The 171-5379 motherboard

The motherboard is Sega's 171-5379, an early design originally used for NTSC Master Systems.

For the French market, Sega reused this board but changed the video configuration considerably.

On the French version:

  • the Sony V7040 video encoder is not fitted;
  • the RF modulator is not fitted;
  • much of the associated video circuitry is absent;
  • RGB is taken directly from the Sega 315-5124 VDP;
  • the console is configured to run at 50 Hz.

So the NTSC marking on the PCB does not mean that the console, as sold in France, originally operated at 60 Hz.

It describes the motherboard design from which the French version was derived.

French Master System 171-5379 motherboard

Not really a "PAL Master System"

Calling this console a PAL Master System is actually slightly misleading.

There is no PAL colour encoder in this version.

The console outputs RGB directly, so there is no PAL or NTSC colour encoding involved at all. What is European about the factory configuration is primarily its 50 Hz video timing.

This distinction becomes particularly interesting when modifying the console for 60 Hz.

Raw RGB, but not television-ready RGB

At first the missing video encoder sounds like a very good thing: RGB comes almost directly from the VDP.

There is a catch.

The RGB output is unamplified and unbuffered and is not intended to drive a normal 75-ohm television input directly.

Sega solved this in France by supplying a special SCART adapter:

SEGA Adapteur R.V.B. 3085

The 3085 contains the electronics needed to condition the RGB signals before feeding them to a television.

This means that an ordinary passive Master System RGB-to-SCART cable is not necessarily suitable for this French console.

Some modern Master System RGB cables contain only passive components such as resistors and capacitors. Those are intended for consoles whose RGB output has already been appropriately conditioned internally.

The early French Master System needs actual amplification/buffering.

I therefore ordered an original Sega 3085 rather than using a normal passive RGB cable.

Audio but no picture

Before getting the proper RGB adapter, I tried the console on an old CRT television.

The result was:

  • audio worked;
  • no usable video appeared.

This was actually encouraging rather than alarming.

The machine was clearly alive and running software, while the lack of video was entirely consistent with trying to use the unusual raw RGB output without the required French adapter.

There is also another complication with SCART: a consumer television normally expects a switching/blanking signal telling it to select RGB mode.

A professional RGB monitor can sometimes be more useful for testing because RGB and external sync can be selected manually, but the signal levels still need to be considered.

Power supply

The Master System 1 uses a centre-negative DC power supply.

The relevant specifications are approximately:

  • 9 V DC;
  • centre negative;
  • 5.5 x 2.1 mm barrel connector;
  • around 850 mA or more.

A power supply capable of delivering more current is fine; the console only draws what it needs.

Polarity, however, matters.

Why 50 Hz matters

Many Master System games were designed around approximately 60 Hz timing.

On a 50 Hz machine, games which were not specifically adjusted for the European market can run noticeably slower.

The basic relationship is:

50 / 60 = 0.8333

So a game whose logic is directly tied to vertical refresh may run about 16.7% slower at 50 Hz.

Depending on the game, this may affect:

  • gameplay speed;
  • animation;
  • music tempo;
  • scrolling;
  • screen borders.

Some European releases were properly adapted for 50 Hz, so the effect is not identical for every game.

A particularly convenient console for a 60 Hz mod

This French motherboard is unusually convenient for converting to 60 Hz.

A conventional European PAL Master System may require additional work because its master clock and video encoder are designed around PAL operation.

The French 171-5379 is different.

It is fundamentally based on the NTSC hardware and already uses the NTSC-style clock arrangement. Since it has no PAL video encoder at all, there is no colour encoder to modify either.

The VDP has simply been configured for 50 Hz.

On the motherboard there is a clearly marked three-pad selection area:

NTSC     PAL
  o   o   o

The centre pad is the VDP mode-select connection.

In the original French configuration, it is linked to the PAL / 50 Hz side.

Changing the connection to the NTSC / 60 Hz side makes the VDP run at 60 Hz.

NTSC PAL jumper on the 171-5379

Installing a 50/60 Hz switch

Rather than permanently converting the console to 60 Hz, I decided to make it switchable.

I removed the original 50 Hz jumper connection and wired the three pads to an SPDT ON-ON switch.

Conceptually the wiring is simply:

          60 Hz / NTSC
                o
                 \
VDP / common -----o
                 /
                o
          50 Hz / PAL

This lets the VDP mode-select pin connect to either the 50 Hz or 60 Hz configuration.

This even works on the fly while playing, slowing down or speeding up as desired.

The modification is extremely simple and, importantly, completely reversible.

Reusing the unused RF opening

One nice feature of this French motherboard is that the RF modulator was never fitted.

That leaves unused openings in the rear of the case.

Rather than drilling another hole in a 1980s console, I wanted to reuse one of those existing openings for the 50/60 Hz selector for a latching pushbutton, ideally:

  • approximately 12 mm mounting diameter;
  • black;
  • no LED;
  • 1NO1NC / SPDT;
  • three terminals.

The finished mod uses the existing unused RF opening, so there was no need to drill the case.

50 Hz and 60 Hz after the modification

The console can now operate in either mode.

At 50 Hz, this is essentially the original French configuration. It is useful for games designed or optimised specifically for European timing (i.e. not that many except maybe Ocean games and the likes)

At 60 Hz, this looks like a great upgrade. Because the French console already has the appropriate NTSC-derived clocking, the result is not merely a crude refresh-rate hack. The CPU and VDP timing are appropriate for the 60 Hz configuration. Because the console outputs RGB rather than encoded composite video, there is no PAL-versus-NTSC colour compatibility problem.

I tested the obligatory Sonic the Hedgehog and it plays like a dream at 60 Hz, despite being a european cartridge. The game is quite slower at 50 Hz!