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Ok so... i got a dead Master System the other day. Ebay, untested, you know how the story goes. So i opened it up, replaced the NEC 7805 Voltage Regulator with a brandnew L7805CV. Now the green LED comes on. The system has power. However the screen stays black. I get no audio. Is the new voltage translator incompatible? Do i have it in the right way? I soldered it in with the metalbit facing outwards, just as the NEC was in there. I saw some PCB photos on this forum where the metal bit of the 7805 faces inwards?

mas.jpg (227.33 KB)

dead nec.jpg (205.8 KB)

new voltage.jpg (117.31 KB)

L7805CV.jpg (3.17 MB)

have you considered replacing the capacitors as well?

Update: all the traces look spotless. I cant find cracks or corrosion of any sort. I also replaced the three 100uf caps, two near the RGB out on position c25 & c28 and the one near the power plug.

At the moment i am wondering if A) the voltage regulator is incompatible somehow, or B) if the z80 is shot?

Is it worth replacing the 4.7uf & 10uf caps as well? I know Game Gears wont switch on due to aging caps.

The 7805 looks fine.. the fact you have a power led means its giving an output.

Have you got an oscilliscope? Id check that the z80 and vdp both have a clock signal before going too much further with replacing caps.

Alas i don't have an oscilliscope. The L7805CV looks OK according to the datasheet. But then again i am no expert.

A decent multimeter should have a basic frequency counter function. You could use that to check the clocks. You should also test the supply rails to make sure the regulator is providing the correct voltage. Due to the age of the board it is highly probable that all of the electrolytic capacitors will need to be replaced.

Without test equipment there isn't much you can do to diagnose faults.

Consoles with a RF modulator sold as "untested" on Ebay don't mean "doesn't work and I'm trying to get away with it" in my book but "I don't know how to make it work"...

My money is on : It actually works fine, take a Megadrive RGB cable and hook it up, you'll see. If it doesn't, open the SCART cable and send a picture.

Your 7805 is indeed in the correct orientation, don't worry about other motherboard revisions ;)

When testing the console, don't leave it running for more than 40 seconds without heat the heat sink. Those things can get hot.

I have the same issue with the same model.

Did you get it resolved.

I have a spare ms2 and have started to swap parts.

The original faulty one so far has had:

All caps replaced
Voltage reg replaced
Swapped z80 for known good (put other one in ms2 and it still worked)
Reflowed most major ICs

Be interested to get this resolved.

The Service Manual is available for download from the SMSPower Development section. It contains troubleshooting steps.

What you can do next is:

1) Check the power rails to each chip for 5 volts.

2) Check the Reset line is 0V at power up for a split second before it returns to a steady 5 volts.

3) Check the clock is running at the VDP and CPU.

4) Check for activity at the BIOS and RAM CE and OE pins.

If you don't have a Oscilloscope to check #3 and #4, you can often use a multimeter as a crude test. For a signal that's toggling between 0 volts and 5 volts a DMM will show an average value somewhere in the middle when on the DC Volts setting e.g. 2.5 volts.

Thanks for the reply,
Iv had a poke about and all seems ok.

My other unit of same model arrived today also.

I swapped the Sony v7040 and it was still dead, the replacement unit still works.

I then had a thought about the crystal, so I swapped crystals. The crystal from working unit did not fix the original one, but I put the crystal into the new unit and it has the same symptoms as the original faulty unit.

So I think the crystal is bad and some components in this area may also be bad.

I’m going to take some readings and compare. If parts are cheap il just order all the parts and see what happens. Definitely need a new crystal, resistors seem ok, But IC11, 12 & 13 maybe faulty or maybe Q146 and / or Q153.

Will be back with results

Author	Message
xxx128 Joined: 26 Jul 2020 Posts: 3	Need help with dead M4 Power Base Pal Posted: Sun Jul 26, 2020 2:32 pm
xxx128 Joined: 26 Jul 2020 Posts: 3	Ok so... i got a dead Master System the other day. Ebay, untested, you know how the story goes. So i opened it up, replaced the NEC 7805 Voltage Regulator with a brandnew L7805CV. Now the green LED comes on. The system has power. However the screen stays black. I get no audio. Is the new voltage translator incompatible? Do i have it in the right way? I soldered it in with the metalbit facing outwards, just as the NEC was in there. I saw some PCB photos on this forum where the metal bit of the 7805 faces inwards? mas.jpg (227.33 KB) dead nec.jpg (205.8 KB) new voltage.jpg (117.31 KB) L7805CV.jpg (3.17 MB)

