I bought an unknown Sega PCB from ebay recently, and it turned out to be the lower PCB of a Frogger.
I assume Sega once had lots of moon cresta PCBs left over, and so designed an add on daughterboard to convert them into Frogger.
I didn’t have the top half for this PCB anyway, so I decided to convert it back to normal galaxian mappings so I could use a multigame on it.
Here is what you need to do :-
- Remove all of the wires added to the top of the board. Most of these end in plugs, so I assume they connect to the daughterboard to allow it to pick up various signals – we don’t need them any more.
- Repair the cut track on the top of the board at 2N near pin 8. The through plated hole is drilled to stop the connection of pin 6 to the program ram select (7N and 7P pin 8) – that re-enables the ram, but it still has the wrong memory map so …
- Underneath the board, cut the track to 8E pin 6, and add a wire link to connect the track to pin 5 instead. The memory is now back in it’s usual galaxian location.
- Now we need to change the NMI enable line back to it’s normal location. so again, on the underside of the board, cut the track to 9N pin 4, and add a wire link to connect it to pin 5 instead.
- Whilst we are working on 9N, look at the track from pin 9 – this is the star enable line, and it’s been cut, so simply add a wire link to repair this track.
- But the stars still don’t work – this is because they have also drilled another through plated hole that feeds the clock to the star generator, this time located on the top of the board at 6D near pins 13 and 14. Now there wasn’t enough track visible to repair this as we did before, so instead turn the board over and add a link from the hole to pin 1. Now we have stars again.
- Now lets link the sound back up, underneath C43 (by the dipswitch) you will see a track has been cut neatly in the middle. This is the input to the amplifier section, so simply scrape each side of the track to get some copper visible, and place a wire link over the gap.
- Finally, they have done some mods to the colour circuitry to change the layout of the colour map. (this is why the original colour prom for FROG looks awful in mame) On the top of the board repair the cut from pin 15. on the underside of the board repair the cuts to tracks from pins 2 and 19.
- The service button does not connect the same as on galaxian, instead it is hard wired to a circuit to reset the board. The edge connector is on the top, to the right of pin L. Turn the board over and you can see the modified track for this bypasses the pin of the inline resistor and instead carries on. Cut this track, and then add a wire link from the through hole of the edge connector to the pin of the inline resistor.
- On my board the IC at 1.2S (shoehorned in sideways between rows 1 and 2 by the sprite ram) was missing. This allows you to invert the signals being output from the sprite ram. If you use D2125 ram, then you put a 7404 into this slot to invert the output. If you use 27LS00 sprite ram then you use a 7407 instead.