Power Mega CD from Mega Drive

Hi Guys,

I was looking into the possibility of powering the MCD from the MD through the card connector. I have read a few threads around the internet, and it seems to come down to replacing one of the 7805s in the MD with a 2676 or similar, and soldering a wire in the MD and MCD.

I have a PAL VA6 MD with 2 7805s, and I think the rear one is the one I should replace. However, what I can't find is any information on where to solder the wire in the MCD v1.

Also, are there any side effects to doing this? For example, is the MCD/connector designed to carry the amount of current required?

 

The MD1 have two 5volt regulators, however, they don't need to be kept separate/isolated.

What I mean, is you can remove both 7805 regulators, and replace them with a single 5volt switchmode regulator

LM2676 is rated at 3amps, so will replace both regulators, and run the MegaCD just fine.

From memory, I measured about 800ma at 5volt current draw in the MegaDrive 1.

This is what I fitted to mine, mainly because I already had a stack of them:

 

Hadn't thought about replacing the 2 7805s with only one regulator. The reason I was only going to replace one of them is that I have read that a switching regulator will introduce interference to the video output. Have you had any problems? If it isn't too bad, I would consider replacing both.

 

The module I'm using is a high quality, low noise device... so its not an issue.
I'm not sure how a different regulator will perform.

edit: I believe that the edge connector, and video chip are run by the same regulator anyway...

 

An LM2676 sounds like a great idea, and I was looking on the datasheet:

http://www.ti.com/lit/ds/symlink/lm2676.pdf

Will it be necessary to apply additional components to the LM2676 or would it work as a "drop in" replacement for the two 7805 regulators?

 

From what I understand, one regulator is for the analogue rail (video, sound etc) and one for the digital rail (edge connector etc). Hence why a switching regulator would affect the video/audio if used for the analogue rail.

Sure I read somewhere that it isn't a drop in replacement, not sure what else is required. Wish I had bookmarked the info I have found.


Edit: Found some good info here: http://www.sega-16.com/forum/showthread.php?10897-Genesis-32X-with-a-single-PSU/page3. Apparently the 2676 needs "a 0.01µF ceramic capacitor, a small ferrite core coil, a schottky barrier diode, a bigger ferrite core coil to filter out high freq noise from the output and a bigger electrolytic capacitor on the output to smoothen out the voltage spikes."

 

I believe you are correct, the audio and video are on a different regulator.
However, remember, the MD2 only have a single regulator

 

Actually, I was saying that the audio and video are on the same regulator.

I have found a drop in replacement for the 7805 which looks to have all of the required components on the board here. Bit expensive though. There are other cheaper versions, but I like the fact this is small and has a 3 pin header.

I have also found this, but it might need extra components to work. The datasheet mentions adding a 47uF cap between +Vin and GND, but there is a diagram showing a more complex circuit? If it only needs one capacitor, I might give it a go.

 

Sorry, I mean the audio and video are on a different regulator, to the expansion slot

You can probably get away with no other external components on that regulator you linked to... they are just to stop noise being introduced BACK into the power supply...

 

Those components are only needed if you're building a new circuit around it; if you're using it to replace a 7805 in a preexisting design you don't need them.

 

Ah ok, I misread your post

So I could just get a simple 2676 and perform a straight swap? That is, if I can find one with only 3 legs?

 

I was talking about the one you linked to in my quote, not the LM2676.

 

I see it now

I'll order one of those then, thanks for the help so far.

Now I need to find where to solder a wire inside the MCD. According to this pinout, the 5v line on the expansion connector is pin 26. If I solder a wire from pin 26 on the MCD side to the output leg of the regulator, it should work?

 

Finally got round to replacing the regulator. In the end I used a powermax 7805-1000, tried 2 SR10S05s and neither worked for whatever reason. I've also replaced the heatsink on the other 7805.

Not sure I'm keeping my MCD, so I've been thinking of other ways to mod the power input.

If I bridge the regulator points by adding a wire from pin 1 to pin 3, and replace the power connector with a microusb connector, is it possible to use a phone charger to power the console (depending on current)?