IllusionOfMana Joined: 08 Sep 2018 Posts: 270	Posted: Sun Jul 26, 2020 6:20 pm
IllusionOfMana Joined: 08 Sep 2018 Posts: 270	have you considered replacing the capacitors as well?

xxx128 Joined: 26 Jul 2020 Posts: 3	Posted: Sun Jul 26, 2020 8:48 pm
xxx128 Joined: 26 Jul 2020 Posts: 3	Update: all the traces look spotless. I cant find cracks or corrosion of any sort. I also replaced the three 100uf caps, two near the RGB out on position c25 & c28 and the one near the power plug. At the moment i am wondering if A) the voltage regulator is incompatible somehow, or B) if the z80 is shot? Is it worth replacing the 4.7uf & 10uf caps as well? I know Game Gears wont switch on due to aging caps.

wasup Joined: 05 Nov 2014 Posts: 435 Location: Auckland - NZ	Posted: Sun Jul 26, 2020 11:28 pm
	The 7805 looks fine.. the fact you have a power led means its giving an output. Have you got an oscilliscope? Id check that the z80 and vdp both have a clock signal before going too much further with replacing caps.

xxx128 Joined: 26 Jul 2020 Posts: 3	Posted: Sun Jul 26, 2020 11:47 pm
xxx128 Joined: 26 Jul 2020 Posts: 3	Alas i don't have an oscilliscope. The L7805CV looks OK according to the datasheet. But then again i am no expert.

Marty Joined: 19 Dec 2005 Posts: 34 Location: United Kingdom	Posted: Sun Aug 02, 2020 11:06 pm
	A decent multimeter should have a basic frequency counter function. You could use that to check the clocks. You should also test the supply rails to make sure the regulator is providing the correct voltage. Due to the age of the board it is highly probable that all of the electrolytic capacitors will need to be replaced. Without test equipment there isn't much you can do to diagnose faults.

SLr Joined: 05 Aug 2020 Posts: 15 Location: York, UK	Posted: Wed Aug 05, 2020 8:40 pm
SLr Joined: 05 Aug 2020 Posts: 15 Location: York, UK	Consoles with a RF modulator sold as "untested" on Ebay don't mean "doesn't work and I'm trying to get away with it" in my book but "I don't know how to make it work"... My money is on : It actually works fine, take a Megadrive RGB cable and hook it up, you'll see. If it doesn't, open the SCART cable and send a picture. Your 7805 is indeed in the correct orientation, don't worry about other motherboard revisions ;) When testing the console, don't leave it running for more than 40 seconds without heat the heat sink. Those things can get hot.

djrookie Joined: 18 Oct 2020 Posts: 2	Posted: Sun Oct 18, 2020 6:31 pm
djrookie Joined: 18 Oct 2020 Posts: 2	I have the same issue with the same model. Did you get it resolved. I have a spare ms2 and have started to swap parts. The original faulty one so far has had: All caps replaced Voltage reg replaced Swapped z80 for known good (put other one in ms2 and it still worked) Reflowed most major ICs Be interested to get this resolved.

asynchronous Joined: 14 Aug 2000 Posts: 742 Location: Adelaide, Australia	Need help with dead M4 Power Base Pal Posted: Tue Oct 20, 2020 1:49 pm
	The Service Manual is available for download from the SMSPower Development section. It contains troubleshooting steps. What you can do next is: 1) Check the power rails to each chip for 5 volts. 2) Check the Reset line is 0V at power up for a split second before it returns to a steady 5 volts. 3) Check the clock is running at the VDP and CPU. 4) Check for activity at the BIOS and RAM CE and OE pins. If you don't have a Oscilloscope to check #3 and #4, you can often use a multimeter as a crude test. For a signal that's toggling between 0 volts and 5 volts a DMM will show an average value somewhere in the middle when on the DC Volts setting e.g. 2.5 volts.

djrookie Joined: 18 Oct 2020 Posts: 2	Posted: Wed Oct 21, 2020 4:46 pm
djrookie Joined: 18 Oct 2020 Posts: 2	Thanks for the reply, Iv had a poke about and all seems ok. My other unit of same model arrived today also. I swapped the Sony v7040 and it was still dead, the replacement unit still works. I then had a thought about the crystal, so I swapped crystals. The crystal from working unit did not fix the original one, but I put the crystal into the new unit and it has the same symptoms as the original faulty unit. So I think the crystal is bad and some components in this area may also be bad. I’m going to take some readings and compare. If parts are cheap il just order all the parts and see what happens. Definitely need a new crystal, resistors seem ok, But IC11, 12 & 13 maybe faulty or maybe Q146 and / or Q153. Will be back with results

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