Mega Drive Repair

Hi guys,

I've just finished recapping my Mk1 mega drive, having just replaced a 7805 with a switching regulator.

Having finished the recap, I put the console away as it was late, and tested it the next day. In my haste to put it away, I made a fatal error - I hadn't clipped the legs off the last cap I replaced (the one on the left right by the RF connector). Presumably the cap shorted against the bottom metal plate, and probably somewhere else on the board. Extremely stupid, I know.

Now all the console will display is a black screen, sometimes with a squealing noise. With a game in the console, thin white lines flicker and roll down the screen. There is no sound either, mainly static though it varies between games.

The 7805 got very hot but is outputting the correct voltage, and all of the ICs are being fed with the correct voltages. They are also all heating up to some degree after being left on for a couple of minutes. The MD still switches on my MCD, though there is still no display.

While I know MDs are fairly cheap, this console has sentimental value to me, so I would really like to get it going again if possible. Any suggestions as to what is likely to have been affected by the short (assuming this is what caused the issue), and what I can do to diagnose/fix the problem?

 

People should stop doing those "mods" without doing proper research.

The original Mega Drive has two power rails (hence why there are two 7805 chips on the heatsink).

You should either put back the 7805 you removed or replace both with the new power supply (tie the +5v from both 7805 chips together and connect to the +5v output of your new power circuit.).

Hope you can get it working again with this little piece of advice.

FYI: There's two power rails because the total power consumption of chips on the Mega Drive is far more than what the 7805 chips can supply. So they split it like this:
Video Display Processor/Video RAM/YM2612(FM synth)/CXA1145/Analog stuff (outputs) on one 7805
68000/Z80/SEGA LSIs/RAMs/Cartridge on the other 7805

 

No its not, Mega Drive 1 draws about 700ma at 5volts.

I believe the reason the MD1 has two regulators, is that they originally intended to power the Mega CD from the Mega Drive itself

Did you replace both regulators? or just one? Although, I have a feeling some MD1's only had one regulator..

 

x

 

Thanks for the reply. I should have mentioned in the original post that I tested the console after replacing the 7805 with a switching regulator, and it was fully working. I had researched the mod - I was going to attempt to power the MCD through the MD. As the console was open, I then decided to do a recap as it would likely need one in the near future anyway. I only mentioned the regulator as it was the reason for doing the recap, not because the console had any bad cap symptoms.

The problem is that I stupidly forgot to snip the legs off the last cap I replaced, which caused a short (hence the 7805 getting very hot). Yes, I know. Extremely dumb mistake. I'm assuming an IC has gone but apart from testing they are getting power, I have no idea how to check them. I have ordered an AV lead in the hope that the issue is with the RF output, but tbh I'm not too hopeful.

Just to clarify, this is the issue: Now all the console will display is a black screen, sometimes with a squealing noise. With a game in the console, thin white lines flicker and roll down the screen. There is no sound either, mainly static though it varies between games. All ICs have power, and the MD still switches on my MCD, though there is still no display.

Any advice on what is likely to have gone given the symptoms is much appreciated.

 

When I attempted that myself I ended with:

Blown 68000
Blown VDP (315-5313)
Blown bus arbitrer (315-5664)
Blown Z80
Blown 7805

If people going to disagree for the simple sake of disagreeing, SEGA used many different 68000 chips on the Mega Drive. The only which matches what was said about the Mega Drive using only 700ma is the Hitachi HD68HC000P8

Those are more common on PAL and Japanese units.

On USA SEGA Genesis the Motorola or Signetics version are more common. Those draw A LOT more power as they're not CMOS. So please think more before posting stuff like that. (lol)

You can tell that a MD1 unit has Hitachi CPU without opening it. It doesn't get very hot after a while playing. Ones with Motorola or Signetics CPU get very very hot on the casing corner with the vents.

 

Additionally, be smart and don't mod the consoles.

Make a powerful 12V power supply (5Amp/12v is fine) and make three "barrel" plugs for it. You can power the MD, the CD and the 32X that way and the risk of screwing up is virtually 0. It will even help with making the grounding connection stronger.

Sadly you already fried stuff. And it was certainly due to difference of voltage on the power rails.

