Return of the Invaders

[Rattanee]

Hey everyone.

Just bougt a bunch of boards for next to nothing and began fixing them as I could. Already have 5 up and running, however I am having a bit of a problem with this one. It's an original boardset made by TAITO. I've looked through the boardset carefully, and compared it with the manual. I replaced the missing z80 chips and all the ceramic capacitors that have been damaged. I still get no picture, but a more in-depth look revealed no clock signal on either of the three cpus. Checked the xtal, it is oscillating.

To make things short, if anyone could point me to a set of schematics for this game, it'd greatly ease my headache.

The second, perhaps harder to fix part would be the fact that the board is missing two smaller bipolar roms, locations u18 and 19 (will have to verify these once I'm back out in the shop, I may be mistaken) however I did check the mame romset and not only these, but some other roms I have on my board aswell are missing from the mame set. If anyone has the knowhow/time to peek into the mame sourcecode to see how it still works without them, or if anyone could get me the images for these roms or perhaps even offer a cheap burning service, I'd greatly appreciate it.

[channelmaniac]

Hmmm...

I don't have a schematic on that one... but I see a 74LS368 on the board layout next to the crystal. That IC will most likely be driving the crystal. Check the inputs/outputs of it and see where they go. They should go to another chip that will divide the clock as necessary to drive all the different subsystems.

Pin 6 on each Z80 should have a clock signal. If they do, your clock isn't the issue.

The Z80 will be dead even with a clock if it's not getting a good reset signal. If that's the case, trace back the reset circuitry and look for a bad IC or missing resistor/capacitor in the reset circuit.

RJ

PS: Manual can be found here: http://www.crazykong.com/manuals/ReturnofInvaders.man.pdf

[Rattanee]

Thanks for the info, fact is I didn't even bother checking a reset signal since neither of the 3 z80's on the board are getting a clock signal, in fact pin 6 is simply stuck high.

The crytal is oscillating, and the signal does get to the first chip it goes to, and that chip does have some activity on some legs, but I'd rather have a full schematic then having to dig through the datasheet of every single logics ic on the board to see which legs are inputs and which are outputs.

If you look at the manual, on the video board layout page you can see A37-19 and A37-20in the upper left corner of the image. Those two are missing, though I'm sure they don1t have an effect on the clock, but if anyone could get me these chips, I'd appreciate it. (Even if I get the images I'm not sure I could burn the chips... or perhaps I could hack an eprom in there...)

[channelmaniac]

The first thing to do is to work backwards to find where the clock is dying. Worry about ROMs later.

Working backwards is easier and quicker than working forwards through an unknown circuit.

[Rattanee]

If anyone happens to have this boardst, I'd appeciate a few high res pictures from the ics just left from the crystal. Turns out some idiot more then slipped with his soldering iron causing some nasty board damage that I did not spot previously from all the dirt on it. (At least 3 broken traces)

Another question. I'm assuming that the 555 timer is wired to oscillate rather then run in timed mode as the trigger seems to be wired for a fixed voltage. I did replace the 555 but it is not willing to oscillate (tried 4 different chips). From the board setup to me it seems that the 555 should take part in the clock signal distribution.

[channelmaniac]

That would be odd if it was. Perhaps it's a one-shot used for generating the power on reset pulse?

[Rattanee]

Unfortunately since I don't know the exact theory behind the more complex logic ics some parts of the board design are less then completely clear. I could clearly trace the clcok signal from the crystal to the 74ls74 right to the left of the 6116 ram. There It seems to be outputting on a leg that it shouldn't be, thus I replaced the IC, but still getting the same results...

No D inputs of the IC are oscillating.

[nightmaretony]

check all inputs/outputs of the 74ls368 used for the clocking. it may divide down from there.

LOVe that game, used to have one