Galaga Starfield Missing - NOT 05xx chip issue

texasmame

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Got a Galaga boardset that isn't displaying the starfield but everything else is fine.

All the research I've done points to the 05xx chip as the starfield generator. Got a new 05xx chip from the person here who sold me this board but still no starfield after swapping.

Reflowed the socket for the 05xx and tested continuity for each leg with the bottom of the PCB - they all test good. So, looks like the socket is not the problem. Legs nice and clean on the chip, too.

Where else to look? The schematic shows several other chips also sending input to the 05xx but I've yet to see a post anywhere online that points to something other than the 05xx as creating issues with the starfield.

The R1 thru R8 all test within tolerance in circuit, too.

Thoughts, suggestions?

TIA!
 
Make sure you have the /VSYNC signal on pin 4, /HSYNC on pin 2, and the 6MHz Clock on pin 1.

If those are good, replace the 74LS259 at 5K.

RJ
 
Make sure you have the /VSYNC signal on pin 4, /HSYNC on pin 2, and the 6MHz Clock on pin 1.

If those are good, replace the 74LS259 at 5K.

RJ

No way for me to test the syncs and clock unless a multimeter can do that.

Can a 257 be used instead - have no 259 on hand but do have 257.
 
The 05XX is VERY close to the actual video output. Are you sure the 05XX is good? (I.e. has it been tested on another Galaga PCB to verify its functionality?)

If you can be certain that it's good, or have a logic probe to check things out, look at:
the LS259 @ 5K, the LS86 @ 4B, the LS20 @5L. The first appears to take commands from the CPU bus and pass them to the SF generator. The latter two appear to be involved in disabling the starfield if there's other video present (to effectively make it the lowest priority, so any text or sprite is always on top of it.) If some of that logic gets "stuck" it might always disable the starfield generator...
 
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The 05XX is VERY close to the actual video output. Are you sure the 05XX is good? (I.e. has it been tested on another Galaga PCB to verify its functionality?)

If you can be certain that it's good, or have a logic probe to check things out, look at:
the LS259 @ 5K, the LS86 @ 4B, the LS20 @5L. Those appear to be involved in disabling the starfield if there's other video present (to effectively make it the lowest priority, so and text or sprite is always on top of it.) If some of that logic gets "stuck" it might always disable the starfield generator...

Well, I've tried two different 05xx and neither work. No logic probe here, just a multimeter. Roger on the 257/259 question, thx.
 
I bought my logic probe from mikes arcade when I ordered once it has been a great tool.
 
Accurate PCB-level testing and diagnosis is difficult without a logic probe or oscilloscope.

+1

you are really kind of blind as to what is actually going on there without a logic probe, scope, logic analyzer.

you really need one of em.
 
Yeah, they aren't nearly as expensive as I was thinking.

Probably get the Rat Shack one as the multimeter I got from there has been with me for years without issue.

In the meantime, waiting for this boardset to be made good by the seller. . .

Great!

the multimeter will have a new friend and room mate and you have another powerful tool in your arsenal.
 
Did you get a 54xx star field generator chip to test? I have several and might take a day or two to find a known working one.
 
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