Read:
http://en.wikipedia.org/wiki/Latchup

"A latchup is a type of short circuit which can occur in an integrated circuit (IC). More specifically it is the inadvertent creation of a low-impedance path between the power supply rails of a MOSFET circuit, triggering a parasitic structure which disrupts proper functioning of the part, possibly even leading to its destruction due to overcurrent."

 

Hi l_olivera,

Are you still referring to the fact I replaced the regulator? If I wasn't clear, I did this before recapping and tested the console for a few hours with the new regulator. At this point it was working perfectly.

I can open it back up to check the regulator (I already checked the remaining 7805), but I have checked the voltages being fed to each IC and they are all good. So I'm confident that the fact I have replaced the regulator isn't the issue.

That said, I know I have fried something, I just don't know what. My first thought was the CXA1145P, but I'm not sure that would affect audio as well.

 

What blew the MD was one power rail having power while the other had no power. (You had a cap shorted, right ?)

So the stuff that blew will be definitely going to be on the power rail which actually had power at the time.

 

Yep, the cap shorted.

So it is the analogue stuff I should be looking at then. Does the fact that the MCD powers on and responds to the start button being pressed rule anything out?

 

That means the 68000, basic I/O stuff is running.

The VDP could still have some damage. (which can only be confirmed if you get picture output to work) To rule that out have the CXA1145 replaced.

Edit: CXA1145 does amplify audio for the A/V connector. Test if the headphones jack has any sound.

 

EIther silence or static/beeping through the headphone jack. The TV doesn't output the same sound through the RF cable, and usually has a faint hum. The volume slider does work though I doubt that means anything.

 

The slider working means the audio amplifier is working properly.

The Mega Drive has two sound sources, on the VDP there's three audio channels (PSG) and there's YM2612 chip.

The MEGA-CD has it's own sound source which is generated at a sampler chip on the CD unit itself. You can check that using the RCA plugs on the CD unit.

You could verify with a cartridge instead of the CD unit to see if it makes any sound.

But really by the sound of it, the unit might have some damage on the VDP chip... :\

 

Mega CD audio is perfect through the RCA plugs, so it sounds like you're right about the VDP. Thanks for your help.

The one currently in the console is a 315-5313A, does it have to be replaced with that exact part?

 

Nice try, but you fail...
I just had a look at my pics, and the console I measured the current draw on, has a Motorola CPU.

So please think more before posting stuff like that.

Don't use 12volts into any console that still has a linear 5volt regulator, all that will do it result in more heat...
You really only need 7volts to get a regulated 5volts inside the mega drive/mega cd, but you'll be fine using 9volts.

So please think more before posting stuff like that.

 

Datasheet say the thing can safely take up to 35 volts. The only actual requirement is that the voltage is about 2.5v over the target voltage.

I've been using a 2.5 amp/12V Toshiba switching power supply from an early 90s laptop on my Mega Drive/Mega-CD since about 1999 and it's going strong since then.
12v does not make my console any hotter than it was when I was still using two linear transformers.

The BTL drivers used on the CD are rated for 18v as well. If "overvoltage" were a issue my console and the CD unit would be blown by now.

 

Yeah, it can take upto 35volts, and every volt results in more heat!

Because they're linear regulators, all the voltage above the output voltage gets turned into heat...

So if you're outputting 5volts at 0.8amps, from 9volts input, this means you have a 4volt drop, 0.8amps x 4volts = 3.2watts of heat...

Increase the input voltage to 12volts, and now you're dropping 7volts x 0.8amps = 5.6watts to be dissipated into the heatsink...

Input 35volts, other than making you an idiot, means you'll have 24watts of heat... :/

So please think more before posting stuff like that.

 

And what is the point here ? You're just wasting my time, your time and the OP time with that. I won't bother replying you again, sir.

 

you really don't get how linear voltage regulators work, do you.. lol

So, as mentioned, and completely unrelated to this topic, if you want your game console with a linear 5volt regulator to run cooler, give it less input voltage, down to 7.5volts or so, not more input voltage!

 

He can't do that. The CD stuff need at least 9v to work properly. (forget what I said the post above lol)

And really what is 2 degrees Celsius more or less ... You think that will make a huge difference on the power bill? The point here was make hooking the system more convenient, no ?