I have a Galaxian machine that I have modified to play 8 different games.  I've had to add
and change a few circuits, but the games all run on basically the same machine.

The Galaxian board is a single PCB that uses the Z80 for the CPU.  It takes AC, and has several
components to produce DC.  There is no seperate sound chip- discrete components are used to
make all the game sounds.  There is room on the board for 3 ROM chips, but a small daughterboard
is plugged into 2 of these sockets, and it has 5 ROMs on it.  There are 2 2102 RAM chips and
4 2114 RAM chips, as well as 5 27LS00 RAM chips.  There are also 2 additional ROMs for
character graphics.  It connects to the wiring harness by a standard 44-pin connector.

I purchased a Frogger boardset off the 'net, and was surprised to see that it is a modified
Galaxian board.  The main difference is the addition of a sound daughterboard that plugs into
the program EPROM sockets and splices into different places on the main board.  Also, the five
27LS00 object RAM chips in the bottom left of the game were replaced by larger 2125s, and a
7400 was added to decode the additional RAM space.  The board plugs directly into my Galaxian
machine and runs.  I modified it for DC operation and put it into a different cabinet.

I bought 3 boards at the Mesquite auction.  Two of them are modified Galaxian boards, and the
other is a Sega/Gremlin Frogger.  One of the Galaxian boards has a slightly larger than normal
ROM daughterboard- it holds 8 ROMs instead of 5, but only 7 are populated.  I plugged it into
my Galaxian machine, and it is a game called Defend the Terra Attack on the Red UFO.

The other Galaxian board was factory set up for DC, and has an elaborate interchangeable ROM
board.  A small daughterboard plugs into the 2 ROM sockets, and provides a 44-pin socket that
another ROM board plugs into.  A ribbon cable from this ROM board also plugs into the 2 graphics
ROM sockets.  The ROM board has room for 12 ROMs- 8 for the game and 4 for the graphics, but
only 7 were populated for the game.  I plugged it into the cabinet that the Frogger went into,
and it came up and ran.  It is a PacMan bootleg.

Lawnmower Man had sent me a diagram of this board, and said that there were a dozen or so games
that ran on it, including a PacMan bootleg.

The Sega/Gremlin Frogger that I bought in Mesquite is basically a new layout of the hacked
Galaxian Frogger that I have.  It has a 36-pin connector instead of Galaxian's 44, but it has
all the same parts, although they are in different locations.

There is an obvious evolution from the original 3-ROM version of Galaxian to the 5-ROM US
production that I have, to the 7-ROM Terra Attack, to the interchangeable 'copy board' to
the Galaxian Frogger and finally to the redesigned Sega/Gremlin Frogger.  There are only minor
design changes, most notably the addition of the AY8910 sound chip.  This means that a basic
Galaxian board can be hacked pretty easily to play a large number of games.