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Gallag "strange" problem
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IZ8DWF
Hi all,
my first post here, I've searched around this forum for similar problems and it seems I have a new entry.
Also, overall, this is the best places in the world for Galaga bootleg boards :)
I have got a two board set Galaga bootleg, looking on the net, it appears to be the "gallag" set, however, the edge pinout doesn't seem to be right, for example, pads E and F are empty. However I managed to make a power and connections adapter, I can set the test/setup mode, hear sounds and see the video output, so far so good.
(You can seen preliminary works on this board here:
Test mode works fine, I can go through all the sounds, exercise all the inputs, I can see the dip switch changes, it does not report any error, but there's a switch that displays a 99999.99999. (more 9) on the bottom of the screen (I can post a picture of this, if it's not something that should happen).
When exiting the test mode, or starting with test switch off, the game displays Credits FF and goes directly on the P1 or P2 start page. Exercising any input, makes a two players game start (even the test/setup switch does it) and what basicly happens is the galaga starship goes right until it stops on the end of the field, fires alone and there's a continous sound of the "coin inserted".
Sometimes this situation returns to normality when I'm probing around with oscilloscope or logic probe, but I couldn't identify a particular PCB zone where it does it (though I'm going to examine the solder joints soon).
When it returns to "normal", the ship can be moved, fired normally and there're no "coins sounds" anymore.
I've dumped all ROMs and run them into MAME, the game runs fine there. It complains about wrong checksum on two of them, I've noticed small differences with the MAME's official "gallag", probably because this romset displays "Galaga" as game name?
I've noticed, when reseating all 4 CPUs, that one socket was marked with D780C-1 but had a D780C like all other, all the same 1982 date codes, so they look like it was always like this. However, I've now put a Zilog Z-80A on this socket (and nothing changed).
However, all the 4 CPUs have the same 3.070 MHz (it's the main crystal oscillator divided by 6 as I tracked it) clock signal, so why only one needs to be a 4 MHz CPU?
I've looked around on the switch buffers (LS244) but they appear to be good, I assume it would not work in test mode otherwise.
I've started to draw the actual PCB schematic in Kicad, since the only "gallag" schematic I've been able to find on the net isn't obviously matching what I have. For example, one CPU in the schematic is reported in row 10, in this PCB all CPUs are in row 6 instead. Also the schematic reports components in row 12 and this pcb only has 11 rows.
I'm going to check the added Z80 (with respect to the original Galaga), since I'm assuming it's not very well exercised from the original self-test mode.
What the experts thinks about this?
Thanks and best regards!
Frank IZ8DWF
Edited by IZ8DWFtypo
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