The TV Tuner Adaptor plugs into the cartridge slot of the Game Gear. It has a UHF/VHF tuner and antenna, an external antenna input, and an external A/V (monaural audio / composite video) input.

There are two revision of the TV tuner, one with an analog tuner that you adjust with a dial manually, and one with a digital tuner that scans for channels based on buttons you push.


The analog TV tuner uses the following parts:

The digital TV tuner uses the following parts:

Functional description

The TV tuner consists of a tuner PCB and digital video PCB joined by a ribbon cable. The tuner PCB contains most of the analog circuitry, ALPS tuner module, M51348 VIF/SIF demodulator, a discrete 4.5V source, and the antenna, external antenna, and A/V connectors. It also has a daughterboard with the channel tuning dial and indicator. The M51348 outputs composite video and composite sync which is fed to the digital video PCB.

The digital video PCB uses the M51405 to convert composite video to analog RGB video, which is passed to the hybrid module and digitized. The hybrid module takes the composite sync output from the tuner board and strips out the vertical sync phase to produce a horizontal sync output that drives the M51405. It also generates the display timing signals (blanking, sync, etc.) specific to the GG chipset and LCD screen.

The TV Tuner switches the Game Gear into a special operating mode by grounding the /TV pin, where the Z80, VDP, and other peripherals are disabled.

Cartridge connections

All unlisted pins are not connected to the TV Tuner.

Ribbon cable pinout

1+34V from cartridge connector pin 1. (Measured 33.6V)
2+5V from cartridge connector pin 2. (Measured 4.98V)
3Active high horizontal sync output (from tuner PCB to hybrid module).
4Composite video, to M51405 "VIDEO IN".
5To M51405 "TINT IN" (this comes from the tint potentiometer on the tuner PCB).
6+4.5V generated on tuner board. (measured 4.54V).
To M51405 "POWER SUPPLY 1", "POWER SUPPLY 2", "COLOR CONT" pins and the hybrid module.
7Analog ground from cartridge connector pin 17
8Monaural audio to cartridge connector pins 44 (TVSNDR) and 45 (TVSNDL)

Test points

Test points around hybrid module:

Active high horizontal sync output, but scaled to a different voltage range and the high part is half as long.
Second leftmost
Active high horizontal sync output from hybrid to M51405 "H. SYNC IN" (63.55us pulse: 4.92us high, 58.64us low)
Top left
Analog blue output from M51405 to hybrid input
Top middle
Analog green output from M51405 to hybrid input
Top right
Analog red output from M51405 to hybrid input

"A/V IN" connector

Composite video
Monaural audio

Plugging a video source into the A/V input jack disconnects the internal composite video output of the M51348 and routes the external video source to the composite video pin of the ribbon cable.

Hybrid module pin assignments:

Top side, left to right

  1 - Digital ground
  2 - Digital ground
  3 - Digital ground
  4 - (N.C.)
  5 - Analog blue input
  6 - Analog green input
  7 - Analog blue input
  8 - Capacitor to digital ground and pull-up resistor to 4.5V

Left side, top to bottom

  1 - 4.5V
  2 - To one pin of oscillator
  3 - To other pin of oscillator and adjacent trim capacitor
  4 - To leftmost test point (Modified horizontal sync output)
  5 - (N.C.)
  6 - To next leftmost test point and M51405 "H. SYNC IN" (Horizontal sync output)
  7 - (N.C.)
  8 - (N.C.)
  9 - Cartridge connector pin A3/CL1B1

Bottom side, left to right

  1 - Cartridge connector pin A4/CL1A1
  2 - Cartridge connector pin A5/CL1B2
  3 - Cartridge connector pin A6/CL1A2
  4 - Cartridge connector pin D6/DO2
  5 - Cartridge connector pin D7/DO4
  6 - Cartridge connector pin A0/DO1
  7 - Cartridge connector pin A1/DO3
  8 - Pull up resistor to 4.5V
  9 - Cartridge connector pin D0/DB
 10 - Cartridge connector pin D1/P3
 11 - Cartridge connector pin D2/P2
 12 - Cartridge connector pin D3/P4
 13 - Cartridge connector pin D4/P1

Right side, top to bottom

  1 - Common with pin 2, otherwise N.C.
  2 - Common with pin 1, otherwise N.C.
  3 - Composite sync input from ribbon cable pin 3
  4 - 4.5V
  5 - Cartridge connector pin A9/TPR1
  6 - Cartridge connector pin A10/TPR2
  7 - (N.C.)
  8 - Cartridge connector pin A2/DW
  9 - Cartridge connector pin A8/CL1A3
 10 - Cartridge connector pin A7/CL1B3
 11 - Cartridge connector pin D5/CL2

Digital video signals

There are six pixel clocks, CL1<A1,A2,A3> and CL1<B1,B2,B3>. On an even scanline during the active display period CL1A1 is clocked 80 times, then A2 is, and finally A3. When not being clocked all of these signals are held low. CL1B1-CL1B3 do the same thing for odd scanlines. Clock cycles are about 185ns (5.36 MHz). All told these provide 240 clock pulses per scanline.

CL2 is a horizontal sync signal. 63.55us period, 60.185us low, 3.37us high.

DW is a vertical blank signal. It is low for 41 scanlines and high for 222 (263 scanlines per frame).

TPR1 is a short positive pulse that is 19.61us wide at 60 Hz. The pulse width is less than a single scanline however, so it can't be used as vertical sync. TPR2 is an exact inversion of TPR1. It occurs at the rising edge of DW (end of V-Blank) Think of it as a 'first scanline of the frame' indicator.

DB changes from frame to frame. It can be high for two scanlines and low for the third, or low for two scanlines and high for the 3rd. The phase changes about 14 lines in to vertical blank (into DW).

P0-0 is a 4-bit down counter that is set to $0F on the rising edge of DW (start of a new scanline) and counts down to $00 in even steps. The clock input to this counter is approximately 256 KHz.

DO3-0 is a 4-bit data bus that contains R,G,B pixel information that is output to the GG LCD screen / ASIC.

Analog video signals

The composite video output from M51348 pin 23 ("Synchronous separation output") consists of active high horizontal sync pulses. The vertical sync period is three scanlines long and during this time the horizontal sync pulses are inverted.

Adjustment settings

The four control inputs of the M51405 are connected as follows:

The lower potentiometer on the digital video board adjusts the peak amplitude of the composite video signal at the "VIDEO IN" pin.


On a Majesco-branded Game Gear (Model 2110G) the TV tuner produces no video, only a white screen. Audio playback still functions correctly.

Chip datasheets

Service manuals

Return to top