across its load range with 7 to 20 V at the input.[7] Sega's flow chart asks for +5 V on both Vcc1 and Vcc2.[4]
  3. If the input is too low for the regulator, look before the regulators. The 7805's input must stay about 2 V above its output.[7] Check the switch SW1 with the console unplugged. A Racketboy moderator's test is a continuity check across the switch: no connection with it off and a dead short (0 Ω) with it on. For a suspect switch he suggests flooding it with isopropyl alcohol or contact cleaner, working it many times and letting it dry fully before testing again.[5] Then check FB1, FB2, the diodes and the tracks for cracks or corrosion.[1][3]
  4. If the input is correct and the output is missing or low, switch off and unplug, and check the output pin to ground for a short with the meter on continuity or resistance, so that a shorted rail does not destroy the new part.[5]

Removing the regulator

[edit | edit source]
  1. Remove one more screw (202) and the five black screws (204), and lift the main board out of the bottom case.[10]
  2. Support the board while it is out. Sega's repair procedure traces several faults to cracked tracks, including cracks under the heat sink.[3]
  3. Remove the M3 × 6 Deltite screw that holds the regulator to the heat sink.[2]
  4. Desolder the regulator's three pins with braid or a pump and lift it off the heat sink.
  5. Clean the old thermal compound off the heat sink with isopropyl alcohol, and inspect the board under the heat sink for cracked tracks or corrosion.[3]

Fitting the new regulator

[edit | edit source]
  1. Apply a thin layer of thermal compound to the back of the new regulator.[2]
  2. Fit it in the same position as the old one, pin 1 (input) and pin 3 (output) in the same holes.[7][1]
  3. Fix it to the heat sink with the M3 × 6 Deltite screw.[2]
  4. Solder the three pins and check for bridges.
  5. Before refitting the board in the case, power it on the bench and repeat the measurements: both regulator outputs should read +5 V.[4][7]

Reassembly

[edit | edit source]
  1. Set the main board on the bottom case with its screw holes over the case's, and fit the five screws (204) and the one screw (202) that hold it.[10]
  2. Fit the shield plate over the bosses of the bottom case, then the screw (203) and seven screws (202). Seat the headphone and AC adaptor sub boards on their bosses.[10]
  3. Set the volume knob to 0 and the power switch knob on the top case to off, reconnect the power LED lead with the LED's longer lead (anode) to the red wire, fit the top case and fit the six screws (201).[10]

Testing

[edit | edit source]
Checks after replacing a regulator
Check Where Expected
Supply at the switch SW1, switch on The adaptor's output[4]
Regulator input IC15 and IC17 pin 1 to pin 2 At least about 2 V above the output[7]
Vcc1 IC15 pin 3 to pin 2 +5 V; 4.8 to 5.2 V for an MC7805C at 25 °C[4][7]
Vcc2 IC17 pin 3 to pin 2 +5 V, as Vcc1; the power LED lights[4][1]

Then run a game and check picture and sound. If the picture is still missing with both rails correct, Sega's flow chart moves on to the crystal and the reset signals; see the Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide.[4]

[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Mega Drive Maintenance Manual, PAL-G, August 1992, section 9, schematic "PC BD M5 PAL VA4 MAIN", sheet 1 (PDF p. 65): CN11, FB1, FB2, SW1, the two 1SR35-100A diodes, C1, IC15 (Vcc1) and IC17 (Vcc2) with heat sinks, and the LED connector CN9 fed from Vcc2 through 120 Ω.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Mega Drive Maintenance Manual, PAL-G, August 1992, section 8-1, parts list IC BD M5 VA6.5 PAL-G, 837-8779: line 2, heat sink M5-02; lines 51-54, IC15 and IC17 (313-0092 MA7805UC or 313-5092 MC7805CT); lines 56 and 59, D1 and D3 1SR-35-100A (PDF p. 99); lines 178-181 and 190-191, screw lock, thermal joint compound (601-0076) and Deltite M3 × 6 screws (029-0227) for IC15 and IC17 (PDF p. 102).
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Mega Drive Maintenance Manual, PAL-G, Sega Enterprises, August 1992, Rev. A, section 5 "PCB Repair Procedure", 3-1-1 "No power LED on" (PDF p. 30). See Mega Drive Maintenance Manual - PAL-G, August, 1992, Rev. A.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Mega Drive Maintenance Manual, PAL-G, August 1992, section 5, "PCB Repair Flow Chart" (PDF p. 28): voltage at the power switch, then Vcc1 and Vcc2 = +5 V with IC15 and IC17 named, then the crystal and reset signals.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 "Friend's Genesis (Model 1, 1601) Died", Racketboy forum, Technical Help, October 2022: measuring the regulators (centre pin ground, input on one outer pin, 5 V on the other), heat sink running hot, regulator inputs reading about 6 V, checking the DC jack, switch and tracks before replacing the regulators, switch continuity test and cleaning. Community discussion.
  6. ↑ 6.0 6.1 Mega Drive Maintenance Manual, PAL-G, August 1992, section 8-1, parts list IC-BD M5 VA4 PAL-G: heat sink M5-02 (530-5038-02) and IC15/IC17 MA7805UC or MC7805CT (PDF pp. 84-85); thermal compound at two positions and Deltite M3 × 6 screws at four (PDF p. 91).
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 onsemi, MC7800, MC7800A, MC7800AE, NCV7800: 1.0 A Positive Voltage Regulators, data sheet: TO-220 pin assignment and tab connection (p. 1), input-output differential of about 2 V (p. 1), MC7805C output voltage limits and dropout (electrical characteristics).
  8. ↑ Mega Drive Maintenance Manual, PAL-G, August 1992, section 8-1, parts list IC-BD M5 VA4 PAL-G: C1, CAP E 220 µF 16 V U-type (150-0159) (PDF p. 97).
  9. ↑ Mega Drive Maintenance Manual, PAL-G, August 1992, section 4 "Accessories List for PAL-G" (PDF p. 23).
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 Mega Drive Maintenance Manual, PAL-G, August 1992, section 2-1 general reference number list (PDF p. 7) and "Procedure of Disassembly and Assembly" (PDF pp. 18-19).
Retrieved from "https://retrotechwiki.com/index.php?title=Sega_Mega_Drive_/_Genesis_(Model_1)_Power_Regulator_Replacement&oldid=29806"