Does the Sega Megadrive 2 have built-in protection against reverse polarity

Question

I have a spare universal power supply with regulated voltage up to 12 volts, max 1500mA. It has replaceable tips, where you can reverse polarity. As I understand the MegaDrive 2 (European / Australian version type MK-1631-50, in my case) requires positive in barrel core and negative on the outside.
If I connect it the other way would any components blow?

Raffzahn · Accepted Answer

TL;DR:

If I connect it other way would any components blow?

No.

Let's have a look at the circuitry
All following schematics are taken from the 1993 manual.
To start with, the wall wart is delivering 9..10V (depending on region *1) DC.

(Page 7 lower right corner, B6/B7)
Polarity is not mentioned at all - not here nor in any other overview or part list. Only the drawing of connector CN7 in the power circuit (D1) seams to indicate plus at the center pin and ground outside:

(Page 11 middle left, C/D-1/2/3)
The circuit itself isn't surprising. There is a diode protecting against reverse polarity, so no need to be afraid of frying the circuit. That is unless it receives high voltage. The Diode is a 1SR35-100A, good for, as the name suggests, 100 Volt (*2). So short cutting it at mains level will fry the circuitry. So lets better stay below.
Having such a diode in line with the regulator, working as half wave rectifier, would allow a feed with AC as well. But I'm not sure if it's sufficient, to me it seems to assume a full wave rectifier, as the 0.022µF capacitor is comparatively small. But it might work.
The 7805 regulator delivers 5 Volt at output. It is split into three separate routed branches (not shown on the insert, check p.11 C3):

VCC for all digital electronics
AVCC for audio components
VVCC for video components

The later two only used by the Video Display Processor (VDP).
To see what the minimum voltage for feeding is we need to look at the 7805. It seams to be a standard type, not an LDO (Low Dropout), so a 2 Volt drop is to be assumed. The diode has a drop of 1.1V. Operating in line with the 7805 we get a minimum input voltage of 8.1 Volt (5 + 2 + 1.1).
Interesting is a tap before the diode named +9V (*3). It is not used anywhere within the MegaDrive, but delivered to the peripheral connector CN2 (see p.9 C/D-3/4) aka expansion port. Tapping it before the diode allows expansions to draw more than the 1A the main system is limited to by diode and regulator. Of course this requires as well a larger PS.
Bottom Line:

It got a protection diode
Reverse polarity will not kill the MegaDrive
Plus seams to be in the middle, ground on the outside
PS should use a full wave rectifier (always better)
It needs at least 8.1 Volt
It shouldn't give more than 10 Volt
It should deliver at least 0.85 Ampere, better might be 1A
Anything above 100 Volt may fry the system

*1 - Strangely enough the 240V UK version is supposed to give 9V while the continental 230V version is rated at 10V. See p.38
*2 - Yes, I know, there is more to it, just details don't matter here.
*3 - The naming is, as so oft only a hint, as this can carry 10V as well, depending on the PS.

Hitek · Answer

Based upon a quick look at the PAL Megadrive II schematics, it appears as if the the power input is first routed through a common mode choke(for which polarity does not matter) before it goes onto the power switch.  After the power switch, it is routed through a reverse polarity protection diode before it is fed to the input of a LM7805 regulator to provide the system's Vcc bus.
The schematic also shows a branch of the 9 Volt source(before the reverse polarity protection diode) that only appears to be connected to a small ceramic bypass capacitor, for which polarity also doesn't matter.
While it's not clear to me what the 9 Volt branch might be used for other than possible availability on an expansion connector(which I did not see) for an external peripheral, it does appear as if the Megadrive II's internal gaming circuitry is protected by a diode on the power input, so at least the internal circuitry should be safe from a power adapter of the wrong polarity.

lvd · Answer

Depends on how powerful your power supply is. Most NMOS chips behave like a big diode from GND to Vcc when polarity is reversed, so weak power supply would simply go into short-circuit protection.
If there is 7805 inside the SMD2, there are chances that 7805 will blow, though.

Does the Sega Megadrive 2 have built-in protection against reverse polarity

3 Answers

TL;DR:

Let's have a look at the circuitry

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